SSL tcp_output.c

Sprache: C

// SPDX-License-Identifier: GPL-2.0-only
/*
* INET An implementation of the TCP/IP protocol suite for the LINUX
* operating system.  INET is implemented using the  BSD Socket
* interface as the means of communication with the user level.
*
* Implementation of the Transmission Control Protocol(TCP).
*
* Authors: Ross Biro
* Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
* Mark Evans, <evansmp@uhura.aston.ac.uk>
* Corey Minyard <wf-rch!minyard@relay.EU.net>
* Florian La Roche, <flla@stud.uni-sb.de>
* Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
* Linus Torvalds, <torvalds@cs.helsinki.fi>
* Alan Cox, <gw4pts@gw4pts.ampr.org>
* Matthew Dillon, <dillon@apollo.west.oic.com>
* Arnt Gulbrandsen, <agulbra@nvg.unit.no>
* Jorge Cwik, <jorge@laser.satlink.net>
*/

/*
* Changes: Pedro Roque : Retransmit queue handled by TCP.
* : Fragmentation on mtu decrease
* : Segment collapse on retransmit
* : AF independence
*
* Linus Torvalds : send_delayed_ack
* David S. Miller : Charge memory using the right skb
* during syn/ack processing.
* David S. Miller : Output engine completely rewritten.
* Andrea Arcangeli: SYNACK carry ts_recent in tsecr.
* Cacophonix Gaul : draft-minshall-nagle-01
* J Hadi Salim : ECN support
*
*/

#define pr_fmt(fmt) "TCP: " fmt

#include <net/tcp.h>
#include <net/mptcp.h>
#include <net/proto_memory.h>

#include <linux/compiler.h>
#include <linux/gfp.h>
#include <linux/module.h>
#include <linux/static_key.h>
#include <linux/skbuff_ref.h>

#include <trace/events/tcp.h>

/* Refresh clocks of a TCP socket,
* ensuring monotically increasing values.
*/
void tcp_mstamp_refresh(struct tcp_sock *tp)
{
u64 val = tcp_clock_ns();

tp->tcp_clock_cache = val;
tp->tcp_mstamp = div_u64(val, NSEC_PER_USEC);
}

static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
      int push_one, gfp_t gfp);

/* Account for new data that has been sent to the network. */
static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
unsigned int prior_packets = tp->packets_out;

WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(skb)->end_seq);

__skb_unlink(skb, &sk->sk_write_queue);
tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);

if (tp->highest_sack == NULL)
  tp->highest_sack = skb;

tp->packets_out += tcp_skb_pcount(skb);
if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
  tcp_rearm_rto(sk);

NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
        tcp_skb_pcount(skb));
tcp_check_space(sk);
}

/* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
* window scaling factor due to loss of precision.
* If window has been shrunk, what should we make? It is not clear at all.
* Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
* Anything in between SND.UNA...SND.UNA+SND.WND also can be already
* invalid. OK, let's make this for now:
*/
static inline __u32 tcp_acceptable_seq(const struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);

if (!before(tcp_wnd_end(tp), tp->snd_nxt) ||
     (tp->rx_opt.wscale_ok &&
      ((tp->snd_nxt - tcp_wnd_end(tp)) < (1 << tp->rx_opt.rcv_wscale))))
  return tp->snd_nxt;
else
  return tcp_wnd_end(tp);
}

/* Calculate mss to advertise in SYN segment.
* RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
*
* 1. It is independent of path mtu.
* 2. Ideally, it is maximal possible segment size i.e. 65535-40.
* 3. For IPv4 it is reasonable to calculate it from maximal MTU of
*    attached devices, because some buggy hosts are confused by
*    large MSS.
* 4. We do not make 3, we advertise MSS, calculated from first
*    hop device mtu, but allow to raise it to ip_rt_min_advmss.
*    This may be overridden via information stored in routing table.
* 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
*    probably even Jumbo".
*/
static __u16 tcp_advertise_mss(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
const struct dst_entry *dst = __sk_dst_get(sk);
int mss = tp->advmss;

if (dst) {
  unsigned int metric = dst_metric_advmss(dst);

  if (metric < mss) {
   mss = metric;
   tp->advmss = mss;
  }
}

return (__u16)mss;
}

/* RFC2861. Reset CWND after idle period longer RTO to "restart window".
* This is the first part of cwnd validation mechanism.
*/
void tcp_cwnd_restart(struct sock *sk, s32 delta)
{
struct tcp_sock *tp = tcp_sk(sk);
u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
u32 cwnd = tcp_snd_cwnd(tp);

tcp_ca_event(sk, CA_EVENT_CWND_RESTART);

tp->snd_ssthresh = tcp_current_ssthresh(sk);
restart_cwnd = min(restart_cwnd, cwnd);

while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
  cwnd >>= 1;
tcp_snd_cwnd_set(tp, max(cwnd, restart_cwnd));
tp->snd_cwnd_stamp = tcp_jiffies32;
tp->snd_cwnd_used = 0;
}

/* Congestion state accounting after a packet has been sent. */
static void tcp_event_data_sent(struct tcp_sock *tp,
    struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
const u32 now = tcp_jiffies32;

if (tcp_packets_in_flight(tp) == 0)
  tcp_ca_event(sk, CA_EVENT_TX_START);

tp->lsndtime = now;

/* If it is a reply for ato after last received
* packet, increase pingpong count.
*/
if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
  inet_csk_inc_pingpong_cnt(sk);
}

/* Account for an ACK we sent. */
static inline void tcp_event_ack_sent(struct sock *sk, u32 rcv_nxt)
{
struct tcp_sock *tp = tcp_sk(sk);

if (unlikely(tp->compressed_ack)) {
  NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
         tp->compressed_ack);
  tp->compressed_ack = 0;
  if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
   __sock_put(sk);
}

if (unlikely(rcv_nxt != tp->rcv_nxt))
  return;  /* Special ACK sent by DCTCP to reflect ECN */
tcp_dec_quickack_mode(sk);
inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
}

/* Determine a window scaling and initial window to offer.
* Based on the assumption that the given amount of space
* will be offered. Store the results in the tp structure.
* NOTE: for smooth operation initial space offering should
* be a multiple of mss if possible. We assume here that mss >= 1.
* This MUST be enforced by all callers.
*/
void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
          __u32 *rcv_wnd, __u32 *__window_clamp,
          int wscale_ok, __u8 *rcv_wscale,
          __u32 init_rcv_wnd)
{
unsigned int space = (__space < 0 ? 0 : __space);
u32 window_clamp = READ_ONCE(*__window_clamp);

/* If no clamp set the clamp to the max possible scaled window */
if (window_clamp == 0)
  window_clamp = (U16_MAX << TCP_MAX_WSCALE);
space = min(window_clamp, space);

/* Quantize space offering to a multiple of mss if possible. */
if (space > mss)
  space = rounddown(space, mss);

/* NOTE: offering an initial window larger than 32767
* will break some buggy TCP stacks. If the admin tells us
* it is likely we could be speaking with such a buggy stack
* we will truncate our initial window offering to 32K-1
* unless the remote has sent us a window scaling option,
* which we interpret as a sign the remote TCP is not
* misinterpreting the window field as a signed quantity.
*/
if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
  (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
else
  (*rcv_wnd) = space;

if (init_rcv_wnd)
  *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);

*rcv_wscale = 0;
if (wscale_ok) {
  /* Set window scaling on max possible window */
  space = max_t(u32, space, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
  space = max_t(u32, space, READ_ONCE(sysctl_rmem_max));
  space = min_t(u32, space, window_clamp);
  *rcv_wscale = clamp_t(int, ilog2(space) - 15,
          0, TCP_MAX_WSCALE);
}
/* Set the clamp no higher than max representable value */
WRITE_ONCE(*__window_clamp,
     min_t(__u32, U16_MAX << (*rcv_wscale), window_clamp));
}
EXPORT_IPV6_MOD(tcp_select_initial_window);

/* Chose a new window to advertise, update state in tcp_sock for the
* socket, and return result with RFC1323 scaling applied.  The return
* value can be stuffed directly into th->window for an outgoing
* frame.
*/
static u16 tcp_select_window(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct net *net = sock_net(sk);
u32 old_win = tp->rcv_wnd;
u32 cur_win, new_win;

/* Make the window 0 if we failed to queue the data because we
* are out of memory.
*/
if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) {
  tp->pred_flags = 0;
  tp->rcv_wnd = 0;
  tp->rcv_wup = tp->rcv_nxt;
  return 0;
}

cur_win = tcp_receive_window(tp);
new_win = __tcp_select_window(sk);
if (new_win < cur_win) {
  /* Danger Will Robinson!
* Don't update rcv_wup/rcv_wnd here or else
* we will not be able to advertise a zero
* window in time.  --DaveM
*
* Relax Will Robinson.
*/
  if (!READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) || !tp->rx_opt.rcv_wscale) {
   /* Never shrink the offered window */
   if (new_win == 0)
    NET_INC_STATS(net, LINUX_MIB_TCPWANTZEROWINDOWADV);
   new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
  }
}

tp->rcv_wnd = new_win;
tp->rcv_wup = tp->rcv_nxt;

/* Make sure we do not exceed the maximum possible
* scaled window.
*/
if (!tp->rx_opt.rcv_wscale &&
     READ_ONCE(net->ipv4.sysctl_tcp_workaround_signed_windows))
  new_win = min(new_win, MAX_TCP_WINDOW);
else
  new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));

/* RFC1323 scaling applied */
new_win >>= tp->rx_opt.rcv_wscale;

/* If we advertise zero window, disable fast path. */
if (new_win == 0) {
  tp->pred_flags = 0;
  if (old_win)
   NET_INC_STATS(net, LINUX_MIB_TCPTOZEROWINDOWADV);
} else if (old_win == 0) {
  NET_INC_STATS(net, LINUX_MIB_TCPFROMZEROWINDOWADV);
}

return new_win;
}

/* Packet ECN state for a SYN-ACK */
static void tcp_ecn_send_synack(struct sock *sk, struct sk_buff *skb)
{
const struct tcp_sock *tp = tcp_sk(sk);

TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
if (tcp_ecn_disabled(tp))
  TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
else if (tcp_ca_needs_ecn(sk) ||
   tcp_bpf_ca_needs_ecn(sk))
  INET_ECN_xmit(sk);
}

/* Packet ECN state for a SYN.  */
static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
bool use_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn) == 1 ||
  tcp_ca_needs_ecn(sk) || bpf_needs_ecn;

if (!use_ecn) {
  const struct dst_entry *dst = __sk_dst_get(sk);

  if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
   use_ecn = true;
}

tp->ecn_flags = 0;

if (use_ecn) {
  TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
  tcp_ecn_mode_set(tp, TCP_ECN_MODE_RFC3168);
  if (tcp_ca_needs_ecn(sk) || bpf_needs_ecn)
   INET_ECN_xmit(sk);
}
}

static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
{
if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback))
  /* tp->ecn_flags are cleared at a later point in time when
* SYN ACK is ultimatively being received.
*/
  TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
}

static void
tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
{
if (inet_rsk(req)->ecn_ok)
  th->ece = 1;
}

/* Set up ECN state for a packet on a ESTABLISHED socket that is about to
* be sent.
*/
static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
    struct tcphdr *th, int tcp_header_len)
{
struct tcp_sock *tp = tcp_sk(sk);

if (tcp_ecn_mode_rfc3168(tp)) {
  /* Not-retransmitted data segment: set ECT and inject CWR. */
  if (skb->len != tcp_header_len &&
      !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
   INET_ECN_xmit(sk);
   if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
    tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
    th->cwr = 1;
    skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
   }
  } else if (!tcp_ca_needs_ecn(sk)) {
   /* ACK or retransmitted segment: clear ECT|CE */
   INET_ECN_dontxmit(sk);
  }
  if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
   th->ece = 1;
}
}

/* Constructs common control bits of non-data skb. If SYN/FIN is present,
* auto increment end seqno.
*/
static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u16 flags)
{
skb->ip_summed = CHECKSUM_PARTIAL;

TCP_SKB_CB(skb)->tcp_flags = flags;

tcp_skb_pcount_set(skb, 1);

TCP_SKB_CB(skb)->seq = seq;
if (flags & (TCPHDR_SYN | TCPHDR_FIN))
  seq++;
TCP_SKB_CB(skb)->end_seq = seq;
}

static inline bool tcp_urg_mode(const struct tcp_sock *tp)
{
return tp->snd_una != tp->snd_up;
}

#define OPTION_SACK_ADVERTISE BIT(0)
#define OPTION_TS  BIT(1)
#define OPTION_MD5  BIT(2)
#define OPTION_WSCALE  BIT(3)
#define OPTION_FAST_OPEN_COOKIE BIT(8)
#define OPTION_SMC  BIT(9)
#define OPTION_MPTCP  BIT(10)
#define OPTION_AO  BIT(11)

static void smc_options_write(__be32 *ptr, u16 *options)
{
#if IS_ENABLED(CONFIG_SMC)
if (static_branch_unlikely(&tcp_have_smc)) {
  if (unlikely(OPTION_SMC & *options)) {
   *ptr++ = htonl((TCPOPT_NOP  << 24) |
           (TCPOPT_NOP  << 16) |
           (TCPOPT_EXP <<  8) |
           (TCPOLEN_EXP_SMC_BASE));
   *ptr++ = htonl(TCPOPT_SMC_MAGIC);
  }
}
#endif
}

struct tcp_out_options {
u16 options;  /* bit field of OPTION_* */
u16 mss;  /* 0 to disable */
u8 ws;   /* window scale, 0 to disable */
u8 num_sack_blocks; /* number of SACK blocks to include */
u8 hash_size;  /* bytes in hash_location */
u8 bpf_opt_len;  /* length of BPF hdr option */
__u8 *hash_location; /* temporary pointer, overloaded */
__u32 tsval, tsecr; /* need to include OPTION_TS */
struct tcp_fastopen_cookie *fastopen_cookie; /* Fast open cookie */
struct mptcp_out_options mptcp;
};

static void mptcp_options_write(struct tcphdr *th, __be32 *ptr,
    struct tcp_sock *tp,
    struct tcp_out_options *opts)
{
#if IS_ENABLED(CONFIG_MPTCP)
if (unlikely(OPTION_MPTCP & opts->options))
  mptcp_write_options(th, ptr, tp, &opts->mptcp);
#endif
}

#ifdef CONFIG_CGROUP_BPF
static int bpf_skops_write_hdr_opt_arg0(struct sk_buff *skb,
     enum tcp_synack_type synack_type)
{
if (unlikely(!skb))
  return BPF_WRITE_HDR_TCP_CURRENT_MSS;

if (unlikely(synack_type == TCP_SYNACK_COOKIE))
  return BPF_WRITE_HDR_TCP_SYNACK_COOKIE;

return 0;
}

/* req, syn_skb and synack_type are used when writing synack */
static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
      struct request_sock *req,
      struct sk_buff *syn_skb,
      enum tcp_synack_type synack_type,
      struct tcp_out_options *opts,
      unsigned int *remaining)
{
struct bpf_sock_ops_kern sock_ops;
int err;

if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
        BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
     !*remaining)
  return;

/* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */

/* init sock_ops */
memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));

sock_ops.op = BPF_SOCK_OPS_HDR_OPT_LEN_CB;

if (req) {
  /* The listen "sk" cannot be passed here because
* it is not locked.  It would not make too much
* sense to do bpf_setsockopt(listen_sk) based
* on individual connection request also.
*
* Thus, "req" is passed here and the cgroup-bpf-progs
* of the listen "sk" will be run.
*
* "req" is also used here for fastopen even the "sk" here is
* a fullsock "child" sk.  It is to keep the behavior
* consistent between fastopen and non-fastopen on
* the bpf programming side.
*/
  sock_ops.sk = (struct sock *)req;
  sock_ops.syn_skb = syn_skb;
} else {
  sock_owned_by_me(sk);

  sock_ops.is_fullsock = 1;
  sock_ops.is_locked_tcp_sock = 1;
  sock_ops.sk = sk;
}

sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
sock_ops.remaining_opt_len = *remaining;
/* tcp_current_mss() does not pass a skb */
if (skb)
  bpf_skops_init_skb(&sock_ops, skb, 0);

err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);

if (err || sock_ops.remaining_opt_len == *remaining)
  return;

opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
/* round up to 4 bytes */
opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;

