Quelle eventpoll.c Sprache: C

// SPDX-License-Identifier: GPL-2.0-or-later
/*
*  fs/eventpoll.c (Efficient event retrieval implementation)
*  Copyright (C) 2001,...,2009 Davide Libenzi
*
*  Davide Libenzi <davidel@xmailserver.org>
*/

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/sched/signal.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/signal.h>
#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/string.h>
#include <linux/list.h>
#include <linux/hash.h>
#include <linux/spinlock.h>
#include <linux/syscalls.h>
#include <linux/rbtree.h>
#include <linux/wait.h>
#include <linux/eventpoll.h>
#include <linux/mount.h>
#include <linux/bitops.h>
#include <linux/mutex.h>
#include <linux/anon_inodes.h>
#include <linux/device.h>
#include <linux/uaccess.h>
#include <asm/io.h>
#include <asm/mman.h>
#include <linux/atomic.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/compat.h>
#include <linux/rculist.h>
#include <linux/capability.h>
#include <net/busy_poll.h>

/*
* LOCKING:
* There are three level of locking required by epoll :
*
* 1) epnested_mutex (mutex)
* 2) ep->mtx (mutex)
* 3) ep->lock (spinlock)
*
* The acquire order is the one listed above, from 1 to 3.
* We need a spinlock (ep->lock) because we manipulate objects
* from inside the poll callback, that might be triggered from
* a wake_up() that in turn might be called from IRQ context.
* So we can't sleep inside the poll callback and hence we need
* a spinlock. During the event transfer loop (from kernel to
* user space) we could end up sleeping due a copy_to_user(), so
* we need a lock that will allow us to sleep. This lock is a
* mutex (ep->mtx). It is acquired during the event transfer loop,
* during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
* The epnested_mutex is acquired when inserting an epoll fd onto another
* epoll fd. We do this so that we walk the epoll tree and ensure that this
* insertion does not create a cycle of epoll file descriptors, which
* could lead to deadlock. We need a global mutex to prevent two
* simultaneous inserts (A into B and B into A) from racing and
* constructing a cycle without either insert observing that it is
* going to.
* It is necessary to acquire multiple "ep->mtx"es at once in the
* case when one epoll fd is added to another. In this case, we
* always acquire the locks in the order of nesting (i.e. after
* epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
* before e2->mtx). Since we disallow cycles of epoll file
* descriptors, this ensures that the mutexes are well-ordered. In
* order to communicate this nesting to lockdep, when walking a tree
* of epoll file descriptors, we use the current recursion depth as
* the lockdep subkey.
* It is possible to drop the "ep->mtx" and to use the global
* mutex "epnested_mutex" (together with "ep->lock") to have it working,
* but having "ep->mtx" will make the interface more scalable.
* Events that require holding "epnested_mutex" are very rare, while for
* normal operations the epoll private "ep->mtx" will guarantee
* a better scalability.
*/

/* Epoll private bits inside the event mask */
#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | EPOLLEXCLUSIVE)

#define EPOLLINOUT_BITS (EPOLLIN | EPOLLOUT)

#define EPOLLEXCLUSIVE_OK_BITS (EPOLLINOUT_BITS | EPOLLERR | EPOLLHUP | \
    EPOLLWAKEUP | EPOLLET | EPOLLEXCLUSIVE)

/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4

#define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))

#define EP_UNACTIVE_PTR ((void *) -1L)

#define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))

struct epoll_filefd {
struct file *file;
int fd;
} __packed;

/* Wait structure used by the poll hooks */
struct eppoll_entry {
/* List header used to link this structure to the "struct epitem" */
struct eppoll_entry *next;

/* The "base" pointer is set to the container "struct epitem" */
struct epitem *base;

/*
* Wait queue item that will be linked to the target file wait
* queue head.
*/
wait_queue_entry_t wait;

/* The wait queue head that linked the "wait" wait queue item */
wait_queue_head_t *whead;
};

/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
* Avoid increasing the size of this struct, there can be many thousands
* of these on a server and we do not want this to take another cache line.
*/
struct epitem {
union {
  /* RB tree node links this structure to the eventpoll RB tree */
  struct rb_node rbn;
  /* Used to free the struct epitem */
  struct rcu_head rcu;
};

/* List header used to link this structure to the eventpoll ready list */
struct list_head rdllink;

/*
* Works together "struct eventpoll"->ovflist in keeping the
* single linked chain of items.
*/
struct epitem *next;

/* The file descriptor information this item refers to */
struct epoll_filefd ffd;

/*
* Protected by file->f_lock, true for to-be-released epitem already
* removed from the "struct file" items list; together with
* eventpoll->refcount orchestrates "struct eventpoll" disposal
*/
bool dying;

/* List containing poll wait queues */
struct eppoll_entry *pwqlist;

/* The "container" of this item */
struct eventpoll *ep;

/* List header used to link this item to the "struct file" items list */
struct hlist_node fllink;

/* wakeup_source used when EPOLLWAKEUP is set */
struct wakeup_source __rcu *ws;

/* The structure that describe the interested events and the source fd */
struct epoll_event event;
};

/*
* This structure is stored inside the "private_data" member of the file
* structure and represents the main data structure for the eventpoll
* interface.
*/
struct eventpoll {
/*
* This mutex is used to ensure that files are not removed
* while epoll is using them. This is held during the event
* collection loop, the file cleanup path, the epoll file exit
* code and the ctl operations.
*/
struct mutex mtx;

/* Wait queue used by sys_epoll_wait() */
wait_queue_head_t wq;

/* Wait queue used by file->poll() */
wait_queue_head_t poll_wait;

/* List of ready file descriptors */
struct list_head rdllist;

/* Lock which protects rdllist and ovflist */
spinlock_t lock;

/* RB tree root used to store monitored fd structs */
struct rb_root_cached rbr;

/*
* This is a single linked list that chains all the "struct epitem" that
* happened while transferring ready events to userspace w/out
* holding ->lock.
*/
struct epitem *ovflist;

/* wakeup_source used when ep_send_events or __ep_eventpoll_poll is running */
struct wakeup_source *ws;

/* The user that created the eventpoll descriptor */
struct user_struct *user;

struct file *file;

/* used to optimize loop detection check */
u64 gen;
struct hlist_head refs;
u8 loop_check_depth;

/*
* usage count, used together with epitem->dying to
* orchestrate the disposal of this struct
*/
refcount_t refcount;

#ifdef CONFIG_NET_RX_BUSY_POLL
/* used to track busy poll napi_id */
unsigned int napi_id;
/* busy poll timeout */
u32 busy_poll_usecs;
/* busy poll packet budget */
u16 busy_poll_budget;
bool prefer_busy_poll;
#endif

#ifdef CONFIG_DEBUG_LOCK_ALLOC
/* tracks wakeup nests for lockdep validation */
u8 nests;
#endif
};