*remaining -= opts->bpf_opt_len;
}

static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
        struct request_sock *req,
        struct sk_buff *syn_skb,
        enum tcp_synack_type synack_type,
        struct tcp_out_options *opts)
{
u8 first_opt_off, nr_written, max_opt_len = opts->bpf_opt_len;
struct bpf_sock_ops_kern sock_ops;
int err;

if (likely(!max_opt_len))
  return;

memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));

sock_ops.op = BPF_SOCK_OPS_WRITE_HDR_OPT_CB;

if (req) {
  sock_ops.sk = (struct sock *)req;
  sock_ops.syn_skb = syn_skb;
} else {
  sock_owned_by_me(sk);

  sock_ops.is_fullsock = 1;
  sock_ops.is_locked_tcp_sock = 1;
  sock_ops.sk = sk;
}

sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
sock_ops.remaining_opt_len = max_opt_len;
first_opt_off = tcp_hdrlen(skb) - max_opt_len;
bpf_skops_init_skb(&sock_ops, skb, first_opt_off);

err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);

if (err)
  nr_written = 0;
else
  nr_written = max_opt_len - sock_ops.remaining_opt_len;

if (nr_written < max_opt_len)
  memset(skb->data + first_opt_off + nr_written, TCPOPT_NOP,
         max_opt_len - nr_written);
}
#else
static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
      struct request_sock *req,
      struct sk_buff *syn_skb,
      enum tcp_synack_type synack_type,
      struct tcp_out_options *opts,
      unsigned int *remaining)
{
}

static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
        struct request_sock *req,
        struct sk_buff *syn_skb,
        enum tcp_synack_type synack_type,
        struct tcp_out_options *opts)
{
}
#endif

static __be32 *process_tcp_ao_options(struct tcp_sock *tp,
          const struct tcp_request_sock *tcprsk,
          struct tcp_out_options *opts,
          struct tcp_key *key, __be32 *ptr)
{
#ifdef CONFIG_TCP_AO
u8 maclen = tcp_ao_maclen(key->ao_key);

if (tcprsk) {
  u8 aolen = maclen + sizeof(struct tcp_ao_hdr);

  *ptr++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
          (tcprsk->ao_keyid << 8) |
          (tcprsk->ao_rcv_next));
} else {
  struct tcp_ao_key *rnext_key;
  struct tcp_ao_info *ao_info;

  ao_info = rcu_dereference_check(tp->ao_info,
   lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
  rnext_key = READ_ONCE(ao_info->rnext_key);
  if (WARN_ON_ONCE(!rnext_key))
   return ptr;
  *ptr++ = htonl((TCPOPT_AO << 24) |
          (tcp_ao_len(key->ao_key) << 16) |
          (key->ao_key->sndid << 8) |
          (rnext_key->rcvid));
}
opts->hash_location = (__u8 *)ptr;
ptr += maclen / sizeof(*ptr);
if (unlikely(maclen % sizeof(*ptr))) {
  memset(ptr, TCPOPT_NOP, sizeof(*ptr));
  ptr++;
}
#endif
return ptr;
}

/* Write previously computed TCP options to the packet.
*
* Beware: Something in the Internet is very sensitive to the ordering of
* TCP options, we learned this through the hard way, so be careful here.
* Luckily we can at least blame others for their non-compliance but from
* inter-operability perspective it seems that we're somewhat stuck with
* the ordering which we have been using if we want to keep working with
* those broken things (not that it currently hurts anybody as there isn't
* particular reason why the ordering would need to be changed).
*
* At least SACK_PERM as the first option is known to lead to a disaster
* (but it may well be that other scenarios fail similarly).
*/
static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
         const struct tcp_request_sock *tcprsk,
         struct tcp_out_options *opts,
         struct tcp_key *key)
{
__be32 *ptr = (__be32 *)(th + 1);
u16 options = opts->options; /* mungable copy */

if (tcp_key_is_md5(key)) {
  *ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
          (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
  /* overload cookie hash location */
  opts->hash_location = (__u8 *)ptr;
  ptr += 4;
} else if (tcp_key_is_ao(key)) {
  ptr = process_tcp_ao_options(tp, tcprsk, opts, key, ptr);
}
if (unlikely(opts->mss)) {
  *ptr++ = htonl((TCPOPT_MSS << 24) |
          (TCPOLEN_MSS << 16) |
          opts->mss);
}

if (likely(OPTION_TS & options)) {
  if (unlikely(OPTION_SACK_ADVERTISE & options)) {
   *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
           (TCPOLEN_SACK_PERM << 16) |
           (TCPOPT_TIMESTAMP << 8) |
           TCPOLEN_TIMESTAMP);
   options &= ~OPTION_SACK_ADVERTISE;
  } else {
   *ptr++ = htonl((TCPOPT_NOP << 24) |
           (TCPOPT_NOP << 16) |
           (TCPOPT_TIMESTAMP << 8) |
           TCPOLEN_TIMESTAMP);
  }
  *ptr++ = htonl(opts->tsval);
  *ptr++ = htonl(opts->tsecr);
}

if (unlikely(OPTION_SACK_ADVERTISE & options)) {
  *ptr++ = htonl((TCPOPT_NOP << 24) |
          (TCPOPT_NOP << 16) |
          (TCPOPT_SACK_PERM << 8) |
          TCPOLEN_SACK_PERM);
}

if (unlikely(OPTION_WSCALE & options)) {
  *ptr++ = htonl((TCPOPT_NOP << 24) |
          (TCPOPT_WINDOW << 16) |
          (TCPOLEN_WINDOW << 8) |
          opts->ws);
}

if (unlikely(opts->num_sack_blocks)) {
  struct tcp_sack_block *sp = tp->rx_opt.dsack ?
   tp->duplicate_sack : tp->selective_acks;
  int this_sack;

  *ptr++ = htonl((TCPOPT_NOP  << 24) |
          (TCPOPT_NOP  << 16) |
          (TCPOPT_SACK <<  8) |
          (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
           TCPOLEN_SACK_PERBLOCK)));

  for (this_sack = 0; this_sack < opts->num_sack_blocks;
       ++this_sack) {
   *ptr++ = htonl(sp[this_sack].start_seq);
   *ptr++ = htonl(sp[this_sack].end_seq);
  }

  tp->rx_opt.dsack = 0;
}

if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
  struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
  u8 *p = (u8 *)ptr;
  u32 len; /* Fast Open option length */

  if (foc->exp) {
   len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
   *ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
         TCPOPT_FASTOPEN_MAGIC);
   p += TCPOLEN_EXP_FASTOPEN_BASE;
  } else {
   len = TCPOLEN_FASTOPEN_BASE + foc->len;
   *p++ = TCPOPT_FASTOPEN;
   *p++ = len;
  }

  memcpy(p, foc->val, foc->len);
  if ((len & 3) == 2) {
   p[foc->len] = TCPOPT_NOP;
   p[foc->len + 1] = TCPOPT_NOP;
  }
  ptr += (len + 3) >> 2;
}

smc_options_write(ptr, &options);

mptcp_options_write(th, ptr, tp, opts);
}

static void smc_set_option(const struct tcp_sock *tp,
      struct tcp_out_options *opts,
      unsigned int *remaining)
{
#if IS_ENABLED(CONFIG_SMC)
if (static_branch_unlikely(&tcp_have_smc)) {
  if (tp->syn_smc) {
   if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
    opts->options |= OPTION_SMC;
    *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
   }
  }
}
#endif
}

static void smc_set_option_cond(const struct tcp_sock *tp,
    const struct inet_request_sock *ireq,
    struct tcp_out_options *opts,
    unsigned int *remaining)
{
#if IS_ENABLED(CONFIG_SMC)
if (static_branch_unlikely(&tcp_have_smc)) {
  if (tp->syn_smc && ireq->smc_ok) {
   if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
    opts->options |= OPTION_SMC;
    *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
   }
  }
}
#endif
}

static void mptcp_set_option_cond(const struct request_sock *req,
      struct tcp_out_options *opts,
      unsigned int *remaining)
{
if (rsk_is_mptcp(req)) {
  unsigned int size;

  if (mptcp_synack_options(req, &size, &opts->mptcp)) {
   if (*remaining >= size) {
    opts->options |= OPTION_MPTCP;
    *remaining -= size;
   }
  }
}
}

/* Compute TCP options for SYN packets. This is not the final
* network wire format yet.
*/
static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
    struct tcp_out_options *opts,
    struct tcp_key *key)
{
struct tcp_sock *tp = tcp_sk(sk);
unsigned int remaining = MAX_TCP_OPTION_SPACE;
struct tcp_fastopen_request *fastopen = tp->fastopen_req;
bool timestamps;

/* Better than switch (key.type) as it has static branches */
if (tcp_key_is_md5(key)) {
  timestamps = false;
  opts->options |= OPTION_MD5;
  remaining -= TCPOLEN_MD5SIG_ALIGNED;
} else {
  timestamps = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps);
  if (tcp_key_is_ao(key)) {
   opts->options |= OPTION_AO;
   remaining -= tcp_ao_len_aligned(key->ao_key);
  }
}

/* We always get an MSS option.  The option bytes which will be seen in
* normal data packets should timestamps be used, must be in the MSS
* advertised.  But we subtract them from tp->mss_cache so that
* calculations in tcp_sendmsg are simpler etc.  So account for this
* fact here if necessary.  If we don't do this correctly, as a
* receiver we won't recognize data packets as being full sized when we
* should, and thus we won't abide by the delayed ACK rules correctly.
* SACKs don't matter, we never delay an ACK when we have any of those
* going out.  */
opts->mss = tcp_advertise_mss(sk);
remaining -= TCPOLEN_MSS_ALIGNED;

if (likely(timestamps)) {
  opts->options |= OPTION_TS;
  opts->tsval = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) + tp->tsoffset;
  opts->tsecr = tp->rx_opt.ts_recent;
  remaining -= TCPOLEN_TSTAMP_ALIGNED;
}
if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling))) {
  opts->ws = tp->rx_opt.rcv_wscale;
  opts->options |= OPTION_WSCALE;
  remaining -= TCPOLEN_WSCALE_ALIGNED;
}
if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_sack))) {
  opts->options |= OPTION_SACK_ADVERTISE;
  if (unlikely(!(OPTION_TS & opts->options)))
   remaining -= TCPOLEN_SACKPERM_ALIGNED;
}

if (fastopen && fastopen->cookie.len >= 0) {
  u32 need = fastopen->cookie.len;

  need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
            TCPOLEN_FASTOPEN_BASE;
  need = (need + 3) & ~3U;  /* Align to 32 bits */
  if (remaining >= need) {
   opts->options |= OPTION_FAST_OPEN_COOKIE;
   opts->fastopen_cookie = &fastopen->cookie;
   remaining -= need;
   tp->syn_fastopen = 1;
   tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
  }
}

smc_set_option(tp, opts, &remaining);

if (sk_is_mptcp(sk)) {
  unsigned int size;

  if (mptcp_syn_options(sk, skb, &size, &opts->mptcp)) {
   if (remaining >= size) {
    opts->options |= OPTION_MPTCP;
    remaining -= size;
   }
  }
}

bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);

return MAX_TCP_OPTION_SPACE - remaining;
}

/* Set up TCP options for SYN-ACKs. */
static unsigned int tcp_synack_options(const struct sock *sk,
           struct request_sock *req,
           unsigned int mss, struct sk_buff *skb,
           struct tcp_out_options *opts,
           const struct tcp_key *key,
           struct tcp_fastopen_cookie *foc,
           enum tcp_synack_type synack_type,
           struct sk_buff *syn_skb)
{
struct inet_request_sock *ireq = inet_rsk(req);
unsigned int remaining = MAX_TCP_OPTION_SPACE;

if (tcp_key_is_md5(key)) {
  opts->options |= OPTION_MD5;
  remaining -= TCPOLEN_MD5SIG_ALIGNED;

  /* We can't fit any SACK blocks in a packet with MD5 + TS
* options. There was discussion about disabling SACK
* rather than TS in order to fit in better with old,
* buggy kernels, but that was deemed to be unnecessary.
*/
  if (synack_type != TCP_SYNACK_COOKIE)
   ireq->tstamp_ok &= !ireq->sack_ok;
} else if (tcp_key_is_ao(key)) {
  opts->options |= OPTION_AO;
  remaining -= tcp_ao_len_aligned(key->ao_key);
  ireq->tstamp_ok &= !ireq->sack_ok;
}

/* We always send an MSS option. */
opts->mss = mss;
remaining -= TCPOLEN_MSS_ALIGNED;

if (likely(ireq->wscale_ok)) {
  opts->ws = ireq->rcv_wscale;
  opts->options |= OPTION_WSCALE;
  remaining -= TCPOLEN_WSCALE_ALIGNED;
}
if (likely(ireq->tstamp_ok)) {
  opts->options |= OPTION_TS;
  opts->tsval = tcp_skb_timestamp_ts(tcp_rsk(req)->req_usec_ts, skb) +
         tcp_rsk(req)->ts_off;
  if (!tcp_rsk(req)->snt_tsval_first) {
   if (!opts->tsval)
    opts->tsval = ~0U;
   tcp_rsk(req)->snt_tsval_first = opts->tsval;
  }
  WRITE_ONCE(tcp_rsk(req)->snt_tsval_last, opts->tsval);
  opts->tsecr = req->ts_recent;
  remaining -= TCPOLEN_TSTAMP_ALIGNED;
}
if (likely(ireq->sack_ok)) {
  opts->options |= OPTION_SACK_ADVERTISE;
  if (unlikely(!ireq->tstamp_ok))
   remaining -= TCPOLEN_SACKPERM_ALIGNED;
}
if (foc != NULL && foc->len >= 0) {
  u32 need = foc->len;

  need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
       TCPOLEN_FASTOPEN_BASE;
  need = (need + 3) & ~3U;  /* Align to 32 bits */
  if (remaining >= need) {
   opts->options |= OPTION_FAST_OPEN_COOKIE;
   opts->fastopen_cookie = foc;
   remaining -= need;
  }
}

mptcp_set_option_cond(req, opts, &remaining);

smc_set_option_cond(tcp_sk(sk), ireq, opts, &remaining);

bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
         synack_type, opts, &remaining);

return MAX_TCP_OPTION_SPACE - remaining;
}

/* Compute TCP options for ESTABLISHED sockets. This is not the
* final wire format yet.
*/
static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
     struct tcp_out_options *opts,
     struct tcp_key *key)
{
struct tcp_sock *tp = tcp_sk(sk);
unsigned int size = 0;
unsigned int eff_sacks;

opts->options = 0;

/* Better than switch (key.type) as it has static branches */
if (tcp_key_is_md5(key)) {
  opts->options |= OPTION_MD5;
  size += TCPOLEN_MD5SIG_ALIGNED;
} else if (tcp_key_is_ao(key)) {
  opts->options |= OPTION_AO;
  size += tcp_ao_len_aligned(key->ao_key);
}

if (likely(tp->rx_opt.tstamp_ok)) {
  opts->options |= OPTION_TS;
  opts->tsval = skb ? tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) +
    tp->tsoffset : 0;
  opts->tsecr = tp->rx_opt.ts_recent;
  size += TCPOLEN_TSTAMP_ALIGNED;
}