/* Wrapper struct used by poll queueing */
struct ep_pqueue {
poll_table pt;
struct epitem *epi;
};

/*
* Configuration options available inside /proc/sys/fs/epoll/
*/
/* Maximum number of epoll watched descriptors, per user */
static long max_user_watches __read_mostly;

/* Used for cycles detection */
static DEFINE_MUTEX(epnested_mutex);

static u64 loop_check_gen = 0;

/* Used to check for epoll file descriptor inclusion loops */
static struct eventpoll *inserting_into;

/* Slab cache used to allocate "struct epitem" */
static struct kmem_cache *epi_cache __ro_after_init;

/* Slab cache used to allocate "struct eppoll_entry" */
static struct kmem_cache *pwq_cache __ro_after_init;

/*
* List of files with newly added links, where we may need to limit the number
* of emanating paths. Protected by the epnested_mutex.
*/
struct epitems_head {
struct hlist_head epitems;
struct epitems_head *next;
};
static struct epitems_head *tfile_check_list = EP_UNACTIVE_PTR;

static struct kmem_cache *ephead_cache __ro_after_init;

static inline void free_ephead(struct epitems_head *head)
{
if (head)
  kmem_cache_free(ephead_cache, head);
}

static void list_file(struct file *file)
{
struct epitems_head *head;

head = container_of(file->f_ep, struct epitems_head, epitems);
if (!head->next) {
  head->next = tfile_check_list;
  tfile_check_list = head;
}
}

static void unlist_file(struct epitems_head *head)
{
struct epitems_head *to_free = head;
struct hlist_node *p = rcu_dereference(hlist_first_rcu(&head->epitems));
if (p) {
  struct epitem *epi= container_of(p, struct epitem, fllink);
  spin_lock(&epi->ffd.file->f_lock);
  if (!hlist_empty(&head->epitems))
   to_free = NULL;
  head->next = NULL;
  spin_unlock(&epi->ffd.file->f_lock);
}
free_ephead(to_free);
}

#ifdef CONFIG_SYSCTL

#include <linux/sysctl.h>

static long long_zero;
static long long_max = LONG_MAX;

static const struct ctl_table epoll_table[] = {
{
  .procname = "max_user_watches",
  .data  = &max_user_watches,
  .maxlen  = sizeof(max_user_watches),
  .mode  = 0644,
  .proc_handler = proc_doulongvec_minmax,
  .extra1  = &long_zero,
  .extra2  = &long_max,
},
};

static void __init epoll_sysctls_init(void)
{
register_sysctl("fs/epoll", epoll_table);
}
#else
#define epoll_sysctls_init() do { } while (0)
#endif /* CONFIG_SYSCTL */

static const struct file_operations eventpoll_fops;

static inline int is_file_epoll(struct file *f)
{
return f->f_op == &eventpoll_fops;
}

/* Setup the structure that is used as key for the RB tree */
static inline void ep_set_ffd(struct epoll_filefd *ffd,
         struct file *file, int fd)
{
ffd->file = file;
ffd->fd = fd;
}

/* Compare RB tree keys */
static inline int ep_cmp_ffd(struct epoll_filefd *p1,
        struct epoll_filefd *p2)
{
return (p1->file > p2->file ? +1:
         (p1->file < p2->file ? -1 : p1->fd - p2->fd));
}

/* Tells us if the item is currently linked */
static inline int ep_is_linked(struct epitem *epi)
{
return !list_empty(&epi->rdllink);
}

static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_entry_t *p)
{
return container_of(p, struct eppoll_entry, wait);
}

/* Get the "struct epitem" from a wait queue pointer */
static inline struct epitem *ep_item_from_wait(wait_queue_entry_t *p)
{
return container_of(p, struct eppoll_entry, wait)->base;
}

/**
* ep_events_available - Checks if ready events might be available.
*
* @ep: Pointer to the eventpoll context.
*
* Return: a value different than %zero if ready events are available,
*          or %zero otherwise.
*/
static inline int ep_events_available(struct eventpoll *ep)
{
return !list_empty_careful(&ep->rdllist) ||
  READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
}

#ifdef CONFIG_NET_RX_BUSY_POLL
/**
* busy_loop_ep_timeout - check if busy poll has timed out. The timeout value
* from the epoll instance ep is preferred, but if it is not set fallback to
* the system-wide global via busy_loop_timeout.
*
* @start_time: The start time used to compute the remaining time until timeout.
* @ep: Pointer to the eventpoll context.
*
* Return: true if the timeout has expired, false otherwise.
*/
static bool busy_loop_ep_timeout(unsigned long start_time,
     struct eventpoll *ep)
{
unsigned long bp_usec = READ_ONCE(ep->busy_poll_usecs);

if (bp_usec) {
  unsigned long end_time = start_time + bp_usec;
  unsigned long now = busy_loop_current_time();

  return time_after(now, end_time);
} else {
  return busy_loop_timeout(start_time);
}
}

static bool ep_busy_loop_on(struct eventpoll *ep)
{
return !!READ_ONCE(ep->busy_poll_usecs) ||
        READ_ONCE(ep->prefer_busy_poll) ||
        net_busy_loop_on();
}

static bool ep_busy_loop_end(void *p, unsigned long start_time)
{
struct eventpoll *ep = p;

return ep_events_available(ep) || busy_loop_ep_timeout(start_time, ep);
}

/*
* Busy poll if globally on and supporting sockets found && no events,
* busy loop will return if need_resched or ep_events_available.
*
* we must do our busy polling with irqs enabled
*/
static bool ep_busy_loop(struct eventpoll *ep)
{
unsigned int napi_id = READ_ONCE(ep->napi_id);
u16 budget = READ_ONCE(ep->busy_poll_budget);
bool prefer_busy_poll = READ_ONCE(ep->prefer_busy_poll);

if (!budget)
  budget = BUSY_POLL_BUDGET;

if (napi_id_valid(napi_id) && ep_busy_loop_on(ep)) {
  napi_busy_loop(napi_id, ep_busy_loop_end,
          ep, prefer_busy_poll, budget);
  if (ep_events_available(ep))
   return true;
  /*
* Busy poll timed out.  Drop NAPI ID for now, we can add
* it back in when we have moved a socket with a valid NAPI
* ID onto the ready list.
*/
  if (prefer_busy_poll)
   napi_resume_irqs(napi_id);
  ep->napi_id = 0;
  return false;
}
return false;
}

/*
* Set epoll busy poll NAPI ID from sk.
*/
static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
{
struct eventpoll *ep = epi->ep;
unsigned int napi_id;
struct socket *sock;
struct sock *sk;

if (!ep_busy_loop_on(ep))
  return;

sock = sock_from_file(epi->ffd.file);
if (!sock)
  return;

sk = sock->sk;
if (!sk)
  return;

napi_id = READ_ONCE(sk->sk_napi_id);