/* MPTCP options have precedence over SACK for the limited TCP
* option space because a MPTCP connection would be forced to
* fall back to regular TCP if a required multipath option is
* missing. SACK still gets a chance to use whatever space is
* left.
*/
if (sk_is_mptcp(sk)) {
  unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
  unsigned int opt_size = 0;

  if (mptcp_established_options(sk, skb, &opt_size, remaining,
           &opts->mptcp)) {
   opts->options |= OPTION_MPTCP;
   size += opt_size;
  }
}

eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
if (unlikely(eff_sacks)) {
  const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
  if (unlikely(remaining < TCPOLEN_SACK_BASE_ALIGNED +
      TCPOLEN_SACK_PERBLOCK))
   return size;

  opts->num_sack_blocks =
   min_t(unsigned int, eff_sacks,
         (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
         TCPOLEN_SACK_PERBLOCK);

  size += TCPOLEN_SACK_BASE_ALIGNED +
   opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
}

if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
         BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
  unsigned int remaining = MAX_TCP_OPTION_SPACE - size;

  bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);

  size = MAX_TCP_OPTION_SPACE - remaining;
}

return size;
}

/* TCP SMALL QUEUES (TSQ)
*
* TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
* to reduce RTT and bufferbloat.
* We do this using a special skb destructor (tcp_wfree).
*
* Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
* needs to be reallocated in a driver.
* The invariant being skb->truesize subtracted from sk->sk_wmem_alloc
*
* Since transmit from skb destructor is forbidden, we use a BH work item
* to process all sockets that eventually need to send more skbs.
* We use one work item per cpu, with its own queue of sockets.
*/
struct tsq_work {
struct work_struct work;
struct list_head head; /* queue of tcp sockets */
};
static DEFINE_PER_CPU(struct tsq_work, tsq_work);

static void tcp_tsq_write(struct sock *sk)
{
if ((1 << sk->sk_state) &
     (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
      TCPF_CLOSE_WAIT  | TCPF_LAST_ACK)) {
  struct tcp_sock *tp = tcp_sk(sk);

  if (tp->lost_out > tp->retrans_out &&
      tcp_snd_cwnd(tp) > tcp_packets_in_flight(tp)) {
   tcp_mstamp_refresh(tp);
   tcp_xmit_retransmit_queue(sk);
  }

  tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
          0, GFP_ATOMIC);
}
}

static void tcp_tsq_handler(struct sock *sk)
{
bh_lock_sock(sk);
if (!sock_owned_by_user(sk))
  tcp_tsq_write(sk);
else if (!test_and_set_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags))
  sock_hold(sk);
bh_unlock_sock(sk);
}
/*
* One work item per cpu tries to send more skbs.
* We run in BH context but need to disable irqs when
* transferring tsq->head because tcp_wfree() might
* interrupt us (non NAPI drivers)
*/
static void tcp_tsq_workfn(struct work_struct *work)
{
struct tsq_work *tsq = container_of(work, struct tsq_work, work);
LIST_HEAD(list);
unsigned long flags;
struct list_head *q, *n;
struct tcp_sock *tp;
struct sock *sk;

local_irq_save(flags);
list_splice_init(&tsq->head, &list);
local_irq_restore(flags);

list_for_each_safe(q, n, &list) {
  tp = list_entry(q, struct tcp_sock, tsq_node);
  list_del(&tp->tsq_node);

  sk = (struct sock *)tp;
  smp_mb__before_atomic();
  clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);

  tcp_tsq_handler(sk);
  sk_free(sk);
}
}

#define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED |  \
     TCPF_WRITE_TIMER_DEFERRED | \
     TCPF_DELACK_TIMER_DEFERRED | \
     TCPF_MTU_REDUCED_DEFERRED | \
     TCPF_ACK_DEFERRED)
/**
* tcp_release_cb - tcp release_sock() callback
* @sk: socket
*
* called from release_sock() to perform protocol dependent
* actions before socket release.
*/
void tcp_release_cb(struct sock *sk)
{
unsigned long flags = smp_load_acquire(&sk->sk_tsq_flags);
unsigned long nflags;

/* perform an atomic operation only if at least one flag is set */
do {
  if (!(flags & TCP_DEFERRED_ALL))
   return;
  nflags = flags & ~TCP_DEFERRED_ALL;
} while (!try_cmpxchg(&sk->sk_tsq_flags, &flags, nflags));

if (flags & TCPF_TSQ_DEFERRED) {
  tcp_tsq_write(sk);
  __sock_put(sk);
}

if (flags & TCPF_WRITE_TIMER_DEFERRED) {
  tcp_write_timer_handler(sk);
  __sock_put(sk);
}
if (flags & TCPF_DELACK_TIMER_DEFERRED) {
  tcp_delack_timer_handler(sk);
  __sock_put(sk);
}
if (flags & TCPF_MTU_REDUCED_DEFERRED) {
  inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
  __sock_put(sk);
}
if ((flags & TCPF_ACK_DEFERRED) && inet_csk_ack_scheduled(sk))
  tcp_send_ack(sk);
}
EXPORT_IPV6_MOD(tcp_release_cb);

void __init tcp_tsq_work_init(void)
{
int i;

for_each_possible_cpu(i) {
  struct tsq_work *tsq = &per_cpu(tsq_work, i);

  INIT_LIST_HEAD(&tsq->head);
  INIT_WORK(&tsq->work, tcp_tsq_workfn);
}
}

/*
* Write buffer destructor automatically called from kfree_skb.
* We can't xmit new skbs from this context, as we might already
* hold qdisc lock.
*/
void tcp_wfree(struct sk_buff *skb)
{
struct sock *sk = skb->sk;
struct tcp_sock *tp = tcp_sk(sk);
unsigned long flags, nval, oval;
struct tsq_work *tsq;
bool empty;

/* Keep one reference on sk_wmem_alloc.
* Will be released by sk_free() from here or tcp_tsq_workfn()
*/
WARN_ON(refcount_sub_and_test(skb->truesize - 1, &sk->sk_wmem_alloc));

/* If this softirq is serviced by ksoftirqd, we are likely under stress.
* Wait until our queues (qdisc + devices) are drained.
* This gives :
* - less callbacks to tcp_write_xmit(), reducing stress (batches)
* - chance for incoming ACK (processed by another cpu maybe)
*   to migrate this flow (skb->ooo_okay will be eventually set)
*/
if (refcount_read(&sk->sk_wmem_alloc) >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
  goto out;

oval = smp_load_acquire(&sk->sk_tsq_flags);
do {
  if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
   goto out;

  nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;
} while (!try_cmpxchg(&sk->sk_tsq_flags, &oval, nval));

/* queue this socket to BH workqueue */
local_irq_save(flags);
tsq = this_cpu_ptr(&tsq_work);
empty = list_empty(&tsq->head);
list_add(&tp->tsq_node, &tsq->head);
if (empty)
  queue_work(system_bh_wq, &tsq->work);
local_irq_restore(flags);
return;
out:
sk_free(sk);
}

/* Note: Called under soft irq.
* We can call TCP stack right away, unless socket is owned by user.
*/
enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
{
struct tcp_sock *tp = container_of(timer, struct tcp_sock, pacing_timer);
struct sock *sk = (struct sock *)tp;

tcp_tsq_handler(sk);
sock_put(sk);

return HRTIMER_NORESTART;
}

static void tcp_update_skb_after_send(struct sock *sk, struct sk_buff *skb,
          u64 prior_wstamp)
{
struct tcp_sock *tp = tcp_sk(sk);

if (sk->sk_pacing_status != SK_PACING_NONE) {
  unsigned long rate = READ_ONCE(sk->sk_pacing_rate);

  /* Original sch_fq does not pace first 10 MSS
* Note that tp->data_segs_out overflows after 2^32 packets,
* this is a minor annoyance.
*/
  if (rate != ~0UL && rate && tp->data_segs_out >= 10) {
   u64 len_ns = div64_ul((u64)skb->len * NSEC_PER_SEC, rate);
   u64 credit = tp->tcp_wstamp_ns - prior_wstamp;

   /* take into account OS jitter */
   len_ns -= min_t(u64, len_ns / 2, credit);
   tp->tcp_wstamp_ns += len_ns;
  }
}
list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
}

INDIRECT_CALLABLE_DECLARE(int ip_queue_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
INDIRECT_CALLABLE_DECLARE(int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
INDIRECT_CALLABLE_DECLARE(void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb));

/* This routine actually transmits TCP packets queued in by
* tcp_do_sendmsg().  This is used by both the initial
* transmission and possible later retransmissions.
* All SKB's seen here are completely headerless.  It is our
* job to build the TCP header, and pass the packet down to
* IP so it can do the same plus pass the packet off to the
* device.
*
* We are working here with either a clone of the original
* SKB, or a fresh unique copy made by the retransmit engine.
*/
static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
         int clone_it, gfp_t gfp_mask, u32 rcv_nxt)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
struct inet_sock *inet;
struct tcp_sock *tp;
struct tcp_skb_cb *tcb;
struct tcp_out_options opts;
unsigned int tcp_options_size, tcp_header_size;
struct sk_buff *oskb = NULL;
struct tcp_key key;
struct tcphdr *th;
u64 prior_wstamp;
int err;

BUG_ON(!skb || !tcp_skb_pcount(skb));
tp = tcp_sk(sk);
prior_wstamp = tp->tcp_wstamp_ns;
tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);
skb_set_delivery_time(skb, tp->tcp_wstamp_ns, SKB_CLOCK_MONOTONIC);
if (clone_it) {
  oskb = skb;

  tcp_skb_tsorted_save(oskb) {
   if (unlikely(skb_cloned(oskb)))
    skb = pskb_copy(oskb, gfp_mask);
   else
    skb = skb_clone(oskb, gfp_mask);
  } tcp_skb_tsorted_restore(oskb);

  if (unlikely(!skb))
   return -ENOBUFS;
  /* retransmit skbs might have a non zero value in skb->dev
* because skb->dev is aliased with skb->rbnode.rb_left
*/
  skb->dev = NULL;
}

inet = inet_sk(sk);
tcb = TCP_SKB_CB(skb);
memset(&opts, 0, sizeof(opts));

tcp_get_current_key(sk, &key);
if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
  tcp_options_size = tcp_syn_options(sk, skb, &opts, &key);
} else {
  tcp_options_size = tcp_established_options(sk, skb, &opts, &key);
  /* Force a PSH flag on all (GSO) packets to expedite GRO flush
* at receiver : This slightly improve GRO performance.
* Note that we do not force the PSH flag for non GSO packets,
* because they might be sent under high congestion events,
* and in this case it is better to delay the delivery of 1-MSS
* packets and thus the corresponding ACK packet that would
* release the following packet.
*/
  if (tcp_skb_pcount(skb) > 1)
   tcb->tcp_flags |= TCPHDR_PSH;
}
tcp_header_size = tcp_options_size + sizeof(struct tcphdr);

/* We set skb->ooo_okay to one if this packet can select
* a different TX queue than prior packets of this flow,
* to avoid self inflicted reorders.
* The 'other' queue decision is based on current cpu number
* if XPS is enabled, or sk->sk_txhash otherwise.
* We can switch to another (and better) queue if:
* 1) No packet with payload is in qdisc/device queues.
*    Delays in TX completion can defeat the test
*    even if packets were already sent.
* 2) Or rtx queue is empty.
*    This mitigates above case if ACK packets for
*    all prior packets were already processed.
*/
skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) ||
   tcp_rtx_queue_empty(sk);

/* If we had to use memory reserve to allocate this skb,
* this might cause drops if packet is looped back :
* Other socket might not have SOCK_MEMALLOC.
* Packets not looped back do not care about pfmemalloc.
*/
skb->pfmemalloc = 0;

skb_push(skb, tcp_header_size);
skb_reset_transport_header(skb);

skb_orphan(skb);
skb->sk = sk;
skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
refcount_add(skb->truesize, &sk->sk_wmem_alloc);

skb_set_dst_pending_confirm(skb, READ_ONCE(sk->sk_dst_pending_confirm));

/* Build TCP header and checksum it. */
th = (struct tcphdr *)skb->data;
th->source  = inet->inet_sport;
th->dest  = inet->inet_dport;
th->seq   = htonl(tcb->seq);
th->ack_seq  = htonl(rcv_nxt);
*(((__be16 *)th) + 6) = htons(((tcp_header_size >> 2) << 12) |
     (tcb->tcp_flags & TCPHDR_FLAGS_MASK));

th->check  = 0;
th->urg_ptr  = 0;

/* The urg_mode check is necessary during a below snd_una win probe */
if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
  if (before(tp->snd_up, tcb->seq + 0x10000)) {
   th->urg_ptr = htons(tp->snd_up - tcb->seq);
   th->urg = 1;
  } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
   th->urg_ptr = htons(0xFFFF);
   th->urg = 1;
  }
}

skb_shinfo(skb)->gso_type = sk->sk_gso_type;
if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
  th->window      = htons(tcp_select_window(sk));
  tcp_ecn_send(sk, skb, th, tcp_header_size);
} else {
  /* RFC1323: The window in SYN & SYN/ACK segments
* is never scaled.
*/
  th->window = htons(min(tp->rcv_wnd, 65535U));
}

tcp_options_write(th, tp, NULL, &opts, &key);

if (tcp_key_is_md5(&key)) {
#ifdef CONFIG_TCP_MD5SIG
  /* Calculate the MD5 hash, as we have all we need now */
  sk_gso_disable(sk);
  tp->af_specific->calc_md5_hash(opts.hash_location,
            key.md5_key, sk, skb);
#endif
} else if (tcp_key_is_ao(&key)) {
  int err;

  err = tcp_ao_transmit_skb(sk, skb, key.ao_key, th,
       opts.hash_location);
  if (err) {
   kfree_skb_reason(skb, SKB_DROP_REASON_NOT_SPECIFIED);
   return -ENOMEM;
  }
}

/* BPF prog is the last one writing header option */
bpf_skops_write_hdr_opt(sk, skb, NULL, NULL, 0, &opts);

INDIRECT_CALL_INET(icsk->icsk_af_ops->send_check,
      tcp_v6_send_check, tcp_v4_send_check,
      sk, skb);

if (likely(tcb->tcp_flags & TCPHDR_ACK))
  tcp_event_ack_sent(sk, rcv_nxt);

if (skb->len != tcp_header_size) {
  tcp_event_data_sent(tp, sk);
  tp->data_segs_out += tcp_skb_pcount(skb);
  tp->bytes_sent += skb->len - tcp_header_size;
}

if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
  TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
         tcp_skb_pcount(skb));

tp->segs_out += tcp_skb_pcount(skb);
skb_set_hash_from_sk(skb, sk);
/* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);

/* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */

/* Cleanup our debris for IP stacks */
memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
          sizeof(struct inet6_skb_parm)));

tcp_add_tx_delay(skb, tp);

err = INDIRECT_CALL_INET(icsk->icsk_af_ops->queue_xmit,
     inet6_csk_xmit, ip_queue_xmit,
     sk, skb, &inet->cork.fl);

if (unlikely(err > 0)) {
  tcp_enter_cwr(sk);
  err = net_xmit_eval(err);
}
if (!err && oskb) {
  tcp_update_skb_after_send(sk, oskb, prior_wstamp);
  tcp_rate_skb_sent(sk, oskb);
}
return err;
}

static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
       gfp_t gfp_mask)
{
return __tcp_transmit_skb(sk, skb, clone_it, gfp_mask,
      tcp_sk(sk)->rcv_nxt);
}

/* This routine just queues the buffer for sending.
*
* NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
* otherwise socket can stall.
*/
static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);

/* Advance write_seq and place onto the write_queue. */
WRITE_ONCE(tp->write_seq, TCP_SKB_CB(skb)->end_seq);
__skb_header_release(skb);
tcp_add_write_queue_tail(sk, skb);
sk_wmem_queued_add(sk, skb->truesize);
sk_mem_charge(sk, skb->truesize);
}

/* Initialize TSO segments for a packet. */
static int tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
{
int tso_segs;

if (skb->len <= mss_now) {
  /* Avoid the costly divide in the normal
* non-TSO case.
*/
  TCP_SKB_CB(skb)->tcp_gso_size = 0;
  tcp_skb_pcount_set(skb, 1);
  return 1;
}
TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
tso_segs = DIV_ROUND_UP(skb->len, mss_now);
tcp_skb_pcount_set(skb, tso_segs);
return tso_segs;
}

/* Pcount in the middle of the write queue got changed, we need to do various
* tweaks to fix counters
*/
static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
{
struct tcp_sock *tp = tcp_sk(sk);

tp->packets_out -= decr;

if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
  tp->sacked_out -= decr;
if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
  tp->retrans_out -= decr;
if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
  tp->lost_out -= decr;

/* Reno case is special. Sigh... */
if (tcp_is_reno(tp) && decr > 0)
  tp->sacked_out -= min_t(u32, tp->sacked_out, decr);

tcp_verify_left_out(tp);
}

static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
{
return TCP_SKB_CB(skb)->txstamp_ack ||
  (skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
}

static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
{
struct skb_shared_info *shinfo = skb_shinfo(skb);

if (unlikely(tcp_has_tx_tstamp(skb)) &&
     !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
  struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
  u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;

  shinfo->tx_flags &= ~tsflags;
  shinfo2->tx_flags |= tsflags;
  swap(shinfo->tskey, shinfo2->tskey);
  TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
  TCP_SKB_CB(skb)->txstamp_ack = 0;
}
}

static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
{
TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
TCP_SKB_CB(skb)->eor = 0;
}

/* Insert buff after skb on the write or rtx queue of sk.  */
static void tcp_insert_write_queue_after(struct sk_buff *skb,
      struct sk_buff *buff,
      struct sock *sk,
      enum tcp_queue tcp_queue)
{
if (tcp_queue == TCP_FRAG_IN_WRITE_QUEUE)
  __skb_queue_after(&sk->sk_write_queue, skb, buff);
else
  tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
}

/* Function to create two new TCP segments.  Shrinks the given segment
* to the specified size and appends a new segment with the rest of the
* packet to the list.  This won't be called frequently, I hope.
* Remember, these are still headerless SKBs at this point.
*/
int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
   struct sk_buff *skb, u32 len,
   unsigned int mss_now, gfp_t gfp)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *buff;
int old_factor;
long limit;
u16 flags;
int nlen;

if (WARN_ON(len > skb->len))
  return -EINVAL;

DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));

/* tcp_sendmsg() can overshoot sk_wmem_queued by one full size skb.
* We need some allowance to not penalize applications setting small
* SO_SNDBUF values.
* Also allow first and last skb in retransmit queue to be split.
*/
limit = sk->sk_sndbuf + 2 * SKB_TRUESIZE(GSO_LEGACY_MAX_SIZE);
if (unlikely((sk->sk_wmem_queued >> 1) > limit &&
       tcp_queue != TCP_FRAG_IN_WRITE_QUEUE &&
       skb != tcp_rtx_queue_head(sk) &&
       skb != tcp_rtx_queue_tail(sk))) {
  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
  return -ENOMEM;
}

if (skb_unclone_keeptruesize(skb, gfp))
  return -ENOMEM;

/* Get a new skb... force flag on. */
buff = tcp_stream_alloc_skb(sk, gfp, true);
if (!buff)
  return -ENOMEM; /* We'll just try again later. */
skb_copy_decrypted(buff, skb);
mptcp_skb_ext_copy(buff, skb);

sk_wmem_queued_add(sk, buff->truesize);
sk_mem_charge(sk, buff->truesize);
nlen = skb->len - len;
buff->truesize += nlen;
skb->truesize -= nlen;

/* Correct the sequence numbers. */
TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

/* PSH and FIN should only be set in the second packet. */
flags = TCP_SKB_CB(skb)->tcp_flags;
TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
TCP_SKB_CB(buff)->tcp_flags = flags;
TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
tcp_skb_fragment_eor(skb, buff);

skb_split(skb, buff, len);

skb_set_delivery_time(buff, skb->tstamp, SKB_CLOCK_MONOTONIC);
tcp_fragment_tstamp(skb, buff);

old_factor = tcp_skb_pcount(skb);

/* Fix up tso_factor for both original and new SKB.  */
tcp_set_skb_tso_segs(skb, mss_now);
tcp_set_skb_tso_segs(buff, mss_now);

/* Update delivered info for the new segment */
TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;

/* If this packet has been sent out already, we must
* adjust the various packet counters.
*/
if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
  int diff = old_factor - tcp_skb_pcount(skb) -
   tcp_skb_pcount(buff);

  if (diff)
   tcp_adjust_pcount(sk, skb, diff);
}

/* Link BUFF into the send queue. */
__skb_header_release(buff);
tcp_insert_write_queue_after(skb, buff, sk, tcp_queue);
if (tcp_queue == TCP_FRAG_IN_RTX_QUEUE)
  list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);

return 0;
}

/* This is similar to __pskb_pull_tail(). The difference is that pulled
* data is not copied, but immediately discarded.
*/
static int __pskb_trim_head(struct sk_buff *skb, int len)
{
struct skb_shared_info *shinfo;
int i, k, eat;

DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
eat = len;
k = 0;
shinfo = skb_shinfo(skb);
for (i = 0; i < shinfo->nr_frags; i++) {
  int size = skb_frag_size(&shinfo->frags[i]);

  if (size <= eat) {
   skb_frag_unref(skb, i);
   eat -= size;
  } else {
   shinfo->frags[k] = shinfo->frags[i];
   if (eat) {
    skb_frag_off_add(&shinfo->frags[k], eat);
    skb_frag_size_sub(&shinfo->frags[k], eat);
    eat = 0;
   }
   k++;
  }
}
shinfo->nr_frags = k;

skb->data_len -= len;
skb->len = skb->data_len;
return len;
}

/* Remove acked data from a packet in the transmit queue. */
int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
{
u32 delta_truesize;

if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
  return -ENOMEM;

delta_truesize = __pskb_trim_head(skb, len);

TCP_SKB_CB(skb)->seq += len;

skb->truesize    -= delta_truesize;
sk_wmem_queued_add(sk, -delta_truesize);
if (!skb_zcopy_pure(skb))
  sk_mem_uncharge(sk, delta_truesize);

/* Any change of skb->len requires recalculation of tso factor. */
if (tcp_skb_pcount(skb) > 1)
  tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));

return 0;
}

/* Calculate MSS not accounting any TCP options.  */
static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
const struct tcp_sock *tp = tcp_sk(sk);
const struct inet_connection_sock *icsk = inet_csk(sk);
int mss_now;

/* Calculate base mss without TCP options:
   It is MMS_S - sizeof(tcphdr) of rfc1122
*/
mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);

/* Clamp it (mss_clamp does not include tcp options) */
if (mss_now > tp->rx_opt.mss_clamp)
  mss_now = tp->rx_opt.mss_clamp;

/* Now subtract optional transport overhead */
mss_now -= icsk->icsk_ext_hdr_len;

/* Then reserve room for full set of TCP options and 8 bytes of data */
mss_now = max(mss_now,
        READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_snd_mss));
return mss_now;
}

/* Calculate MSS. Not accounting for SACKs here.  */
int tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
/* Subtract TCP options size, not including SACKs */
return __tcp_mtu_to_mss(sk, pmtu) -
        (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
}
EXPORT_IPV6_MOD(tcp_mtu_to_mss);

/* Inverse of above */
int tcp_mss_to_mtu(struct sock *sk, int mss)
{
const struct tcp_sock *tp = tcp_sk(sk);
const struct inet_connection_sock *icsk = inet_csk(sk);

return mss +
       tp->tcp_header_len +
       icsk->icsk_ext_hdr_len +
       icsk->icsk_af_ops->net_header_len;
}
EXPORT_SYMBOL(tcp_mss_to_mtu);

/* MTU probing init per socket */
void tcp_mtup_init(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct inet_connection_sock *icsk = inet_csk(sk);
struct net *net = sock_net(sk);

icsk->icsk_mtup.enabled = READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing) > 1;
icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
          icsk->icsk_af_ops->net_header_len;
icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, READ_ONCE(net->ipv4.sysctl_tcp_base_mss));
icsk->icsk_mtup.probe_size = 0;
if (icsk->icsk_mtup.enabled)
  icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
}

/* This function synchronize snd mss to current pmtu/exthdr set.

   tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
   for TCP options, but includes only bare TCP header.

   tp->rx_opt.mss_clamp is mss negotiated at connection setup.
   It is minimum of user_mss and mss received with SYN.
   It also does not include TCP options.

   inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.

   tp->mss_cache is current effective sending mss, including
   all tcp options except for SACKs. It is evaluated,
   taking into account current pmtu, but never exceeds
   tp->rx_opt.mss_clamp.

   NOTE1. rfc1122 clearly states that advertised MSS
   DOES NOT include either tcp or ip options.

   NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
   are READ ONLY outside this function. --ANK (980731)
*/
unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
{
struct tcp_sock *tp = tcp_sk(sk);
struct inet_connection_sock *icsk = inet_csk(sk);
int mss_now;

if (icsk->icsk_mtup.search_high > pmtu)
  icsk->icsk_mtup.search_high = pmtu;

mss_now = tcp_mtu_to_mss(sk, pmtu);
mss_now = tcp_bound_to_half_wnd(tp, mss_now);

/* And store cached results */
icsk->icsk_pmtu_cookie = pmtu;
if (icsk->icsk_mtup.enabled)
  mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
tp->mss_cache = mss_now;

return mss_now;
}
EXPORT_IPV6_MOD(tcp_sync_mss);

/* Compute the current effective MSS, taking SACKs and IP options,
* and even PMTU discovery events into account.
*/
unsigned int tcp_current_mss(struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
const struct dst_entry *dst = __sk_dst_get(sk);
u32 mss_now;
unsigned int header_len;
struct tcp_out_options opts;
struct tcp_key key;

mss_now = tp->mss_cache;

if (dst) {
  u32 mtu = dst_mtu(dst);
  if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
   mss_now = tcp_sync_mss(sk, mtu);
}
tcp_get_current_key(sk, &key);
header_len = tcp_established_options(sk, NULL, &opts, &key) +
       sizeof(struct tcphdr);
/* The mss_cache is sized based on tp->tcp_header_len, which assumes
* some common options. If this is an odd packet (because we have SACK
* blocks etc) then our calculated header_len will be different, and
* we have to adjust mss_now correspondingly */
if (header_len != tp->tcp_header_len) {
  int delta = (int) header_len - tp->tcp_header_len;
  mss_now -= delta;
}

return mss_now;
}

/* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
* As additional protections, we do not touch cwnd in retransmission phases,
* and if application hit its sndbuf limit recently.
*/
static void tcp_cwnd_application_limited(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);

if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
     sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
  /* Limited by application or receiver window. */
  u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
  u32 win_used = max(tp->snd_cwnd_used, init_win);
  if (win_used < tcp_snd_cwnd(tp)) {
   tp->snd_ssthresh = tcp_current_ssthresh(sk);
   tcp_snd_cwnd_set(tp, (tcp_snd_cwnd(tp) + win_used) >> 1);
  }
  tp->snd_cwnd_used = 0;
}
tp->snd_cwnd_stamp = tcp_jiffies32;
}

static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
{
const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
struct tcp_sock *tp = tcp_sk(sk);

/* Track the strongest available signal of the degree to which the cwnd
* is fully utilized. If cwnd-limited then remember that fact for the
* current window. If not cwnd-limited then track the maximum number of
* outstanding packets in the current window. (If cwnd-limited then we
* chose to not update tp->max_packets_out to avoid an extra else
* clause with no functional impact.)
*/
if (!before(tp->snd_una, tp->cwnd_usage_seq) ||
     is_cwnd_limited ||
     (!tp->is_cwnd_limited &&
      tp->packets_out > tp->max_packets_out)) {
  tp->is_cwnd_limited = is_cwnd_limited;
  tp->max_packets_out = tp->packets_out;
  tp->cwnd_usage_seq = tp->snd_nxt;
}

if (tcp_is_cwnd_limited(sk)) {
  /* Network is feed fully. */
  tp->snd_cwnd_used = 0;
  tp->snd_cwnd_stamp = tcp_jiffies32;
} else {
  /* Network starves. */
  if (tp->packets_out > tp->snd_cwnd_used)
   tp->snd_cwnd_used = tp->packets_out;

  if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_slow_start_after_idle) &&
      (s32)(tcp_jiffies32 - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
      !ca_ops->cong_control)
   tcp_cwnd_application_limited(sk);

  /* The following conditions together indicate the starvation
* is caused by insufficient sender buffer:
* 1) just sent some data (see tcp_write_xmit)
* 2) not cwnd limited (this else condition)
* 3) no more data to send (tcp_write_queue_empty())
* 4) application is hitting buffer limit (SOCK_NOSPACE)
*/
  if (tcp_write_queue_empty(sk) && sk->sk_socket &&
      test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
      (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
   tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
}
}

/* Minshall's variant of the Nagle send check. */
static bool tcp_minshall_check(const struct tcp_sock *tp)
{
return after(tp->snd_sml, tp->snd_una) &&
  !after(tp->snd_sml, tp->snd_nxt);
}

/* Update snd_sml if this skb is under mss
* Note that a TSO packet might end with a sub-mss segment
* The test is really :
* if ((skb->len % mss) != 0)
*        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
* But we can avoid doing the divide again given we already have
*  skb_pcount = skb->len / mss_now
*/
static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
    const struct sk_buff *skb)
{
if (skb->len < tcp_skb_pcount(skb) * mss_now)
  tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
}

/* Return false, if packet can be sent now without violation Nagle's rules:
* 1. It is full sized. (provided by caller in %partial bool)
* 2. Or it contains FIN. (already checked by caller)
* 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
* 4. Or TCP_CORK is not set, and all sent packets are ACKed.
*    With Minshall's modification: all sent small packets are ACKed.
*/
static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
       int nonagle)
{
return partial &&
  ((nonagle & TCP_NAGLE_CORK) ||
   (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
}

/* Return how many segs we'd like on a TSO packet,
* depending on current pacing rate, and how close the peer is.
*
* Rationale is:
* - For close peers, we rather send bigger packets to reduce
*   cpu costs, because occasional losses will be repaired fast.
* - For long distance/rtt flows, we would like to get ACK clocking
*   with 1 ACK per ms.
*
* Use min_rtt to help adapt TSO burst size, with smaller min_rtt resulting
* in bigger TSO bursts. We we cut the RTT-based allowance in half
* for every 2^9 usec (aka 512 us) of RTT, so that the RTT-based allowance
* is below 1500 bytes after 6 * ~500 usec = 3ms.
*/
static u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
       int min_tso_segs)
{
unsigned long bytes;
u32 r;

bytes = READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift);

r = tcp_min_rtt(tcp_sk(sk)) >> READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_rtt_log);
if (r < BITS_PER_TYPE(sk->sk_gso_max_size))
  bytes += sk->sk_gso_max_size >> r;

bytes = min_t(unsigned long, bytes, sk->sk_gso_max_size);

return max_t(u32, bytes / mss_now, min_tso_segs);
}

/* Return the number of segments we want in the skb we are transmitting.
* See if congestion control module wants to decide; otherwise, autosize.
*/
static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
{
const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
u32 min_tso, tso_segs;

min_tso = ca_ops->min_tso_segs ?
   ca_ops->min_tso_segs(sk) :
   READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);

tso_segs = tcp_tso_autosize(sk, mss_now, min_tso);
return min_t(u32, tso_segs, sk->sk_gso_max_segs);
}

/* Returns the portion of skb which can be sent right away */
static unsigned int tcp_mss_split_point(const struct sock *sk,
     const struct sk_buff *skb,
     unsigned int mss_now,
     unsigned int max_segs,
     int nonagle)
{
const struct tcp_sock *tp = tcp_sk(sk);
u32 partial, needed, window, max_len;

window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
max_len = mss_now * max_segs;

if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
  return max_len;

needed = min(skb->len, window);

if (max_len <= needed)
  return max_len;

partial = needed % mss_now;
/* If last segment is not a full MSS, check if Nagle rules allow us
* to include this last segment in this skb.
* Otherwise, we'll split the skb at last MSS boundary
*/
if (tcp_nagle_check(partial != 0, tp, nonagle))
  return needed - partial;

return needed;
}

/* Can at least one segment of SKB be sent right now, according to the
* congestion window rules?  If so, return how many segments are allowed.
*/
static u32 tcp_cwnd_test(const struct tcp_sock *tp)
{
u32 in_flight, cwnd, halfcwnd;

in_flight = tcp_packets_in_flight(tp);
cwnd = tcp_snd_cwnd(tp);
if (in_flight >= cwnd)
  return 0;

/* For better scheduling, ensure we have at least
* 2 GSO packets in flight.
*/
halfcwnd = max(cwnd >> 1, 1U);
return min(halfcwnd, cwnd - in_flight);
}

/* Initialize TSO state of a skb.
* This must be invoked the first time we consider transmitting
* SKB onto the wire.
*/
static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
{
int tso_segs = tcp_skb_pcount(skb);

if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now))
  return tcp_set_skb_tso_segs(skb, mss_now);

return tso_segs;
}

/* Return true if the Nagle test allows this packet to be
* sent now.
*/
static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
      unsigned int cur_mss, int nonagle)
{
/* Nagle rule does not apply to frames, which sit in the middle of the
* write_queue (they have no chances to get new data).
*
* This is implemented in the callers, where they modify the 'nonagle'
* argument based upon the location of SKB in the send queue.
*/
if (nonagle & TCP_NAGLE_PUSH)
  return true;

/* Don't use the nagle rule for urgent data (or for the final FIN). */
if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
  return true;

if (!tcp_nagle_check(skb->len < cur_mss, tp, nonagle))
  return true;

return false;
}

/* Does at least the first segment of SKB fit into the send window? */
static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
        const struct sk_buff *skb,
        unsigned int cur_mss)
{
u32 end_seq = TCP_SKB_CB(skb)->end_seq;

if (skb->len > cur_mss)
  end_seq = TCP_SKB_CB(skb)->seq + cur_mss;

return !after(end_seq, tcp_wnd_end(tp));
}

/* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
* which is put after SKB on the list.  It is very much like
* tcp_fragment() except that it may make several kinds of assumptions
* in order to speed up the splitting operation.  In particular, we
* know that all the data is in scatter-gather pages, and that the
* packet has never been sent out before (and thus is not cloned).
*/
static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
   unsigned int mss_now, gfp_t gfp)
{
int nlen = skb->len - len;
struct sk_buff *buff;
u16 flags;

/* All of a TSO frame must be composed of paged data.  */
DEBUG_NET_WARN_ON_ONCE(skb->len != skb->data_len);

buff = tcp_stream_alloc_skb(sk, gfp, true);
if (unlikely(!buff))
  return -ENOMEM;
skb_copy_decrypted(buff, skb);
mptcp_skb_ext_copy(buff, skb);

sk_wmem_queued_add(sk, buff->truesize);
sk_mem_charge(sk, buff->truesize);
buff->truesize += nlen;
skb->truesize -= nlen;

/* Correct the sequence numbers. */
TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

/* PSH and FIN should only be set in the second packet. */
flags = TCP_SKB_CB(skb)->tcp_flags;
TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
TCP_SKB_CB(buff)->tcp_flags = flags;

tcp_skb_fragment_eor(skb, buff);

skb_split(skb, buff, len);
tcp_fragment_tstamp(skb, buff);

/* Fix up tso_factor for both original and new SKB.  */
tcp_set_skb_tso_segs(skb, mss_now);
tcp_set_skb_tso_segs(buff, mss_now);

/* Link BUFF into the send queue. */
__skb_header_release(buff);
tcp_insert_write_queue_after(skb, buff, sk, TCP_FRAG_IN_WRITE_QUEUE);

return 0;
}

/* Try to defer sending, if possible, in order to minimize the amount
* of TSO splitting we do.  View it as a kind of TSO Nagle test.
*
* This algorithm is from John Heffner.
*/
static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb,
     bool *is_cwnd_limited,
     bool *is_rwnd_limited,
     u32 max_segs)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
u32 send_win, cong_win, limit, in_flight, threshold;
u64 srtt_in_ns, expected_ack, how_far_is_the_ack;
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *head;
int win_divisor;
s64 delta;

if (icsk->icsk_ca_state >= TCP_CA_Recovery)
  goto send_now;

/* Avoid bursty behavior by allowing defer
* only if the last write was recent (1 ms).
* Note that tp->tcp_wstamp_ns can be in the future if we have
* packets waiting in a qdisc or device for EDT delivery.
*/
delta = tp->tcp_clock_cache - tp->tcp_wstamp_ns - NSEC_PER_MSEC;
if (delta > 0)
  goto send_now;

in_flight = tcp_packets_in_flight(tp);

BUG_ON(tcp_skb_pcount(skb) <= 1);
BUG_ON(tcp_snd_cwnd(tp) <= in_flight);

send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

/* From in_flight test above, we know that cwnd > in_flight.  */
cong_win = (tcp_snd_cwnd(tp) - in_flight) * tp->mss_cache;

limit = min(send_win, cong_win);

/* If a full-sized TSO skb can be sent, do it. */
if (limit >= max_segs * tp->mss_cache)
  goto send_now;

/* Middle in queue won't get any more data, full sendable already? */
if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
  goto send_now;

win_divisor = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_win_divisor);
if (win_divisor) {
  u32 chunk = min(tp->snd_wnd, tcp_snd_cwnd(tp) * tp->mss_cache);

  /* If at least some fraction of a window is available,
* just use it.
*/
  chunk /= win_divisor;
  if (limit >= chunk)
   goto send_now;
} else {
  /* Different approach, try not to defer past a single
* ACK.  Receiver should ACK every other full sized
* frame, so if we have space for more than 3 frames
* then send now.
*/
  if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
   goto send_now;
}

/* TODO : use tsorted_sent_queue ? */
head = tcp_rtx_queue_head(sk);
if (!head)
  goto send_now;

srtt_in_ns = (u64)(NSEC_PER_USEC >> 3) * tp->srtt_us;
/* When is the ACK expected ? */
expected_ack = head->tstamp + srtt_in_ns;
/* How far from now is the ACK expected ? */
how_far_is_the_ack = expected_ack - tp->tcp_clock_cache;

/* If next ACK is likely to come too late,
* ie in more than min(1ms, half srtt), do not defer.
*/
threshold = min(srtt_in_ns >> 1, NSEC_PER_MSEC);

if ((s64)(how_far_is_the_ack - threshold) > 0)
  goto send_now;

/* Ok, it looks like it is advisable to defer.
* Three cases are tracked :
* 1) We are cwnd-limited
* 2) We are rwnd-limited
* 3) We are application limited.
*/
if (cong_win < send_win) {
  if (cong_win <= skb->len) {
   *is_cwnd_limited = true;
   return true;
  }
} else {
  if (send_win <= skb->len) {
   *is_rwnd_limited = true;
   return true;
  }
}

/* If this packet won't get more data, do not wait. */
if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) ||
     TCP_SKB_CB(skb)->eor)
  goto send_now;

return true;

send_now:
return false;
}

static inline void tcp_mtu_check_reprobe(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct net *net = sock_net(sk);
u32 interval;
s32 delta;

interval = READ_ONCE(net->ipv4.sysctl_tcp_probe_interval);
delta = tcp_jiffies32 - icsk->icsk_mtup.probe_timestamp;
if (unlikely(delta >= interval * HZ)) {
  int mss = tcp_current_mss(sk);

  /* Update current search range */
  icsk->icsk_mtup.probe_size = 0;
  icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp +
   sizeof(struct tcphdr) +
   icsk->icsk_af_ops->net_header_len;
  icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);

  /* Update probe time stamp */
  icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
}
}

static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
{
struct sk_buff *skb, *next;

skb = tcp_send_head(sk);
tcp_for_write_queue_from_safe(skb, next, sk) {
  if (len <= skb->len)
   break;

  if (tcp_has_tx_tstamp(skb) || !tcp_skb_can_collapse(skb, next))
   return false;

  len -= skb->len;
}

return true;
}

static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
        int probe_size)
{
skb_frag_t *lastfrag = NULL, *fragto = skb_shinfo(to)->frags;
int i, todo, len = 0, nr_frags = 0;
const struct sk_buff *skb;

if (!sk_wmem_schedule(sk, to->truesize + probe_size))
  return -ENOMEM;

skb_queue_walk(&sk->sk_write_queue, skb) {
  const skb_frag_t *fragfrom = skb_shinfo(skb)->frags;

  if (skb_headlen(skb))
   return -EINVAL;

  for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, fragfrom++) {
   if (len >= probe_size)
    goto commit;
   todo = min_t(int, skb_frag_size(fragfrom),
         probe_size - len);
   len += todo;
   if (lastfrag &&
       skb_frag_page(fragfrom) == skb_frag_page(lastfrag) &&
       skb_frag_off(fragfrom) == skb_frag_off(lastfrag) +
            skb_frag_size(lastfrag)) {
    skb_frag_size_add(lastfrag, todo);
    continue;
   }
   if (unlikely(nr_frags == MAX_SKB_FRAGS))
    return -E2BIG;
   skb_frag_page_copy(fragto, fragfrom);
   skb_frag_off_copy(fragto, fragfrom);
   skb_frag_size_set(fragto, todo);
   nr_frags++;
   lastfrag = fragto++;
  }
}
commit:
WARN_ON_ONCE(len != probe_size);
for (i = 0; i < nr_frags; i++)
  skb_frag_ref(to, i);

skb_shinfo(to)->nr_frags = nr_frags;
to->truesize += probe_size;
to->len += probe_size;
to->data_len += probe_size;
__skb_header_release(to);
return 0;
}

/* tcp_mtu_probe() and tcp_grow_skb() can both eat an skb (src) if
* all its payload was moved to another one (dst).
* Make sure to transfer tcp_flags, eor, and tstamp.
*/
static void tcp_eat_one_skb(struct sock *sk,
       struct sk_buff *dst,
       struct sk_buff *src)
{
TCP_SKB_CB(dst)->tcp_flags |= TCP_SKB_CB(src)->tcp_flags;
TCP_SKB_CB(dst)->eor = TCP_SKB_CB(src)->eor;
tcp_skb_collapse_tstamp(dst, src);
tcp_unlink_write_queue(src, sk);
tcp_wmem_free_skb(sk, src);
}

/* Create a new MTU probe if we are ready.
* MTU probe is regularly attempting to increase the path MTU by
* deliberately sending larger packets.  This discovers routing
* changes resulting in larger path MTUs.
*
* Returns 0 if we should wait to probe (no cwnd available),
*         1 if a probe was sent,
*         -1 otherwise
*/
static int tcp_mtu_probe(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb, *nskb, *next;
struct net *net = sock_net(sk);
int probe_size;
int size_needed;
int copy, len;
int mss_now;
int interval;

/* Not currently probing/verifying,
* not in recovery,
* have enough cwnd, and
* not SACKing (the variable headers throw things off)
*/
if (likely(!icsk->icsk_mtup.enabled ||
     icsk->icsk_mtup.probe_size ||
     inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
     tcp_snd_cwnd(tp) < 11 ||
     tp->rx_opt.num_sacks || tp->rx_opt.dsack))
  return -1;

/* Use binary search for probe_size between tcp_mss_base,
* and current mss_clamp. if (search_high - search_low)
* smaller than a threshold, backoff from probing.
*/
mss_now = tcp_current_mss(sk);
probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
        icsk->icsk_mtup.search_low) >> 1);
size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
/* When misfortune happens, we are reprobing actively,
* and then reprobe timer has expired. We stick with current
* probing process by not resetting search range to its orignal.
*/
if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
     interval < READ_ONCE(net->ipv4.sysctl_tcp_probe_threshold)) {
  /* Check whether enough time has elaplased for
* another round of probing.
*/
  tcp_mtu_check_reprobe(sk);
  return -1;
}

/* Have enough data in the send queue to probe? */
if (tp->write_seq - tp->snd_nxt < size_needed)
  return -1;

if (tp->snd_wnd < size_needed)
  return -1;
if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
  return 0;

/* Do we need to wait to drain cwnd? With none in flight, don't stall */
if (tcp_packets_in_flight(tp) + 2 > tcp_snd_cwnd(tp)) {
  if (!tcp_packets_in_flight(tp))
   return -1;
  else
   return 0;
}

if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
  return -1;

/* We're allowed to probe.  Build it now. */
nskb = tcp_stream_alloc_skb(sk, GFP_ATOMIC, false);
if (!nskb)
  return -1;

/* build the payload, and be prepared to abort if this fails. */
if (tcp_clone_payload(sk, nskb, probe_size)) {
  tcp_skb_tsorted_anchor_cleanup(nskb);
  consume_skb(nskb);
  return -1;
}
sk_wmem_queued_add(sk, nskb->truesize);
sk_mem_charge(sk, nskb->truesize);

skb = tcp_send_head(sk);
skb_copy_decrypted(nskb, skb);
mptcp_skb_ext_copy(nskb, skb);

TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;

tcp_insert_write_queue_before(nskb, skb, sk);
tcp_highest_sack_replace(sk, skb, nskb);

len = 0;
tcp_for_write_queue_from_safe(skb, next, sk) {
  copy = min_t(int, skb->len, probe_size - len);

  if (skb->len <= copy) {
   tcp_eat_one_skb(sk, nskb, skb);
  } else {
   TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
         ~(TCPHDR_FIN|TCPHDR_PSH);
   __pskb_trim_head(skb, copy);
   tcp_set_skb_tso_segs(skb, mss_now);
   TCP_SKB_CB(skb)->seq += copy;
  }

  len += copy;

  if (len >= probe_size)
   break;
}
tcp_init_tso_segs(nskb, nskb->len);

/* We're ready to send.  If this fails, the probe will
* be resegmented into mss-sized pieces by tcp_write_xmit().
*/
if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
  /* Decrement cwnd here because we are sending
* effectively two packets. */
  tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
  tcp_event_new_data_sent(sk, nskb);

  icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
  tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
  tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;

  return 1;
}

return -1;
}

static bool tcp_pacing_check(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);

if (!tcp_needs_internal_pacing(sk))
  return false;

if (tp->tcp_wstamp_ns <= tp->tcp_clock_cache)
  return false;

if (!hrtimer_is_queued(&tp->pacing_timer)) {
  hrtimer_start(&tp->pacing_timer,
         ns_to_ktime(tp->tcp_wstamp_ns),
         HRTIMER_MODE_ABS_PINNED_SOFT);
  sock_hold(sk);
}
return true;
}

static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
{
const struct rb_node *node = sk->tcp_rtx_queue.rb_node;

/* No skb in the rtx queue. */
if (!node)
  return true;

/* Only one skb in rtx queue. */
return !node->rb_left && !node->rb_right;
}

/* TCP Small Queues :
* Control number of packets in qdisc/devices to two packets / or ~1 ms.
* (These limits are doubled for retransmits)
* This allows for :
*  - better RTT estimation and ACK scheduling
*  - faster recovery
*  - high rates
* Alas, some drivers / subsystems require a fair amount
* of queued bytes to ensure line rate.
* One example is wifi aggregation (802.11 AMPDU)
*/
static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
      unsigned int factor)
{
unsigned long limit;

limit = max_t(unsigned long,
        2 * skb->truesize,
        READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift));
limit = min_t(unsigned long, limit,
        READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
limit <<= factor;

if (static_branch_unlikely(&tcp_tx_delay_enabled) &&
     tcp_sk(sk)->tcp_tx_delay) {
  u64 extra_bytes = (u64)READ_ONCE(sk->sk_pacing_rate) *
      tcp_sk(sk)->tcp_tx_delay;

  /* TSQ is based on skb truesize sum (sk_wmem_alloc), so we
* approximate our needs assuming an ~100% skb->truesize overhead.
* USEC_PER_SEC is approximated by 2^20.
* do_div(extra_bytes, USEC_PER_SEC/2) is replaced by a right shift.
*/
  extra_bytes >>= (20 - 1);
  limit += extra_bytes;
}
if (refcount_read(&sk->sk_wmem_alloc) > limit) {
  /* Always send skb if rtx queue is empty or has one skb.
* No need to wait for TX completion to call us back,
* after softirq schedule.
* This helps when TX completions are delayed too much.
*/
  if (tcp_rtx_queue_empty_or_single_skb(sk))
   return false;

  set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
  /* It is possible TX completion already happened
* before we set TSQ_THROTTLED, so we must
* test again the condition.
*/
  smp_mb__after_atomic();
  if (refcount_read(&sk->sk_wmem_alloc) > limit)
   return true;
}
return false;
}

static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
{
const u32 now = tcp_jiffies32;
enum tcp_chrono old = tp->chrono_type;

if (old > TCP_CHRONO_UNSPEC)
  tp->chrono_stat[old - 1] += now - tp->chrono_start;
tp->chrono_start = now;
tp->chrono_type = new;
}

void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type)
{
struct tcp_sock *tp = tcp_sk(sk);

/* If there are multiple conditions worthy of tracking in a
* chronograph then the highest priority enum takes precedence
* over the other conditions. So that if something "more interesting"
* starts happening, stop the previous chrono and start a new one.
*/
if (type > tp->chrono_type)
  tcp_chrono_set(tp, type);
}

void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
{
struct tcp_sock *tp = tcp_sk(sk);

/* There are multiple conditions worthy of tracking in a
* chronograph, so that the highest priority enum takes
* precedence over the other conditions (see tcp_chrono_start).
* If a condition stops, we only stop chrono tracking if
* it's the "most interesting" or current chrono we are
* tracking and starts busy chrono if we have pending data.
*/
if (tcp_rtx_and_write_queues_empty(sk))
  tcp_chrono_set(tp, TCP_CHRONO_UNSPEC);
else if (type == tp->chrono_type)
  tcp_chrono_set(tp, TCP_CHRONO_BUSY);
}

/* First skb in the write queue is smaller than ideal packet size.
* Check if we can move payload from the second skb in the queue.
*/
static void tcp_grow_skb(struct sock *sk, struct sk_buff *skb, int amount)
{
struct sk_buff *next_skb = skb->next;
unsigned int nlen;

if (tcp_skb_is_last(sk, skb))
  return;

if (!tcp_skb_can_collapse(skb, next_skb))
  return;

nlen = min_t(u32, amount, next_skb->len);
if (!nlen || !skb_shift(skb, next_skb, nlen))
  return;

TCP_SKB_CB(skb)->end_seq += nlen;
TCP_SKB_CB(next_skb)->seq += nlen;

if (!next_skb->len) {
  /* In case FIN is set, we need to update end_seq */
  TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

  tcp_eat_one_skb(sk, skb, next_skb);
}
}

/* This routine writes packets to the network.  It advances the
* send_head.  This happens as incoming acks open up the remote
* window for us.
*
* LARGESEND note: !tcp_urg_mode is overkill, only frames between
* snd_up-64k-mss .. snd_up cannot be large. However, taking into
* account rare use of URG, this is not a big flaw.
*
* Send at most one packet when push_one > 0. Temporarily ignore
* cwnd limit to force at most one packet out when push_one == 2.