/* Non-NAPI IDs can be rejected
* or
* Nothing to do if we already have this ID
*/
if (!napi_id_valid(napi_id) || napi_id == ep->napi_id)
  return;

/* record NAPI ID for use in next busy poll */
ep->napi_id = napi_id;
}

static long ep_eventpoll_bp_ioctl(struct file *file, unsigned int cmd,
      unsigned long arg)
{
struct eventpoll *ep = file->private_data;
void __user *uarg = (void __user *)arg;
struct epoll_params epoll_params;

switch (cmd) {
case EPIOCSPARAMS:
  if (copy_from_user(&epoll_params, uarg, sizeof(epoll_params)))
   return -EFAULT;

  /* pad byte must be zero */
  if (epoll_params.__pad)
   return -EINVAL;

  if (epoll_params.busy_poll_usecs > S32_MAX)
   return -EINVAL;

  if (epoll_params.prefer_busy_poll > 1)
   return -EINVAL;

  if (epoll_params.busy_poll_budget > NAPI_POLL_WEIGHT &&
      !capable(CAP_NET_ADMIN))
   return -EPERM;

  WRITE_ONCE(ep->busy_poll_usecs, epoll_params.busy_poll_usecs);
  WRITE_ONCE(ep->busy_poll_budget, epoll_params.busy_poll_budget);
  WRITE_ONCE(ep->prefer_busy_poll, epoll_params.prefer_busy_poll);
  return 0;
case EPIOCGPARAMS:
  memset(&epoll_params, 0, sizeof(epoll_params));
  epoll_params.busy_poll_usecs = READ_ONCE(ep->busy_poll_usecs);
  epoll_params.busy_poll_budget = READ_ONCE(ep->busy_poll_budget);
  epoll_params.prefer_busy_poll = READ_ONCE(ep->prefer_busy_poll);
  if (copy_to_user(uarg, &epoll_params, sizeof(epoll_params)))
   return -EFAULT;
  return 0;
default:
  return -ENOIOCTLCMD;
}
}

static void ep_suspend_napi_irqs(struct eventpoll *ep)
{
unsigned int napi_id = READ_ONCE(ep->napi_id);

if (napi_id_valid(napi_id) && READ_ONCE(ep->prefer_busy_poll))
  napi_suspend_irqs(napi_id);
}

static void ep_resume_napi_irqs(struct eventpoll *ep)
{
unsigned int napi_id = READ_ONCE(ep->napi_id);

if (napi_id_valid(napi_id) && READ_ONCE(ep->prefer_busy_poll))
  napi_resume_irqs(napi_id);
}

#else

static inline bool ep_busy_loop(struct eventpoll *ep)
{
return false;
}

static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
{
}

static long ep_eventpoll_bp_ioctl(struct file *file, unsigned int cmd,
      unsigned long arg)
{
return -EOPNOTSUPP;
}

static void ep_suspend_napi_irqs(struct eventpoll *ep)
{
}

static void ep_resume_napi_irqs(struct eventpoll *ep)
{
}

#endif /* CONFIG_NET_RX_BUSY_POLL */

/*
* As described in commit 0ccf831cb lockdep: annotate epoll
* the use of wait queues used by epoll is done in a very controlled
* manner. Wake ups can nest inside each other, but are never done
* with the same locking. For example:
*
*   dfd = socket(...);
*   efd1 = epoll_create();
*   efd2 = epoll_create();
*   epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
*   epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
*
* When a packet arrives to the device underneath "dfd", the net code will
* issue a wake_up() on its poll wake list. Epoll (efd1) has installed a
* callback wakeup entry on that queue, and the wake_up() performed by the
* "dfd" net code will end up in ep_poll_callback(). At this point epoll
* (efd1) notices that it may have some event ready, so it needs to wake up
* the waiters on its poll wait list (efd2). So it calls ep_poll_safewake()
* that ends up in another wake_up(), after having checked about the
* recursion constraints. That are, no more than EP_MAX_NESTS, to avoid
* stack blasting.
*
* When CONFIG_DEBUG_LOCK_ALLOC is enabled, make sure lockdep can handle
* this special case of epoll.
*/
#ifdef CONFIG_DEBUG_LOCK_ALLOC

static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
        unsigned pollflags)
{
struct eventpoll *ep_src;
unsigned long flags;
u8 nests = 0;

/*
* To set the subclass or nesting level for spin_lock_irqsave_nested()
* it might be natural to create a per-cpu nest count. However, since
* we can recurse on ep->poll_wait.lock, and a non-raw spinlock can
* schedule() in the -rt kernel, the per-cpu variable are no longer
* protected. Thus, we are introducing a per eventpoll nest field.
* If we are not being call from ep_poll_callback(), epi is NULL and
* we are at the first level of nesting, 0. Otherwise, we are being
* called from ep_poll_callback() and if a previous wakeup source is
* not an epoll file itself, we are at depth 1 since the wakeup source
* is depth 0. If the wakeup source is a previous epoll file in the
* wakeup chain then we use its nests value and record ours as
* nests + 1. The previous epoll file nests value is stable since its
* already holding its own poll_wait.lock.
*/
if (epi) {
  if ((is_file_epoll(epi->ffd.file))) {
   ep_src = epi->ffd.file->private_data;
   nests = ep_src->nests;
  } else {
   nests = 1;
  }
}
spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);
ep->nests = nests + 1;
wake_up_locked_poll(&ep->poll_wait, EPOLLIN | pollflags);
ep->nests = 0;
spin_unlock_irqrestore(&ep->poll_wait.lock, flags);
}

#else

static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
        __poll_t pollflags)
{
wake_up_poll(&ep->poll_wait, EPOLLIN | pollflags);
}

#endif

static void ep_remove_wait_queue(struct eppoll_entry *pwq)
{
wait_queue_head_t *whead;

rcu_read_lock();
/*
* If it is cleared by POLLFREE, it should be rcu-safe.
* If we read NULL we need a barrier paired with
* smp_store_release() in ep_poll_callback(), otherwise
* we rely on whead->lock.
*/
whead = smp_load_acquire(&pwq->whead);
if (whead)
  remove_wait_queue(whead, &pwq->wait);
rcu_read_unlock();
}

/*
* This function unregisters poll callbacks from the associated file
* descriptor.  Must be called with "mtx" held.
*/
static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
{
struct eppoll_entry **p = &epi->pwqlist;
struct eppoll_entry *pwq;

while ((pwq = *p) != NULL) {
  *p = pwq->next;
  ep_remove_wait_queue(pwq);
  kmem_cache_free(pwq_cache, pwq);
}
}

/* call only when ep->mtx is held */
static inline struct wakeup_source *ep_wakeup_source(struct epitem *epi)
{
return rcu_dereference_check(epi->ws, lockdep_is_held(&epi->ep->mtx));
}

/* call only when ep->mtx is held */
static inline void ep_pm_stay_awake(struct epitem *epi)
{
struct wakeup_source *ws = ep_wakeup_source(epi);

if (ws)
  __pm_stay_awake(ws);
}

static inline bool ep_has_wakeup_source(struct epitem *epi)
{
return rcu_access_pointer(epi->ws) ? true : false;
}

/* call when ep->mtx cannot be held (ep_poll_callback) */
static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
{
struct wakeup_source *ws;

rcu_read_lock();
ws = rcu_dereference(epi->ws);
if (ws)
  __pm_stay_awake(ws);
rcu_read_unlock();
}

/*
* ep->mutex needs to be held because we could be hit by
* eventpoll_release_file() and epoll_ctl().
*/
static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist)
{
/*
* Steal the ready list, and re-init the original one to the
* empty list. Also, set ep->ovflist to NULL so that events
* happening while looping w/out locks, are not lost. We cannot
* have the poll callback to queue directly on ep->rdllist,
* because we want the "sproc" callback to be able to do it
* in a lockless way.
*/
lockdep_assert_irqs_enabled();
spin_lock_irq(&ep->lock);
list_splice_init(&ep->rdllist, txlist);
WRITE_ONCE(ep->ovflist, NULL);
spin_unlock_irq(&ep->lock);
}

static void ep_done_scan(struct eventpoll *ep,
    struct list_head *txlist)
{
struct epitem *epi, *nepi;

spin_lock_irq(&ep->lock);
/*
* During the time we spent inside the "sproc" callback, some
* other events might have been queued by the poll callback.
* We re-insert them inside the main ready-list here.
*/
for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;
      nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
  /*
* We need to check if the item is already in the list.
* During the "sproc" callback execution time, items are
* queued into ->ovflist but the "txlist" might already
* contain them, and the list_splice() below takes care of them.
*/
  if (!ep_is_linked(epi)) {
   /*
* ->ovflist is LIFO, so we have to reverse it in order
* to keep in FIFO.
*/
   list_add(&epi->rdllink, &ep->rdllist);
   ep_pm_stay_awake(epi);
  }
}
/*
* We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
* releasing the lock, events will be queued in the normal way inside
* ep->rdllist.
*/
WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);

/*
* Quickly re-inject items left on "txlist".
*/
list_splice(txlist, &ep->rdllist);
__pm_relax(ep->ws);

if (!list_empty(&ep->rdllist)) {
  if (waitqueue_active(&ep->wq))
   wake_up(&ep->wq);
}

spin_unlock_irq(&ep->lock);
}

static void ep_get(struct eventpoll *ep)
{
refcount_inc(&ep->refcount);
}

/*
* Returns true if the event poll can be disposed
*/
static bool ep_refcount_dec_and_test(struct eventpoll *ep)
{
if (!refcount_dec_and_test(&ep->refcount))
  return false;

WARN_ON_ONCE(!RB_EMPTY_ROOT(&ep->rbr.rb_root));
return true;
}

static void ep_free(struct eventpoll *ep)
{
ep_resume_napi_irqs(ep);
mutex_destroy(&ep->mtx);
free_uid(ep->user);
wakeup_source_unregister(ep->ws);
kfree(ep);
}

/*
* Removes a "struct epitem" from the eventpoll RB tree and deallocates
* all the associated resources. Must be called with "mtx" held.
* If the dying flag is set, do the removal only if force is true.
* This prevents ep_clear_and_put() from dropping all the ep references
* while running concurrently with eventpoll_release_file().
* Returns true if the eventpoll can be disposed.
*/
static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
{
struct file *file = epi->ffd.file;
struct epitems_head *to_free;
struct hlist_head *head;

lockdep_assert_irqs_enabled();

/*
* Removes poll wait queue hooks.
*/
ep_unregister_pollwait(ep, epi);

/* Remove the current item from the list of epoll hooks */
spin_lock(&file->f_lock);
if (epi->dying && !force) {
  spin_unlock(&file->f_lock);
  return false;
}

to_free = NULL;
head = file->f_ep;
if (head->first == &epi->fllink && !epi->fllink.next) {
  /* See eventpoll_release() for details. */
  WRITE_ONCE(file->f_ep, NULL);
  if (!is_file_epoll(file)) {
   struct epitems_head *v;
   v = container_of(head, struct epitems_head, epitems);
   if (!smp_load_acquire(&v->next))
    to_free = v;
  }
}
hlist_del_rcu(&epi->fllink);
spin_unlock(&file->f_lock);
free_ephead(to_free);

rb_erase_cached(&epi->rbn, &ep->rbr);

spin_lock_irq(&ep->lock);
if (ep_is_linked(epi))
  list_del_init(&epi->rdllink);
spin_unlock_irq(&ep->lock);

wakeup_source_unregister(ep_wakeup_source(epi));
/*
* At this point it is safe to free the eventpoll item. Use the union
* field epi->rcu, since we are trying to minimize the size of
* 'struct epitem'. The 'rbn' field is no longer in use. Protected by
* ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
* use of the rbn field.
*/
kfree_rcu(epi, rcu);

percpu_counter_dec(&ep->user->epoll_watches);
return true;
}

/*
* ep_remove variant for callers owing an additional reference to the ep
*/
static void ep_remove_safe(struct eventpoll *ep, struct epitem *epi)
{
if (__ep_remove(ep, epi, false))
  WARN_ON_ONCE(ep_refcount_dec_and_test(ep));
}

static void ep_clear_and_put(struct eventpoll *ep)
{
struct rb_node *rbp, *next;
struct epitem *epi;

/* We need to release all tasks waiting for these file */
if (waitqueue_active(&ep->poll_wait))
  ep_poll_safewake(ep, NULL, 0);

mutex_lock(&ep->mtx);

/*
* Walks through the whole tree by unregistering poll callbacks.
*/
for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  epi = rb_entry(rbp, struct epitem, rbn);

  ep_unregister_pollwait(ep, epi);
  cond_resched();
}

/*
* Walks through the whole tree and try to free each "struct epitem".
* Note that ep_remove_safe() will not remove the epitem in case of a
* racing eventpoll_release_file(); the latter will do the removal.
* At this point we are sure no poll callbacks will be lingering around.
* Since we still own a reference to the eventpoll struct, the loop can't
* dispose it.
*/
for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = next) {
  next = rb_next(rbp);
  epi = rb_entry(rbp, struct epitem, rbn);
  ep_remove_safe(ep, epi);
  cond_resched();
}

mutex_unlock(&ep->mtx);
if (ep_refcount_dec_and_test(ep))
  ep_free(ep);
}

static long ep_eventpoll_ioctl(struct file *file, unsigned int cmd,
          unsigned long arg)
{
int ret;

if (!is_file_epoll(file))
  return -EINVAL;

switch (cmd) {
case EPIOCSPARAMS:
case EPIOCGPARAMS:
  ret = ep_eventpoll_bp_ioctl(file, cmd, arg);
  break;
default:
  ret = -EINVAL;
  break;
}

return ret;
}

static int ep_eventpoll_release(struct inode *inode, struct file *file)
{
struct eventpoll *ep = file->private_data;

if (ep)
  ep_clear_and_put(ep);

return 0;
}

static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt, int depth);

static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int depth)
{
struct eventpoll *ep = file->private_data;
LIST_HEAD(txlist);
struct epitem *epi, *tmp;
poll_table pt;
__poll_t res = 0;

init_poll_funcptr(&pt, NULL);