* Returns true, if no segments are in flight and we have queued segments,
* but cannot send anything now because of SWS or another problem.
*/
static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
      int push_one, gfp_t gfp)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
unsigned int tso_segs, sent_pkts;
u32 cwnd_quota, max_segs;
int result;
bool is_cwnd_limited = false, is_rwnd_limited = false;

sent_pkts = 0;

tcp_mstamp_refresh(tp);
if (!push_one) {
  /* Do MTU probing. */
  result = tcp_mtu_probe(sk);
  if (!result) {
   return false;
  } else if (result > 0) {
   sent_pkts = 1;
  }
}

max_segs = tcp_tso_segs(sk, mss_now);
while ((skb = tcp_send_head(sk))) {
  unsigned int limit;
  int missing_bytes;

  if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
   /* "skb_mstamp_ns" is used as a start point for the retransmit timer */
   tp->tcp_wstamp_ns = tp->tcp_clock_cache;
   skb_set_delivery_time(skb, tp->tcp_wstamp_ns, SKB_CLOCK_MONOTONIC);
   list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
   tcp_init_tso_segs(skb, mss_now);
   goto repair; /* Skip network transmission */
  }

  if (tcp_pacing_check(sk))
   break;

  cwnd_quota = tcp_cwnd_test(tp);
  if (!cwnd_quota) {
   if (push_one == 2)
    /* Force out a loss probe pkt. */
    cwnd_quota = 1;
   else
    break;
  }
  cwnd_quota = min(cwnd_quota, max_segs);
  missing_bytes = cwnd_quota * mss_now - skb->len;
  if (missing_bytes > 0)
   tcp_grow_skb(sk, skb, missing_bytes);

  tso_segs = tcp_set_skb_tso_segs(skb, mss_now);

  if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
   is_rwnd_limited = true;
   break;
  }

  if (tso_segs == 1) {
   if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
           (tcp_skb_is_last(sk, skb) ?
            nonagle : TCP_NAGLE_PUSH))))
    break;
  } else {
   if (!push_one &&
       tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
       &is_rwnd_limited, max_segs))
    break;
  }

  limit = mss_now;
  if (tso_segs > 1 && !tcp_urg_mode(tp))
   limit = tcp_mss_split_point(sk, skb, mss_now,
          cwnd_quota,
          nonagle);

  if (skb->len > limit &&
      unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
   break;

  if (tcp_small_queue_check(sk, skb, 0))
   break;

  /* Argh, we hit an empty skb(), presumably a thread
* is sleeping in sendmsg()/sk_stream_wait_memory().
* We do not want to send a pure-ack packet and have
* a strange looking rtx queue with empty packet(s).
*/
  if (TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq)
   break;

  if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
   break;

repair:
  /* Advance the send_head.  This one is sent out.
* This call will increment packets_out.
*/
  tcp_event_new_data_sent(sk, skb);

  tcp_minshall_update(tp, mss_now, skb);
  sent_pkts += tcp_skb_pcount(skb);

  if (push_one)
   break;
}

if (is_rwnd_limited)
  tcp_chrono_start(sk, TCP_CHRONO_RWND_LIMITED);
else
  tcp_chrono_stop(sk, TCP_CHRONO_RWND_LIMITED);

is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp));
if (likely(sent_pkts || is_cwnd_limited))
  tcp_cwnd_validate(sk, is_cwnd_limited);

if (likely(sent_pkts)) {
  if (tcp_in_cwnd_reduction(sk))
   tp->prr_out += sent_pkts;

  /* Send one loss probe per tail loss episode. */
  if (push_one != 2)
   tcp_schedule_loss_probe(sk, false);
  return false;
}
return !tp->packets_out && !tcp_write_queue_empty(sk);
}

bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
u32 timeout, timeout_us, rto_delta_us;
int early_retrans;

/* Don't do any loss probe on a Fast Open connection before 3WHS
* finishes.
*/
if (rcu_access_pointer(tp->fastopen_rsk))
  return false;

early_retrans = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_early_retrans);
/* Schedule a loss probe in 2*RTT for SACK capable connections
* not in loss recovery, that are either limited by cwnd or application.
*/
if ((early_retrans != 3 && early_retrans != 4) ||
     !tp->packets_out || !tcp_is_sack(tp) ||
     (icsk->icsk_ca_state != TCP_CA_Open &&
      icsk->icsk_ca_state != TCP_CA_CWR))
  return false;

/* Probe timeout is 2*rtt. Add minimum RTO to account
* for delayed ack when there's one outstanding packet. If no RTT
* sample is available then probe after TCP_TIMEOUT_INIT.
*/
if (tp->srtt_us) {
  timeout_us = tp->srtt_us >> 2;
  if (tp->packets_out == 1)
   timeout_us += tcp_rto_min_us(sk);
  else
   timeout_us += TCP_TIMEOUT_MIN_US;
  timeout = usecs_to_jiffies(timeout_us);
} else {
  timeout = TCP_TIMEOUT_INIT;
}

/* If the RTO formula yields an earlier time, then use that time. */
rto_delta_us = advancing_rto ?
   jiffies_to_usecs(inet_csk(sk)->icsk_rto) :
   tcp_rto_delta_us(sk);  /* How far in future is RTO? */
if (rto_delta_us > 0)
  timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us));

tcp_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout, true);
return true;
}

/* Thanks to skb fast clones, we can detect if a prior transmit of
* a packet is still in a qdisc or driver queue.
* In this case, there is very little point doing a retransmit !
*/
static bool skb_still_in_host_queue(struct sock *sk,
        const struct sk_buff *skb)
{
if (unlikely(skb_fclone_busy(sk, skb))) {
  set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
  smp_mb__after_atomic();
  if (skb_fclone_busy(sk, skb)) {
   NET_INC_STATS(sock_net(sk),
          LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
   return true;
  }
}
return false;
}

/* When probe timeout (PTO) fires, try send a new segment if possible, else
* retransmit the last segment.
*/
void tcp_send_loss_probe(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
int pcount;
int mss = tcp_current_mss(sk);

/* At most one outstanding TLP */
if (tp->tlp_high_seq)
  goto rearm_timer;

tp->tlp_retrans = 0;
skb = tcp_send_head(sk);
if (skb && tcp_snd_wnd_test(tp, skb, mss)) {
  pcount = tp->packets_out;
  tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
  if (tp->packets_out > pcount)
   goto probe_sent;
  goto rearm_timer;
}
skb = skb_rb_last(&sk->tcp_rtx_queue);
if (unlikely(!skb)) {
  tcp_warn_once(sk, tp->packets_out, "invalid inflight: ");
  smp_store_release(&inet_csk(sk)->icsk_pending, 0);
  return;
}

if (skb_still_in_host_queue(sk, skb))
  goto rearm_timer;

pcount = tcp_skb_pcount(skb);
if (WARN_ON(!pcount))
  goto rearm_timer;

if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
  if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
       (pcount - 1) * mss, mss,
       GFP_ATOMIC)))
   goto rearm_timer;
  skb = skb_rb_next(skb);
}

if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
  goto rearm_timer;

if (__tcp_retransmit_skb(sk, skb, 1))
  goto rearm_timer;

tp->tlp_retrans = 1;

probe_sent:
/* Record snd_nxt for loss detection. */
tp->tlp_high_seq = tp->snd_nxt;

NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
/* Reset s.t. tcp_rearm_rto will restart timer from now */
smp_store_release(&inet_csk(sk)->icsk_pending, 0);
rearm_timer:
tcp_rearm_rto(sk);
}

/* Push out any pending frames which were held back due to
* TCP_CORK or attempt at coalescing tiny packets.
* The socket must be locked by the caller.
*/
void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
          int nonagle)
{
/* If we are closed, the bytes will have to remain here.
* In time closedown will finish, we empty the write queue and
* all will be happy.
*/
if (unlikely(sk->sk_state == TCP_CLOSE))
  return;

if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
      sk_gfp_mask(sk, GFP_ATOMIC)))
  tcp_check_probe_timer(sk);
}

/* Send _single_ skb sitting at the send head. This function requires
* true push pending frames to setup probe timer etc.
*/
void tcp_push_one(struct sock *sk, unsigned int mss_now)
{
struct sk_buff *skb = tcp_send_head(sk);

BUG_ON(!skb || skb->len < mss_now);

tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
}

/* This function returns the amount that we can raise the
* usable window based on the following constraints
*
* 1. The window can never be shrunk once it is offered (RFC 793)
* 2. We limit memory per socket
*
* RFC 1122:
* "the suggested [SWS] avoidance algorithm for the receiver is to keep
*  RECV.NEXT + RCV.WIN fixed until:
*  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
*
* i.e. don't raise the right edge of the window until you can raise
* it at least MSS bytes.
*
* Unfortunately, the recommended algorithm breaks header prediction,
* since header prediction assumes th->window stays fixed.
*
* Strictly speaking, keeping th->window fixed violates the receiver
* side SWS prevention criteria. The problem is that under this rule
* a stream of single byte packets will cause the right side of the
* window to always advance by a single byte.
*
* Of course, if the sender implements sender side SWS prevention
* then this will not be a problem.
*
* BSD seems to make the following compromise:
*
* If the free space is less than the 1/4 of the maximum
* space available and the free space is less than 1/2 mss,
* then set the window to 0.
* [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
* Otherwise, just prevent the window from shrinking
* and from being larger than the largest representable value.
*
* This prevents incremental opening of the window in the regime
* where TCP is limited by the speed of the reader side taking
* data out of the TCP receive queue. It does nothing about
* those cases where the window is constrained on the sender side
* because the pipeline is full.
*
* BSD also seems to "accidentally" limit itself to windows that are a
* multiple of MSS, at least until the free space gets quite small.
* This would appear to be a side effect of the mbuf implementation.
* Combining these two algorithms results in the observed behavior
* of having a fixed window size at almost all times.
*
* Below we obtain similar behavior by forcing the offered window to
* a multiple of the mss when it is feasible to do so.
*
* Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
* Regular options like TIMESTAMP are taken into account.
*/
u32 __tcp_select_window(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct net *net = sock_net(sk);
/* MSS for the peer's data.  Previous versions used mss_clamp
* here.  I don't know if the value based on our guesses
* of peer's MSS is better for the performance.  It's more correct
* but may be worse for the performance because of rcv_mss
* fluctuations.  --SAW  1998/11/1
*/
int mss = icsk->icsk_ack.rcv_mss;
int free_space = tcp_space(sk);
int allowed_space = tcp_full_space(sk);
int full_space, window;

if (sk_is_mptcp(sk))
  mptcp_space(sk, &free_space, &allowed_space);

full_space = min_t(int, tp->window_clamp, allowed_space);

if (unlikely(mss > full_space)) {
  mss = full_space;
  if (mss <= 0)
   return 0;
}

/* Only allow window shrink if the sysctl is enabled and we have
* a non-zero scaling factor in effect.
*/
if (READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) && tp->rx_opt.rcv_wscale)
  goto shrink_window_allowed;

/* do not allow window to shrink */

if (free_space < (full_space >> 1)) {
  icsk->icsk_ack.quick = 0;

  if (tcp_under_memory_pressure(sk))
   tcp_adjust_rcv_ssthresh(sk);

  /* free_space might become our new window, make sure we don't
* increase it due to wscale.
*/
  free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);

  /* if free space is less than mss estimate, or is below 1/16th
* of the maximum allowed, try to move to zero-window, else
* tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
* new incoming data is dropped due to memory limits.
* With large window, mss test triggers way too late in order
* to announce zero window in time before rmem limit kicks in.
*/
  if (free_space < (allowed_space >> 4) || free_space < mss)
   return 0;
}

if (free_space > tp->rcv_ssthresh)
  free_space = tp->rcv_ssthresh;

/* Don't do rounding if we are using window scaling, since the
* scaled window will not line up with the MSS boundary anyway.
*/
if (tp->rx_opt.rcv_wscale) {
  window = free_space;

  /* Advertise enough space so that it won't get scaled away.
* Import case: prevent zero window announcement if
* 1<<rcv_wscale > mss.
*/
  window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
} else {
  window = tp->rcv_wnd;
  /* Get the largest window that is a nice multiple of mss.
* Window clamp already applied above.
* If our current window offering is within 1 mss of the
* free space we just keep it. This prevents the divide
* and multiply from happening most of the time.
* We also don't do any window rounding when the free space
* is too small.
*/
  if (window <= free_space - mss || window > free_space)
   window = rounddown(free_space, mss);
  else if (mss == full_space &&
    free_space > window + (full_space >> 1))
   window = free_space;
}

return window;

shrink_window_allowed:
/* new window should always be an exact multiple of scaling factor */
free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);

if (free_space < (full_space >> 1)) {
  icsk->icsk_ack.quick = 0;

  if (tcp_under_memory_pressure(sk))
   tcp_adjust_rcv_ssthresh(sk);

  /* if free space is too low, return a zero window */
  if (free_space < (allowed_space >> 4) || free_space < mss ||
   free_space < (1 << tp->rx_opt.rcv_wscale))
   return 0;
}

if (free_space > tp->rcv_ssthresh) {
  free_space = tp->rcv_ssthresh;
  /* new window should always be an exact multiple of scaling factor
*
* For this case, we ALIGN "up" (increase free_space) because
* we know free_space is not zero here, it has been reduced from
* the memory-based limit, and rcv_ssthresh is not a hard limit
* (unlike sk_rcvbuf).
*/
  free_space = ALIGN(free_space, (1 << tp->rx_opt.rcv_wscale));
}

return free_space;
}

void tcp_skb_collapse_tstamp(struct sk_buff *skb,
        const struct sk_buff *next_skb)
{
if (unlikely(tcp_has_tx_tstamp(next_skb))) {
  const struct skb_shared_info *next_shinfo =
   skb_shinfo(next_skb);
  struct skb_shared_info *shinfo = skb_shinfo(skb);

  shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
  shinfo->tskey = next_shinfo->tskey;
  TCP_SKB_CB(skb)->txstamp_ack |=
   TCP_SKB_CB(next_skb)->txstamp_ack;
}
}

/* Collapses two adjacent SKB's during retransmission. */
static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *next_skb = skb_rb_next(skb);
int next_skb_size;

next_skb_size = next_skb->len;

BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);

if (next_skb_size && !tcp_skb_shift(skb, next_skb, 1, next_skb_size))
  return false;

tcp_highest_sack_replace(sk, next_skb, skb);

/* Update sequence range on original skb. */
TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

/* Merge over control information. This moves PSH/FIN etc. over */
TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;

/* All done, get rid of second SKB and account for it so
* packet counting does not break.
*/
TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;

/* changed transmit queue under us so clear hints */
if (next_skb == tp->retransmit_skb_hint)
  tp->retransmit_skb_hint = skb;

tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));

tcp_skb_collapse_tstamp(skb, next_skb);

tcp_rtx_queue_unlink_and_free(next_skb, sk);
return true;
}

/* Check if coalescing SKBs is legal. */
static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
{
if (tcp_skb_pcount(skb) > 1)
  return false;
if (skb_cloned(skb))
  return false;
if (!skb_frags_readable(skb))
  return false;
/* Some heuristics for collapsing over SACK'd could be invented */
if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
  return false;

return true;
}

/* Collapse packets in the retransmit queue to make to create
* less packets on the wire. This is only done on retransmission.
*/
static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
         int space)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb = to, *tmp;
bool first = true;

if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse))
  return;
if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
  return;

skb_rbtree_walk_from_safe(skb, tmp) {
  if (!tcp_can_collapse(sk, skb))
   break;

  if (!tcp_skb_can_collapse(to, skb))
   break;

  space -= skb->len;

  if (first) {
   first = false;
   continue;
  }

  if (space < 0)
   break;

  if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
   break;

  if (!tcp_collapse_retrans(sk, to))
   break;
}
}

/* This retransmits one SKB.  Policy decisions and retransmit queue
* state updates are done by the caller.  Returns non-zero if an
* error occurred which prevented the send.
*/
int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
unsigned int cur_mss;
int diff, len, err;
int avail_wnd;

/* Inconclusive MTU probe */
if (icsk->icsk_mtup.probe_size)
  icsk->icsk_mtup.probe_size = 0;

if (skb_still_in_host_queue(sk, skb)) {
  err = -EBUSY;
  goto out;
}

start:
if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
  if (unlikely(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
   TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_SYN;
   TCP_SKB_CB(skb)->seq++;
   goto start;
  }
  if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
   WARN_ON_ONCE(1);
   err = -EINVAL;
   goto out;
  }
  if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq)) {
   err = -ENOMEM;
   goto out;
  }
}

if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk)) {
  err = -EHOSTUNREACH; /* Routing failure or similar. */
  goto out;
}

cur_mss = tcp_current_mss(sk);
avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