/* Insert inside our poll wait queue */
poll_wait(file, &ep->poll_wait, wait);

/*
* Proceed to find out if wanted events are really available inside
* the ready list.
*/
mutex_lock_nested(&ep->mtx, depth);
ep_start_scan(ep, &txlist);
list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
  if (ep_item_poll(epi, &pt, depth + 1)) {
   res = EPOLLIN | EPOLLRDNORM;
   break;
  } else {
   /*
* Item has been dropped into the ready list by the poll
* callback, but it's not actually ready, as far as
* caller requested events goes. We can remove it here.
*/
   __pm_relax(ep_wakeup_source(epi));
   list_del_init(&epi->rdllink);
  }
}
ep_done_scan(ep, &txlist);
mutex_unlock(&ep->mtx);
return res;
}

/*
* The ffd.file pointer may be in the process of being torn down due to
* being closed, but we may not have finished eventpoll_release() yet.
*
* Normally, even with the atomic_long_inc_not_zero, the file may have
* been free'd and then gotten re-allocated to something else (since
* files are not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU).
*
* But for epoll, users hold the ep->mtx mutex, and as such any file in
* the process of being free'd will block in eventpoll_release_file()
* and thus the underlying file allocation will not be free'd, and the
* file re-use cannot happen.
*
* For the same reason we can avoid a rcu_read_lock() around the
* operation - 'ffd.file' cannot go away even if the refcount has
* reached zero (but we must still not call out to ->poll() functions
* etc).
*/
static struct file *epi_fget(const struct epitem *epi)
{
struct file *file;

file = epi->ffd.file;
if (!file_ref_get(&file->f_ref))
  file = NULL;
return file;
}

/*
* Differs from ep_eventpoll_poll() in that internal callers already have
* the ep->mtx so we need to start from depth=1, such that mutex_lock_nested()
* is correctly annotated.
*/
static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
     int depth)
{
struct file *file = epi_fget(epi);
__poll_t res;

/*
* We could return EPOLLERR | EPOLLHUP or something, but let's
* treat this more as "file doesn't exist, poll didn't happen".
*/
if (!file)
  return 0;

pt->_key = epi->event.events;
if (!is_file_epoll(file))
  res = vfs_poll(file, pt);
else
  res = __ep_eventpoll_poll(file, pt, depth);
fput(file);
return res & epi->event.events;
}

static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
{
return __ep_eventpoll_poll(file, wait, 0);
}

#ifdef CONFIG_PROC_FS
static void ep_show_fdinfo(struct seq_file *m, struct file *f)
{
struct eventpoll *ep = f->private_data;
struct rb_node *rbp;

mutex_lock(&ep->mtx);
for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  struct epitem *epi = rb_entry(rbp, struct epitem, rbn);
  struct inode *inode = file_inode(epi->ffd.file);

  seq_printf(m, "tfd: %8d events: %8x data: %16llx "
      " pos:%lli ino:%lx sdev:%x\n",
      epi->ffd.fd, epi->event.events,
      (long long)epi->event.data,
      (long long)epi->ffd.file->f_pos,
      inode->i_ino, inode->i_sb->s_dev);
  if (seq_has_overflowed(m))
   break;
}
mutex_unlock(&ep->mtx);
}
#endif

/* File callbacks that implement the eventpoll file behaviour */
static const struct file_operations eventpoll_fops = {
#ifdef CONFIG_PROC_FS
.show_fdinfo = ep_show_fdinfo,
#endif
.release = ep_eventpoll_release,
.poll  = ep_eventpoll_poll,
.llseek  = noop_llseek,
.unlocked_ioctl = ep_eventpoll_ioctl,
.compat_ioctl   = compat_ptr_ioctl,
};

/*
* This is called from eventpoll_release() to unlink files from the eventpoll
* interface. We need to have this facility to cleanup correctly files that are
* closed without being removed from the eventpoll interface.
*/
void eventpoll_release_file(struct file *file)
{
struct eventpoll *ep;
struct epitem *epi;
bool dispose;

/*
* Use the 'dying' flag to prevent a concurrent ep_clear_and_put() from
* touching the epitems list before eventpoll_release_file() can access
* the ep->mtx.
*/
again:
spin_lock(&file->f_lock);
if (file->f_ep && file->f_ep->first) {
  epi = hlist_entry(file->f_ep->first, struct epitem, fllink);
  epi->dying = true;
  spin_unlock(&file->f_lock);

  /*
* ep access is safe as we still own a reference to the ep
* struct
*/
  ep = epi->ep;
  mutex_lock(&ep->mtx);
  dispose = __ep_remove(ep, epi, true);
  mutex_unlock(&ep->mtx);

  if (dispose && ep_refcount_dec_and_test(ep))
   ep_free(ep);
  goto again;
}
spin_unlock(&file->f_lock);
}

static int ep_alloc(struct eventpoll **pep)
{
struct eventpoll *ep;

ep = kzalloc(sizeof(*ep), GFP_KERNEL);
if (unlikely(!ep))
  return -ENOMEM;

mutex_init(&ep->mtx);
spin_lock_init(&ep->lock);
init_waitqueue_head(&ep->wq);
init_waitqueue_head(&ep->poll_wait);
INIT_LIST_HEAD(&ep->rdllist);
ep->rbr = RB_ROOT_CACHED;
ep->ovflist = EP_UNACTIVE_PTR;
ep->user = get_current_user();
refcount_set(&ep->refcount, 1);

*pep = ep;

return 0;
}

/*
* Search the file inside the eventpoll tree. The RB tree operations
* are protected by the "mtx" mutex, and ep_find() must be called with
* "mtx" held.
*/
static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
{
int kcmp;
struct rb_node *rbp;
struct epitem *epi, *epir = NULL;
struct epoll_filefd ffd;

ep_set_ffd(&ffd, file, fd);
for (rbp = ep->rbr.rb_root.rb_node; rbp; ) {
  epi = rb_entry(rbp, struct epitem, rbn);
  kcmp = ep_cmp_ffd(&ffd, &epi->ffd);
  if (kcmp > 0)
   rbp = rbp->rb_right;
  else if (kcmp < 0)
   rbp = rbp->rb_left;
  else {
   epir = epi;
   break;
  }
}

return epir;
}

#ifdef CONFIG_KCMP
static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long toff)
{
struct rb_node *rbp;
struct epitem *epi;

for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  epi = rb_entry(rbp, struct epitem, rbn);
  if (epi->ffd.fd == tfd) {
   if (toff == 0)
    return epi;
   else
    toff--;
  }
  cond_resched();
}

return NULL;
}

struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
         unsigned long toff)
{
struct file *file_raw;
struct eventpoll *ep;
struct epitem *epi;

if (!is_file_epoll(file))
  return ERR_PTR(-EINVAL);

ep = file->private_data;

mutex_lock(&ep->mtx);
epi = ep_find_tfd(ep, tfd, toff);
if (epi)
  file_raw = epi->ffd.file;
else
  file_raw = ERR_PTR(-ENOENT);
mutex_unlock(&ep->mtx);

return file_raw;
}
#endif /* CONFIG_KCMP */

/*
* This is the callback that is passed to the wait queue wakeup
* mechanism. It is called by the stored file descriptors when they
* have events to report.
*/
static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
{
int pwake = 0;
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi->ep;
__poll_t pollflags = key_to_poll(key);
unsigned long flags;
int ewake = 0;

spin_lock_irqsave(&ep->lock, flags);

ep_set_busy_poll_napi_id(epi);

/*
* If the event mask does not contain any poll(2) event, we consider the
* descriptor to be disabled. This condition is likely the effect of the
* EPOLLONESHOT bit that disables the descriptor when an event is received,
* until the next EPOLL_CTL_MOD will be issued.
*/
if (!(epi->event.events & ~EP_PRIVATE_BITS))
  goto out_unlock;

/*
* Check the events coming with the callback. At this stage, not
* every device reports the events in the "key" parameter of the
* callback. We need to be able to handle both cases here, hence the
* test for "key" != NULL before the event match test.
*/
if (pollflags && !(pollflags & epi->event.events))
  goto out_unlock;

/*
* If we are transferring events to userspace, we can hold no locks
* (because we're accessing user memory, and because of linux f_op->poll()
* semantics). All the events that happen during that period of time are
* chained in ep->ovflist and requeued later on.
*/
if (READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR) {
  if (epi->next == EP_UNACTIVE_PTR) {
   epi->next = READ_ONCE(ep->ovflist);
   WRITE_ONCE(ep->ovflist, epi);
   ep_pm_stay_awake_rcu(epi);
  }
} else if (!ep_is_linked(epi)) {
  /* In the usual case, add event to ready list. */
  list_add_tail(&epi->rdllink, &ep->rdllist);
  ep_pm_stay_awake_rcu(epi);
}

/*
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
* wait list.
*/
if (waitqueue_active(&ep->wq)) {
  if ((epi->event.events & EPOLLEXCLUSIVE) &&
     !(pollflags & POLLFREE)) {
   switch (pollflags & EPOLLINOUT_BITS) {
   case EPOLLIN:
    if (epi->event.events & EPOLLIN)
     ewake = 1;
    break;
   case EPOLLOUT:
    if (epi->event.events & EPOLLOUT)
     ewake = 1;
    break;
   case 0:
    ewake = 1;
    break;
   }
  }
  if (sync)
   wake_up_sync(&ep->wq);
  else
   wake_up(&ep->wq);
}
if (waitqueue_active(&ep->poll_wait))
  pwake++;

out_unlock:
spin_unlock_irqrestore(&ep->lock, flags);

/* We have to call this outside the lock */
if (pwake)
  ep_poll_safewake(ep, epi, pollflags & EPOLL_URING_WAKE);

if (!(epi->event.events & EPOLLEXCLUSIVE))
  ewake = 1;

if (pollflags & POLLFREE) {
  /*
* If we race with ep_remove_wait_queue() it can miss
* ->whead = NULL and do another remove_wait_queue() after
* us, so we can't use __remove_wait_queue().
*/
  list_del_init(&wait->entry);
  /*
* ->whead != NULL protects us from the race with
* ep_clear_and_put() or ep_remove(), ep_remove_wait_queue()
* takes whead->lock held by the caller. Once we nullify it,
* nothing protects ep/epi or even wait.
*/
  smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);
}

return ewake;
}

/*
* This is the callback that is used to add our wait queue to the
* target file wakeup lists.
*/
static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
     poll_table *pt)
{
struct ep_pqueue *epq = container_of(pt, struct ep_pqueue, pt);
struct epitem *epi = epq->epi;
struct eppoll_entry *pwq;

if (unlikely(!epi)) // an earlier allocation has failed
  return;

pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL);
if (unlikely(!pwq)) {
  epq->epi = NULL;
  return;
}

init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
pwq->whead = whead;
pwq->base = epi;
if (epi->event.events & EPOLLEXCLUSIVE)
  add_wait_queue_exclusive(whead, &pwq->wait);
else
  add_wait_queue(whead, &pwq->wait);
pwq->next = epi->pwqlist;
epi->pwqlist = pwq;
}

static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
{
int kcmp;
struct rb_node **p = &ep->rbr.rb_root.rb_node, *parent = NULL;
struct epitem *epic;
bool leftmost = true;

while (*p) {
  parent = *p;
  epic = rb_entry(parent, struct epitem, rbn);
  kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);
  if (kcmp > 0) {
   p = &parent->rb_right;
   leftmost = false;
  } else
   p = &parent->rb_left;
}
rb_link_node(&epi->rbn, parent, p);
rb_insert_color_cached(&epi->rbn, &ep->rbr, leftmost);
}

#define PATH_ARR_SIZE 5
/*
* These are the number paths of length 1 to 5, that we are allowing to emanate
* from a single file of interest. For example, we allow 1000 paths of length
* 1, to emanate from each file of interest. This essentially represents the
* potential wakeup paths, which need to be limited in order to avoid massive
* uncontrolled wakeup storms. The common use case should be a single ep which
* is connected to n file sources. In this case each file source has 1 path
* of length 1. Thus, the numbers below should be more than sufficient. These
* path limits are enforced during an EPOLL_CTL_ADD operation, since a modify
* and delete can't add additional paths. Protected by the epnested_mutex.
*/
static const int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };
static int path_count[PATH_ARR_SIZE];

static int path_count_inc(int nests)
{
/* Allow an arbitrary number of depth 1 paths */
if (nests == 0)
  return 0;

if (++path_count[nests] > path_limits[nests])
  return -1;
return 0;
}

static void path_count_init(void)
{
int i;

for (i = 0; i < PATH_ARR_SIZE; i++)
  path_count[i] = 0;
}

static int reverse_path_check_proc(struct hlist_head *refs, int depth)
{
int error = 0;
struct epitem *epi;

if (depth > EP_MAX_NESTS) /* too deep nesting */
  return -1;

/* CTL_DEL can remove links here, but that can't increase our count */
hlist_for_each_entry_rcu(epi, refs, fllink) {
  struct hlist_head *refs = &epi->ep->refs;
  if (hlist_empty(refs))
   error = path_count_inc(depth);
  else
   error = reverse_path_check_proc(refs, depth + 1);
  if (error != 0)
   break;
}
return error;
}