/* If receiver has shrunk his window, and skb is out of
* new window, do not retransmit it. The exception is the
* case, when window is shrunk to zero. In this case
* our retransmit of one segment serves as a zero window probe.
*/
if (avail_wnd <= 0) {
  if (TCP_SKB_CB(skb)->seq != tp->snd_una) {
   err = -EAGAIN;
   goto out;
  }
  avail_wnd = cur_mss;
}

len = cur_mss * segs;
if (len > avail_wnd) {
  len = rounddown(avail_wnd, cur_mss);
  if (!len)
   len = avail_wnd;
}
if (skb->len > len) {
  if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
     cur_mss, GFP_ATOMIC)) {
   err = -ENOMEM;  /* We'll try again later. */
   goto out;
  }
} else {
  if (skb_unclone_keeptruesize(skb, GFP_ATOMIC)) {
   err = -ENOMEM;
   goto out;
  }

  diff = tcp_skb_pcount(skb);
  tcp_set_skb_tso_segs(skb, cur_mss);
  diff -= tcp_skb_pcount(skb);
  if (diff)
   tcp_adjust_pcount(sk, skb, diff);
  avail_wnd = min_t(int, avail_wnd, cur_mss);
  if (skb->len < avail_wnd)
   tcp_retrans_try_collapse(sk, skb, avail_wnd);
}

/* RFC3168, section 6.1.1.1. ECN fallback */
if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
  tcp_ecn_clear_syn(sk, skb);

/* Update global and local TCP statistics. */
segs = tcp_skb_pcount(skb);
TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
  __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
tp->total_retrans += segs;
tp->bytes_retrans += skb->len;

/* make sure skb->data is aligned on arches that require it
* and check if ack-trimming & collapsing extended the headroom
* beyond what csum_start can cover.
*/
if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
       skb_headroom(skb) >= 0xFFFF)) {
  struct sk_buff *nskb;

  tcp_skb_tsorted_save(skb) {
   nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
   if (nskb) {
    nskb->dev = NULL;
    err = tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC);
   } else {
    err = -ENOBUFS;
   }
  } tcp_skb_tsorted_restore(skb);

  if (!err) {
   tcp_update_skb_after_send(sk, skb, tp->tcp_wstamp_ns);
   tcp_rate_skb_sent(sk, skb);
  }
} else {
  err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
}

if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
  tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
      TCP_SKB_CB(skb)->seq, segs, err);

if (unlikely(err) && err != -EBUSY)
  NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);

/* To avoid taking spuriously low RTT samples based on a timestamp
* for a transmit that never happened, always mark EVER_RETRANS
*/
TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;

out:
trace_tcp_retransmit_skb(sk, skb, err);
return err;
}

int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
{
struct tcp_sock *tp = tcp_sk(sk);
int err = __tcp_retransmit_skb(sk, skb, segs);

if (err == 0) {
#if FASTRETRANS_DEBUG > 0
  if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
   net_dbg_ratelimited("retrans_out leaked\n");
  }
#endif
  TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
  tp->retrans_out += tcp_skb_pcount(skb);
}

/* Save stamp of the first (attempted) retransmit. */
if (!tp->retrans_stamp)
  tp->retrans_stamp = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb);

if (tp->undo_retrans < 0)
  tp->undo_retrans = 0;
tp->undo_retrans += tcp_skb_pcount(skb);
return err;
}

/* This gets called after a retransmit timeout, and the initially
* retransmitted data is acknowledged.  It tries to continue
* resending the rest of the retransmit queue, until either
* we've sent it all or the congestion window limit is reached.
*/
void tcp_xmit_retransmit_queue(struct sock *sk)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
struct sk_buff *skb, *rtx_head, *hole = NULL;
struct tcp_sock *tp = tcp_sk(sk);
bool rearm_timer = false;
u32 max_segs;
int mib_idx;

if (!tp->packets_out)
  return;

rtx_head = tcp_rtx_queue_head(sk);
skb = tp->retransmit_skb_hint ?: rtx_head;
max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
skb_rbtree_walk_from(skb) {
  __u8 sacked;
  int segs;

  if (tcp_pacing_check(sk))
   break;

  /* we could do better than to assign each time */
  if (!hole)
   tp->retransmit_skb_hint = skb;

  segs = tcp_snd_cwnd(tp) - tcp_packets_in_flight(tp);
  if (segs <= 0)
   break;
  sacked = TCP_SKB_CB(skb)->sacked;
  /* In case tcp_shift_skb_data() have aggregated large skbs,
* we need to make sure not sending too bigs TSO packets
*/
  segs = min_t(int, segs, max_segs);

  if (tp->retrans_out >= tp->lost_out) {
   break;
  } else if (!(sacked & TCPCB_LOST)) {
   if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
    hole = skb;
   continue;

  } else {
   if (icsk->icsk_ca_state != TCP_CA_Loss)
    mib_idx = LINUX_MIB_TCPFASTRETRANS;
   else
    mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
  }

  if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
   continue;

  if (tcp_small_queue_check(sk, skb, 1))
   break;

  if (tcp_retransmit_skb(sk, skb, segs))
   break;

  NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));

  if (tcp_in_cwnd_reduction(sk))
   tp->prr_out += tcp_skb_pcount(skb);

  if (skb == rtx_head &&
      icsk->icsk_pending != ICSK_TIME_REO_TIMEOUT)
   rearm_timer = true;

}
if (rearm_timer)
  tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
         inet_csk(sk)->icsk_rto, true);
}

/* We allow to exceed memory limits for FIN packets to expedite
* connection tear down and (memory) recovery.
* Otherwise tcp_send_fin() could be tempted to either delay FIN
* or even be forced to close flow without any FIN.
* In general, we want to allow one skb per socket to avoid hangs
* with edge trigger epoll()
*/
void sk_forced_mem_schedule(struct sock *sk, int size)
{
int delta, amt;

delta = size - sk->sk_forward_alloc;
if (delta <= 0)
  return;
amt = sk_mem_pages(delta);
sk_forward_alloc_add(sk, amt << PAGE_SHIFT);
sk_memory_allocated_add(sk, amt);

if (mem_cgroup_sockets_enabled && sk->sk_memcg)
  mem_cgroup_charge_skmem(sk->sk_memcg, amt,
     gfp_memcg_charge() | __GFP_NOFAIL);
}

/* Send a FIN. The caller locks the socket for us.
* We should try to send a FIN packet really hard, but eventually give up.
*/
void tcp_send_fin(struct sock *sk)
{
struct sk_buff *skb, *tskb, *tail = tcp_write_queue_tail(sk);
struct tcp_sock *tp = tcp_sk(sk);

/* Optimization, tack on the FIN if we have one skb in write queue and
* this skb was not yet sent, or we are under memory pressure.
* Note: in the latter case, FIN packet will be sent after a timeout,
* as TCP stack thinks it has already been transmitted.
*/
tskb = tail;
if (!tskb && tcp_under_memory_pressure(sk))
  tskb = skb_rb_last(&sk->tcp_rtx_queue);

if (tskb) {
  TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
  TCP_SKB_CB(tskb)->end_seq++;
  tp->write_seq++;
  if (!tail) {
   /* This means tskb was already sent.
* Pretend we included the FIN on previous transmit.
* We need to set tp->snd_nxt to the value it would have
* if FIN had been sent. This is because retransmit path
* does not change tp->snd_nxt.
*/
   WRITE_ONCE(tp->snd_nxt, tp->snd_nxt + 1);
   return;
  }
} else {
  skb = alloc_skb_fclone(MAX_TCP_HEADER,
           sk_gfp_mask(sk, GFP_ATOMIC |
             __GFP_NOWARN));
  if (unlikely(!skb))
   return;

  INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
  skb_reserve(skb, MAX_TCP_HEADER);
  sk_forced_mem_schedule(sk, skb->truesize);
  /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
  tcp_init_nondata_skb(skb, tp->write_seq,
         TCPHDR_ACK | TCPHDR_FIN);
  tcp_queue_skb(sk, skb);
}
__tcp_push_pending_frames(sk, tcp_current_mss(sk), TCP_NAGLE_OFF);
}

/* We get here when a process closes a file descriptor (either due to
* an explicit close() or as a byproduct of exit()'ing) and there
* was unread data in the receive queue.  This behavior is recommended
* by RFC 2525, section 2.17.  -DaveM
*/
void tcp_send_active_reset(struct sock *sk, gfp_t priority,
      enum sk_rst_reason reason)
{
struct sk_buff *skb;

TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);

/* NOTE: No TCP options attached and we never retransmit this. */
skb = alloc_skb(MAX_TCP_HEADER, priority);
if (!skb) {
  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
  return;
}

/* Reserve space for headers and prepare control bits. */
skb_reserve(skb, MAX_TCP_HEADER);
tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
        TCPHDR_ACK | TCPHDR_RST);
tcp_mstamp_refresh(tcp_sk(sk));
/* Send it off. */
if (tcp_transmit_skb(sk, skb, 0, priority))
  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);

/* skb of trace_tcp_send_reset() keeps the skb that caused RST,
* skb here is different to the troublesome skb, so use NULL
*/
trace_tcp_send_reset(sk, NULL, reason);
}

/* Send a crossed SYN-ACK during socket establishment.
* WARNING: This routine must only be called when we have already sent
* a SYN packet that crossed the incoming SYN that caused this routine
* to get called. If this assumption fails then the initial rcv_wnd
* and rcv_wscale values will not be correct.
*/
int tcp_send_synack(struct sock *sk)
{
struct sk_buff *skb;

skb = tcp_rtx_queue_head(sk);
if (!skb || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
  pr_err("%s: wrong queue state\n", __func__);
  return -EFAULT;
}
if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
  if (skb_cloned(skb)) {
   struct sk_buff *nskb;

   tcp_skb_tsorted_save(skb) {
    nskb = skb_copy(skb, GFP_ATOMIC);
   } tcp_skb_tsorted_restore(skb);
   if (!nskb)
    return -ENOMEM;
   INIT_LIST_HEAD(&nskb->tcp_tsorted_anchor);
   tcp_highest_sack_replace(sk, skb, nskb);
   tcp_rtx_queue_unlink_and_free(skb, sk);
   __skb_header_release(nskb);
   tcp_rbtree_insert(&sk->tcp_rtx_queue, nskb);
   sk_wmem_queued_add(sk, nskb->truesize);
   sk_mem_charge(sk, nskb->truesize);
   skb = nskb;
  }

  TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
  tcp_ecn_send_synack(sk, skb);
}
return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
}

/**
* tcp_make_synack - Allocate one skb and build a SYNACK packet.
* @sk: listener socket
* @dst: dst entry attached to the SYNACK. It is consumed and caller
*       should not use it again.
* @req: request_sock pointer
* @foc: cookie for tcp fast open
* @synack_type: Type of synack to prepare
* @syn_skb: SYN packet just received.  It could be NULL for rtx case.
*/
struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
    struct request_sock *req,
    struct tcp_fastopen_cookie *foc,
    enum tcp_synack_type synack_type,
    struct sk_buff *syn_skb)
{
struct inet_request_sock *ireq = inet_rsk(req);
const struct tcp_sock *tp = tcp_sk(sk);
struct tcp_out_options opts;
struct tcp_key key = {};
struct sk_buff *skb;
int tcp_header_size;
struct tcphdr *th;
int mss;
u64 now;

skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
if (unlikely(!skb)) {
  dst_release(dst);
  return NULL;
}
/* Reserve space for headers. */
skb_reserve(skb, MAX_TCP_HEADER);

switch (synack_type) {
case TCP_SYNACK_NORMAL:
  skb_set_owner_edemux(skb, req_to_sk(req));
  break;
case TCP_SYNACK_COOKIE:
  /* Under synflood, we do not attach skb to a socket,
* to avoid false sharing.
*/
  break;
case TCP_SYNACK_FASTOPEN:
  /* sk is a const pointer, because we want to express multiple
* cpu might call us concurrently.
* sk->sk_wmem_alloc in an atomic, we can promote to rw.
*/
  skb_set_owner_w(skb, (struct sock *)sk);
  break;
}
skb_dst_set(skb, dst);

mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));

memset(&opts, 0, sizeof(opts));
now = tcp_clock_ns();
#ifdef CONFIG_SYN_COOKIES
if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))
  skb_set_delivery_time(skb, cookie_init_timestamp(req, now),
          SKB_CLOCK_MONOTONIC);
else
#endif
{
  skb_set_delivery_time(skb, now, SKB_CLOCK_MONOTONIC);
  if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */
   tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb);
}

#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
rcu_read_lock();
#endif
if (tcp_rsk_used_ao(req)) {
#ifdef CONFIG_TCP_AO
  struct tcp_ao_key *ao_key = NULL;
  u8 keyid = tcp_rsk(req)->ao_keyid;
  u8 rnext = tcp_rsk(req)->ao_rcv_next;

  ao_key = tcp_sk(sk)->af_specific->ao_lookup(sk, req_to_sk(req),
           keyid, -1);
  /* If there is no matching key - avoid sending anything,
* especially usigned segments. It could try harder and lookup
* for another peer-matching key, but the peer has requested
* ao_keyid (RFC5925 RNextKeyID), so let's keep it simple here.
*/
  if (unlikely(!ao_key)) {
   trace_tcp_ao_synack_no_key(sk, keyid, rnext);
   rcu_read_unlock();
   kfree_skb(skb);
   net_warn_ratelimited("TCP-AO: the keyid %u from SYN packet is not present - not sending SYNACK\n",
          keyid);
   return NULL;
  }
  key.ao_key = ao_key;
  key.type = TCP_KEY_AO;
#endif
} else {
#ifdef CONFIG_TCP_MD5SIG
  key.md5_key = tcp_rsk(req)->af_specific->req_md5_lookup(sk,
     req_to_sk(req));
  if (key.md5_key)
   key.type = TCP_KEY_MD5;
#endif
}
skb_set_hash(skb, READ_ONCE(tcp_rsk(req)->txhash), PKT_HASH_TYPE_L4);
/* bpf program will be interested in the tcp_flags */
TCP_SKB_CB(skb)->tcp_flags = TCPHDR_SYN | TCPHDR_ACK;
tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts,
          &key, foc, synack_type, syn_skb)
     + sizeof(*th);

skb_push(skb, tcp_header_size);
skb_reset_transport_header(skb);

th = (struct tcphdr *)skb->data;
memset(th, 0, sizeof(struct tcphdr));
th->syn = 1;
th->ack = 1;
tcp_ecn_make_synack(req, th);
th->source = htons(ireq->ir_num);
th->dest = ireq->ir_rmt_port;
skb->mark = ireq->ir_mark;
skb->ip_summed = CHECKSUM_PARTIAL;
th->seq = htonl(tcp_rsk(req)->snt_isn);
/* XXX data is queued and acked as is. No buffer/window check */
th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);

/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
th->window = htons(min(req->rsk_rcv_wnd, 65535U));
tcp_options_write(th, NULL, tcp_rsk(req), &opts, &key);
th->doff = (tcp_header_size >> 2);
TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);

/* Okay, we have all we need - do the md5 hash if needed */
if (tcp_key_is_md5(&key)) {
#ifdef CONFIG_TCP_MD5SIG
  tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
     key.md5_key, req_to_sk(req), skb);
#endif
} else if (tcp_key_is_ao(&key)) {
#ifdef CONFIG_TCP_AO
  tcp_rsk(req)->af_specific->ao_synack_hash(opts.hash_location,
     key.ao_key, req, skb,
     opts.hash_location - (u8 *)th, 0);
#endif
}
#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
rcu_read_unlock();
#endif

bpf_skops_write_hdr_opt((struct sock *)sk, skb, req, syn_skb,
    synack_type, &opts);

skb_set_delivery_time(skb, now, SKB_CLOCK_MONOTONIC);
tcp_add_tx_delay(skb, tp);

return skb;
}
EXPORT_IPV6_MOD(tcp_make_synack);

static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
{
struct inet_connection_sock *icsk = inet_csk(sk);
const struct tcp_congestion_ops *ca;
u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);

if (ca_key == TCP_CA_UNSPEC)
  return;

rcu_read_lock();
ca = tcp_ca_find_key(ca_key);
if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
  bpf_module_put(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner);
  icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
  icsk->icsk_ca_ops = ca;
}
rcu_read_unlock();
}