/**
* reverse_path_check - The tfile_check_list is list of epitem_head, which have
*                      links that are proposed to be newly added. We need to
*                      make sure that those added links don't add too many
*                      paths such that we will spend all our time waking up
*                      eventpoll objects.
*
* Return: %zero if the proposed links don't create too many paths,
*     %-1 otherwise.
*/
static int reverse_path_check(void)
{
struct epitems_head *p;

for (p = tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {
  int error;
  path_count_init();
  rcu_read_lock();
  error = reverse_path_check_proc(&p->epitems, 0);
  rcu_read_unlock();
  if (error)
   return error;
}
return 0;
}

static int ep_create_wakeup_source(struct epitem *epi)
{
struct name_snapshot n;
struct wakeup_source *ws;

if (!epi->ep->ws) {
  epi->ep->ws = wakeup_source_register(NULL, "eventpoll");
  if (!epi->ep->ws)
   return -ENOMEM;
}

take_dentry_name_snapshot(&n, epi->ffd.file->f_path.dentry);
ws = wakeup_source_register(NULL, n.name.name);
release_dentry_name_snapshot(&n);

if (!ws)
  return -ENOMEM;
rcu_assign_pointer(epi->ws, ws);

return 0;
}

/* rare code path, only used when EPOLL_CTL_MOD removes a wakeup source */
static noinline void ep_destroy_wakeup_source(struct epitem *epi)
{
struct wakeup_source *ws = ep_wakeup_source(epi);

RCU_INIT_POINTER(epi->ws, NULL);

/*
* wait for ep_pm_stay_awake_rcu to finish, synchronize_rcu is
* used internally by wakeup_source_remove, too (called by
* wakeup_source_unregister), so we cannot use call_rcu
*/
synchronize_rcu();
wakeup_source_unregister(ws);
}

static int attach_epitem(struct file *file, struct epitem *epi)
{
struct epitems_head *to_free = NULL;
struct hlist_head *head = NULL;
struct eventpoll *ep = NULL;

if (is_file_epoll(file))
  ep = file->private_data;

if (ep) {
  head = &ep->refs;
} else if (!READ_ONCE(file->f_ep)) {
allocate:
  to_free = kmem_cache_zalloc(ephead_cache, GFP_KERNEL);
  if (!to_free)
   return -ENOMEM;
  head = &to_free->epitems;
}
spin_lock(&file->f_lock);
if (!file->f_ep) {
  if (unlikely(!head)) {
   spin_unlock(&file->f_lock);
   goto allocate;
  }
  /* See eventpoll_release() for details. */
  WRITE_ONCE(file->f_ep, head);
  to_free = NULL;
}
hlist_add_head_rcu(&epi->fllink, file->f_ep);
spin_unlock(&file->f_lock);
free_ephead(to_free);
return 0;
}

/*
* Must be called with "mtx" held.
*/
static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
       struct file *tfile, int fd, int full_check)
{
int error, pwake = 0;
__poll_t revents;
struct epitem *epi;
struct ep_pqueue epq;
struct eventpoll *tep = NULL;

if (is_file_epoll(tfile))
  tep = tfile->private_data;

lockdep_assert_irqs_enabled();

if (unlikely(percpu_counter_compare(&ep->user->epoll_watches,
         max_user_watches) >= 0))
  return -ENOSPC;
percpu_counter_inc(&ep->user->epoll_watches);

if (!(epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL))) {
  percpu_counter_dec(&ep->user->epoll_watches);
  return -ENOMEM;
}

/* Item initialization follow here ... */
INIT_LIST_HEAD(&epi->rdllink);
epi->ep = ep;
ep_set_ffd(&epi->ffd, tfile, fd);
epi->event = *event;
epi->next = EP_UNACTIVE_PTR;

if (tep)
  mutex_lock_nested(&tep->mtx, 1);
/* Add the current item to the list of active epoll hook for this file */
if (unlikely(attach_epitem(tfile, epi) < 0)) {
  if (tep)
   mutex_unlock(&tep->mtx);
  kmem_cache_free(epi_cache, epi);
  percpu_counter_dec(&ep->user->epoll_watches);
  return -ENOMEM;
}

if (full_check && !tep)
  list_file(tfile);

/*
* Add the current item to the RB tree. All RB tree operations are
* protected by "mtx", and ep_insert() is called with "mtx" held.
*/
ep_rbtree_insert(ep, epi);
if (tep)
  mutex_unlock(&tep->mtx);

/*
* ep_remove_safe() calls in the later error paths can't lead to
* ep_free() as the ep file itself still holds an ep reference.
*/
ep_get(ep);

/* now check if we've created too many backpaths */
if (unlikely(full_check && reverse_path_check())) {
  ep_remove_safe(ep, epi);
  return -EINVAL;
}

if (epi->event.events & EPOLLWAKEUP) {
  error = ep_create_wakeup_source(epi);
  if (error) {
   ep_remove_safe(ep, epi);
   return error;
  }
}

/* Initialize the poll table using the queue callback */
epq.epi = epi;
init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);

/*
* Attach the item to the poll hooks and get current event bits.
* We can safely use the file* here because its usage count has
* been increased by the caller of this function. Note that after
* this operation completes, the poll callback can start hitting
* the new item.
*/
revents = ep_item_poll(epi, &epq.pt, 1);

/*
* We have to check if something went wrong during the poll wait queue
* install process. Namely an allocation for a wait queue failed due
* high memory pressure.
*/
if (unlikely(!epq.epi)) {
  ep_remove_safe(ep, epi);
  return -ENOMEM;
}

/* We have to drop the new item inside our item list to keep track of it */
spin_lock_irq(&ep->lock);

/* record NAPI ID of new item if present */
ep_set_busy_poll_napi_id(epi);

/* If the file is already "ready" we drop it inside the ready list */
if (revents && !ep_is_linked(epi)) {
  list_add_tail(&epi->rdllink, &ep->rdllist);
  ep_pm_stay_awake(epi);

  /* Notify waiting tasks that events are available */
  if (waitqueue_active(&ep->wq))
   wake_up(&ep->wq);
  if (waitqueue_active(&ep->poll_wait))
   pwake++;
}

spin_unlock_irq(&ep->lock);

/* We have to call this outside the lock */
if (pwake)
  ep_poll_safewake(ep, NULL, 0);

return 0;
}

/*
* Modify the interest event mask by dropping an event if the new mask
* has a match in the current file status. Must be called with "mtx" held.
*/
static int ep_modify(struct eventpoll *ep, struct epitem *epi,
       const struct epoll_event *event)
{
int pwake = 0;
poll_table pt;

lockdep_assert_irqs_enabled();

init_poll_funcptr(&pt, NULL);