/* Do all connect socket setups that can be done AF independent. */
static void tcp_connect_init(struct sock *sk)
{
const struct dst_entry *dst = __sk_dst_get(sk);
struct tcp_sock *tp = tcp_sk(sk);
__u8 rcv_wscale;
u32 rcv_wnd;

/* We'll fix this up when we get a response from the other end.
* See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
*/
tp->tcp_header_len = sizeof(struct tcphdr);
if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps))
  tp->tcp_header_len += TCPOLEN_TSTAMP_ALIGNED;

tcp_ao_connect_init(sk);

/* If user gave his TCP_MAXSEG, record it to clamp */
if (tp->rx_opt.user_mss)
  tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
tp->max_window = 0;
tcp_mtup_init(sk);
tcp_sync_mss(sk, dst_mtu(dst));

tcp_ca_dst_init(sk, dst);

if (!tp->window_clamp)
  WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));
tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));

tcp_initialize_rcv_mss(sk);

/* limit the window selection if the user enforce a smaller rx buffer */
if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
     (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
  WRITE_ONCE(tp->window_clamp, tcp_full_space(sk));

rcv_wnd = tcp_rwnd_init_bpf(sk);
if (rcv_wnd == 0)
  rcv_wnd = dst_metric(dst, RTAX_INITRWND);

tcp_select_initial_window(sk, tcp_full_space(sk),
      tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
      &tp->rcv_wnd,
      &tp->window_clamp,
      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling),
      &rcv_wscale,
      rcv_wnd);

tp->rx_opt.rcv_wscale = rcv_wscale;
tp->rcv_ssthresh = tp->rcv_wnd;

WRITE_ONCE(sk->sk_err, 0);
sock_reset_flag(sk, SOCK_DONE);
tp->snd_wnd = 0;
tcp_init_wl(tp, 0);
tcp_write_queue_purge(sk);
tp->snd_una = tp->write_seq;
tp->snd_sml = tp->write_seq;
tp->snd_up = tp->write_seq;
WRITE_ONCE(tp->snd_nxt, tp->write_seq);

if (likely(!tp->repair))
  tp->rcv_nxt = 0;
else
  tp->rcv_tstamp = tcp_jiffies32;
tp->rcv_wup = tp->rcv_nxt;
WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);

inet_csk(sk)->icsk_rto = tcp_timeout_init(sk);
inet_csk(sk)->icsk_retransmits = 0;
tcp_clear_retrans(tp);
}

static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
{
struct tcp_sock *tp = tcp_sk(sk);
struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);

tcb->end_seq += skb->len;
__skb_header_release(skb);
sk_wmem_queued_add(sk, skb->truesize);
sk_mem_charge(sk, skb->truesize);
WRITE_ONCE(tp->write_seq, tcb->end_seq);
tp->packets_out += tcp_skb_pcount(skb);
}

/* Build and send a SYN with data and (cached) Fast Open cookie. However,
* queue a data-only packet after the regular SYN, such that regular SYNs
* are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
* only the SYN sequence, the data are retransmitted in the first ACK.
* If cookie is not cached or other error occurs, falls back to send a
* regular SYN with Fast Open cookie request option.
*/
static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct tcp_fastopen_request *fo = tp->fastopen_req;
struct page_frag *pfrag = sk_page_frag(sk);
struct sk_buff *syn_data;
int space, err = 0;

tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
if (!tcp_fastopen_cookie_check(sk, &tp->rx_opt.mss_clamp, &fo->cookie))
  goto fallback;

/* MSS for SYN-data is based on cached MSS and bounded by PMTU and
* user-MSS. Reserve maximum option space for middleboxes that add
* private TCP options. The cost is reduced data space in SYN :(
*/
tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
/* Sync mss_cache after updating the mss_clamp */
tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);

space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) -
  MAX_TCP_OPTION_SPACE;

space = min_t(size_t, space, fo->size);

if (space &&
     !skb_page_frag_refill(min_t(size_t, space, PAGE_SIZE),
      pfrag, sk->sk_allocation))
  goto fallback;
syn_data = tcp_stream_alloc_skb(sk, sk->sk_allocation, false);
if (!syn_data)
  goto fallback;
memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
if (space) {
  space = min_t(size_t, space, pfrag->size - pfrag->offset);
  space = tcp_wmem_schedule(sk, space);
}
if (space) {
  space = copy_page_from_iter(pfrag->page, pfrag->offset,
         space, &fo->data->msg_iter);
  if (unlikely(!space)) {
   tcp_skb_tsorted_anchor_cleanup(syn_data);
   kfree_skb(syn_data);
   goto fallback;
  }
  skb_fill_page_desc(syn_data, 0, pfrag->page,
       pfrag->offset, space);
  page_ref_inc(pfrag->page);
  pfrag->offset += space;
  skb_len_add(syn_data, space);
  skb_zcopy_set(syn_data, fo->uarg, NULL);
}
/* No more data pending in inet_wait_for_connect() */
if (space == fo->size)
  fo->data = NULL;
fo->copied = space;

tcp_connect_queue_skb(sk, syn_data);
if (syn_data->len)
  tcp_chrono_start(sk, TCP_CHRONO_BUSY);

err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);

skb_set_delivery_time(syn, syn_data->skb_mstamp_ns, SKB_CLOCK_MONOTONIC);

/* Now full SYN+DATA was cloned and sent (or not),
* remove the SYN from the original skb (syn_data)
* we keep in write queue in case of a retransmit, as we
* also have the SYN packet (with no data) in the same queue.
*/
TCP_SKB_CB(syn_data)->seq++;
TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
if (!err) {
  tp->syn_data = (fo->copied > 0);
  tcp_rbtree_insert(&sk->tcp_rtx_queue, syn_data);
  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
  goto done;
}

/* data was not sent, put it in write_queue */
__skb_queue_tail(&sk->sk_write_queue, syn_data);
tp->packets_out -= tcp_skb_pcount(syn_data);

fallback:
/* Send a regular SYN with Fast Open cookie request option */
if (fo->cookie.len > 0)
  fo->cookie.len = 0;
err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
if (err)
  tp->syn_fastopen = 0;
done:
fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
return err;
}

/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *buff;
int err;

tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);

#if defined(CONFIG_TCP_MD5SIG) && defined(CONFIG_TCP_AO)
/* Has to be checked late, after setting daddr/saddr/ops.
* Return error if the peer has both a md5 and a tcp-ao key
* configured as this is ambiguous.
*/
if (unlikely(rcu_dereference_protected(tp->md5sig_info,
            lockdep_sock_is_held(sk)))) {
  bool needs_ao = !!tp->af_specific->ao_lookup(sk, sk, -1, -1);
  bool needs_md5 = !!tp->af_specific->md5_lookup(sk, sk);
  struct tcp_ao_info *ao_info;

  ao_info = rcu_dereference_check(tp->ao_info,
      lockdep_sock_is_held(sk));
  if (ao_info) {
   /* This is an extra check: tcp_ao_required() in
* tcp_v{4,6}_parse_md5_keys() should prevent adding
* md5 keys on ao_required socket.
*/
   needs_ao |= ao_info->ao_required;
   WARN_ON_ONCE(ao_info->ao_required && needs_md5);
  }
  if (needs_md5 && needs_ao)
   return -EKEYREJECTED;

  /* If we have a matching md5 key and no matching tcp-ao key
* then free up ao_info if allocated.
*/
  if (needs_md5) {
   tcp_ao_destroy_sock(sk, false);
  } else if (needs_ao) {
   tcp_clear_md5_list(sk);
   kfree(rcu_replace_pointer(tp->md5sig_info, NULL,
        lockdep_sock_is_held(sk)));
  }
}
#endif
#ifdef CONFIG_TCP_AO
if (unlikely(rcu_dereference_protected(tp->ao_info,
            lockdep_sock_is_held(sk)))) {
  /* Don't allow connecting if ao is configured but no
* matching key is found.
*/
  if (!tp->af_specific->ao_lookup(sk, sk, -1, -1))
   return -EKEYREJECTED;
}
#endif

if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
  return -EHOSTUNREACH; /* Routing failure or similar. */

tcp_connect_init(sk);

if (unlikely(tp->repair)) {
  tcp_finish_connect(sk, NULL);
  return 0;
}

buff = tcp_stream_alloc_skb(sk, sk->sk_allocation, true);
if (unlikely(!buff))
  return -ENOBUFS;

/* SYN eats a sequence byte, write_seq updated by
* tcp_connect_queue_skb().
*/
tcp_init_nondata_skb(buff, tp->write_seq, TCPHDR_SYN);
tcp_mstamp_refresh(tp);
tp->retrans_stamp = tcp_time_stamp_ts(tp);
tcp_connect_queue_skb(sk, buff);
tcp_ecn_send_syn(sk, buff);
tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);

/* Send off SYN; include data in Fast Open. */
err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
       tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
if (err == -ECONNREFUSED)
  return err;

/* We change tp->snd_nxt after the tcp_transmit_skb() call
* in order to make this packet get counted in tcpOutSegs.
*/
WRITE_ONCE(tp->snd_nxt, tp->write_seq);
tp->pushed_seq = tp->write_seq;
buff = tcp_send_head(sk);
if (unlikely(buff)) {
  WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(buff)->seq);
  tp->pushed_seq = TCP_SKB_CB(buff)->seq;
}
TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);

/* Timer for repeating the SYN until an answer. */
tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
        inet_csk(sk)->icsk_rto, false);
return 0;
}
EXPORT_SYMBOL(tcp_connect);

u32 tcp_delack_max(const struct sock *sk)
{
u32 delack_from_rto_min = max(tcp_rto_min(sk), 2) - 1;

return min(READ_ONCE(inet_csk(sk)->icsk_delack_max), delack_from_rto_min);
}

/* Send out a delayed ack, the caller does the policy checking
* to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
* for details.
*/
void tcp_send_delayed_ack(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
int ato = icsk->icsk_ack.ato;
unsigned long timeout;

if (ato > TCP_DELACK_MIN) {
  const struct tcp_sock *tp = tcp_sk(sk);
  int max_ato = HZ / 2;

  if (inet_csk_in_pingpong_mode(sk) ||
      (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
   max_ato = TCP_DELACK_MAX;

  /* Slow path, intersegment interval is "high". */

  /* If some rtt estimate is known, use it to bound delayed ack.
* Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
* directly.
*/
  if (tp->srtt_us) {
   int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
     TCP_DELACK_MIN);

   if (rtt < max_ato)
    max_ato = rtt;
  }

  ato = min(ato, max_ato);
}

ato = min_t(u32, ato, tcp_delack_max(sk));

/* Stay within the limit we were given */
timeout = jiffies + ato;

/* Use new timeout only if there wasn't a older one earlier. */
if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
  /* If delack timer is about to expire, send ACK now. */
  if (time_before_eq(icsk_delack_timeout(icsk), jiffies + (ato >> 2))) {
   tcp_send_ack(sk);
   return;
  }

  if (!time_before(timeout, icsk_delack_timeout(icsk)))
   timeout = icsk_delack_timeout(icsk);
}
smp_store_release(&icsk->icsk_ack.pending,
     icsk->icsk_ack.pending | ICSK_ACK_SCHED | ICSK_ACK_TIMER);
sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
}

/* This routine sends an ack and also updates the window. */
void __tcp_send_ack(struct sock *sk, u32 rcv_nxt, u16 flags)
{
struct sk_buff *buff;

/* If we have been reset, we may not send again. */
if (sk->sk_state == TCP_CLOSE)
  return;

/* We are not putting this on the write queue, so
* tcp_transmit_skb() will set the ownership to this
* sock.
*/
buff = alloc_skb(MAX_TCP_HEADER,
    sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
if (unlikely(!buff)) {
  struct inet_connection_sock *icsk = inet_csk(sk);
  unsigned long delay;

  delay = TCP_DELACK_MAX << icsk->icsk_ack.retry;
  if (delay < tcp_rto_max(sk))
   icsk->icsk_ack.retry++;
  inet_csk_schedule_ack(sk);
  icsk->icsk_ack.ato = TCP_ATO_MIN;
  tcp_reset_xmit_timer(sk, ICSK_TIME_DACK, delay, false);
  return;
}

/* Reserve space for headers and prepare control bits. */
skb_reserve(buff, MAX_TCP_HEADER);
tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK | flags);

/* We do not want pure acks influencing TCP Small Queues or fq/pacing
* too much.
* SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
*/
skb_set_tcp_pure_ack(buff);

/* Send it off, this clears delayed acks for us. */
__tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0, rcv_nxt);
}
EXPORT_SYMBOL_GPL(__tcp_send_ack);

void tcp_send_ack(struct sock *sk)
{
__tcp_send_ack(sk, tcp_sk(sk)->rcv_nxt, 0);
}

/* This routine sends a packet with an out of date sequence
* number. It assumes the other end will try to ack it.
*
* Question: what should we make while urgent mode?
* 4.4BSD forces sending single byte of data. We cannot send
* out of window data, because we have SND.NXT==SND.MAX...
*
* Current solution: to send TWO zero-length segments in urgent mode:
* one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
* out-of-date with SND.UNA-1 to probe window.
*/
static int tcp_xmit_probe_skb(struct sock *sk, int urgent, int mib)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;

/* We don't queue it, tcp_transmit_skb() sets ownership. */
skb = alloc_skb(MAX_TCP_HEADER,
   sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
if (!skb)
  return -1;

/* Reserve space for headers and set control bits. */
skb_reserve(skb, MAX_TCP_HEADER);
/* Use a previous sequence.  This should cause the other
* end to send an ack.  Don't queue or clone SKB, just
* send it.
*/
tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
NET_INC_STATS(sock_net(sk), mib);
return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
}

/* Called from setsockopt( ... TCP_REPAIR ) */
void tcp_send_window_probe(struct sock *sk)
{
if (sk->sk_state == TCP_ESTABLISHED) {
  tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
  tcp_mstamp_refresh(tcp_sk(sk));
  tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
}
}

/* Initiate keepalive or window probe from timer. */
int tcp_write_wakeup(struct sock *sk, int mib)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;

if (sk->sk_state == TCP_CLOSE)
  return -1;

skb = tcp_send_head(sk);
if (skb && before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
  int err;
  unsigned int mss = tcp_current_mss(sk);
  unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

  if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
   tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;

  /* We are probing the opening of a window
* but the window size is != 0
* must have been a result SWS avoidance ( sender )
*/
  if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
      skb->len > mss) {
   seg_size = min(seg_size, mss);
   TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
   if (tcp_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
      skb, seg_size, mss, GFP_ATOMIC))
    return -1;
  } else if (!tcp_skb_pcount(skb))
   tcp_set_skb_tso_segs(skb, mss);

  TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
  err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
  if (!err)
   tcp_event_new_data_sent(sk, skb);
  return err;
} else {
  if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
   tcp_xmit_probe_skb(sk, 1, mib);
  return tcp_xmit_probe_skb(sk, 0, mib);
}
}

/* A window probe timeout has occurred.  If window is not closed send
* a partial packet else a zero probe.
*/
void tcp_send_probe0(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct net *net = sock_net(sk);
unsigned long timeout;
int err;

err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);

if (tp->packets_out || tcp_write_queue_empty(sk)) {
  /* Cancel probe timer, if it is not required. */
  icsk->icsk_probes_out = 0;
  icsk->icsk_backoff = 0;
  icsk->icsk_probes_tstamp = 0;
  return;
}

icsk->icsk_probes_out++;
if (err <= 0) {
  if (icsk->icsk_backoff < READ_ONCE(net->ipv4.sysctl_tcp_retries2))
   icsk->icsk_backoff++;
  timeout = tcp_probe0_when(sk, tcp_rto_max(sk));
} else {
  /* If packet was not sent due to local congestion,
* Let senders fight for local resources conservatively.
*/
  timeout = TCP_RESOURCE_PROBE_INTERVAL;
}

timeout = tcp_clamp_probe0_to_user_timeout(sk, timeout);
tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, timeout, true);
}

int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
{
const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
struct flowi fl;
int res;

/* Paired with WRITE_ONCE() in sock_setsockopt() */
if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
  WRITE_ONCE(tcp_rsk(req)->txhash, net_tx_rndhash());
res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
      NULL);
if (!res) {
  TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
  if (unlikely(tcp_passive_fastopen(sk))) {
   /* sk has const attribute because listeners are lockless.
* However in this case, we are dealing with a passive fastopen
* socket thus we can change total_retrans value.
*/
   tcp_sk_rw(sk)->total_retrans++;
  }
  trace_tcp_retransmit_synack(sk, req);
  req->num_retrans++;
}
return res;
}
EXPORT_IPV6_MOD(tcp_rtx_synack);

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