/*
* Set the new event interest mask before calling f_op->poll();
* otherwise we might miss an event that happens between the
* f_op->poll() call and the new event set registering.
*/
epi->event.events = event->events; /* need barrier below */
epi->event.data = event->data; /* protected by mtx */
if (epi->event.events & EPOLLWAKEUP) {
  if (!ep_has_wakeup_source(epi))
   ep_create_wakeup_source(epi);
} else if (ep_has_wakeup_source(epi)) {
  ep_destroy_wakeup_source(epi);
}

/*
* The following barrier has two effects:
*
* 1) Flush epi changes above to other CPUs.  This ensures
*    we do not miss events from ep_poll_callback if an
*    event occurs immediately after we call f_op->poll().
*    We need this because we did not take ep->lock while
*    changing epi above (but ep_poll_callback does take
*    ep->lock).
*
* 2) We also need to ensure we do not miss _past_ events
*    when calling f_op->poll().  This barrier also
*    pairs with the barrier in wq_has_sleeper (see
*    comments for wq_has_sleeper).
*
* This barrier will now guarantee ep_poll_callback or f_op->poll
* (or both) will notice the readiness of an item.
*/
smp_mb();

/*
* Get current event bits. We can safely use the file* here because
* its usage count has been increased by the caller of this function.
* If the item is "hot" and it is not registered inside the ready
* list, push it inside.
*/
if (ep_item_poll(epi, &pt, 1)) {
  spin_lock_irq(&ep->lock);
  if (!ep_is_linked(epi)) {
   list_add_tail(&epi->rdllink, &ep->rdllist);
   ep_pm_stay_awake(epi);

   /* Notify waiting tasks that events are available */
   if (waitqueue_active(&ep->wq))
    wake_up(&ep->wq);
   if (waitqueue_active(&ep->poll_wait))
    pwake++;
  }
  spin_unlock_irq(&ep->lock);
}

/* We have to call this outside the lock */
if (pwake)
  ep_poll_safewake(ep, NULL, 0);

return 0;
}

static int ep_send_events(struct eventpoll *ep,
     struct epoll_event __user *events, int maxevents)
{
struct epitem *epi, *tmp;
LIST_HEAD(txlist);
poll_table pt;
int res = 0;

/*
* Always short-circuit for fatal signals to allow threads to make a
* timely exit without the chance of finding more events available and
* fetching repeatedly.
*/
if (fatal_signal_pending(current))
  return -EINTR;

init_poll_funcptr(&pt, NULL);

mutex_lock(&ep->mtx);
ep_start_scan(ep, &txlist);

/*
* We can loop without lock because we are passed a task private list.
* Items cannot vanish during the loop we are holding ep->mtx.
*/
list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
  struct wakeup_source *ws;
  __poll_t revents;

  if (res >= maxevents)
   break;

  /*
* Activate ep->ws before deactivating epi->ws to prevent
* triggering auto-suspend here (in case we reactive epi->ws
* below).
*
* This could be rearranged to delay the deactivation of epi->ws
* instead, but then epi->ws would temporarily be out of sync
* with ep_is_linked().
*/
  ws = ep_wakeup_source(epi);
  if (ws) {
   if (ws->active)
    __pm_stay_awake(ep->ws);
   __pm_relax(ws);
  }

  list_del_init(&epi->rdllink);

  /*
* If the event mask intersect the caller-requested one,
* deliver the event to userspace. Again, we are holding ep->mtx,
* so no operations coming from userspace can change the item.
*/
  revents = ep_item_poll(epi, &pt, 1);
  if (!revents)
   continue;

  events = epoll_put_uevent(revents, epi->event.data, events);
  if (!events) {
   list_add(&epi->rdllink, &txlist);
   ep_pm_stay_awake(epi);
   if (!res)
    res = -EFAULT;
   break;
  }
  res++;
  if (epi->event.events & EPOLLONESHOT)
   epi->event.events &= EP_PRIVATE_BITS;
  else if (!(epi->event.events & EPOLLET)) {
   /*
* If this file has been added with Level
* Trigger mode, we need to insert back inside
* the ready list, so that the next call to
* epoll_wait() will check again the events
* availability. At this point, no one can insert
* into ep->rdllist besides us. The epoll_ctl()
* callers are locked out by
* ep_send_events() holding "mtx" and the
* poll callback will queue them in ep->ovflist.
*/
   list_add_tail(&epi->rdllink, &ep->rdllist);
   ep_pm_stay_awake(epi);
  }
}
ep_done_scan(ep, &txlist);
mutex_unlock(&ep->mtx);

return res;
}

static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms)
{
struct timespec64 now;

if (ms < 0)
  return NULL;

if (!ms) {
  to->tv_sec = 0;
  to->tv_nsec = 0;
  return to;
}

to->tv_sec = ms / MSEC_PER_SEC;
to->tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC);

ktime_get_ts64(&now);
*to = timespec64_add_safe(now, *to);
return to;
}

/*
* autoremove_wake_function, but remove even on failure to wake up, because we
* know that default_wake_function/ttwu will only fail if the thread is already
* woken, and in that case the ep_poll loop will remove the entry anyways, not
* try to reuse it.
*/
static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
           unsigned int mode, int sync, void *key)
{
int ret = default_wake_function(wq_entry, mode, sync, key);

/*
* Pairs with list_empty_careful in ep_poll, and ensures future loop
* iterations see the cause of this wakeup.
*/
list_del_init_careful(&wq_entry->entry);
return ret;
}

static int ep_try_send_events(struct eventpoll *ep,
         struct epoll_event __user *events, int maxevents)
{
int res;

/*
* Try to transfer events to user space. In case we get 0 events and
* there's still timeout left over, we go trying again in search of
* more luck.
*/
res = ep_send_events(ep, events, maxevents);
if (res > 0)
  ep_suspend_napi_irqs(ep);
return res;
}

static int ep_schedule_timeout(ktime_t *to)
{
if (to)
  return ktime_after(*to, ktime_get());
else
  return 1;
}

/**
* ep_poll - Retrieves ready events, and delivers them to the caller-supplied
*           event buffer.
*
* @ep: Pointer to the eventpoll context.
* @events: Pointer to the userspace buffer where the ready events should be
*          stored.
* @maxevents: Size (in terms of number of events) of the caller event buffer.
* @timeout: Maximum timeout for the ready events fetch operation, in
*           timespec. If the timeout is zero, the function will not block,
*           while if the @timeout ptr is NULL, the function will block
*           until at least one event has been retrieved (or an error
*           occurred).
*
* Return: the number of ready events which have been fetched, or an
*          error code, in case of error.
*/
static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
     int maxevents, struct timespec64 *timeout)
{
int res, eavail, timed_out = 0;
u64 slack = 0;
wait_queue_entry_t wait;
ktime_t expires, *to = NULL;

lockdep_assert_irqs_enabled();

if (timeout && (timeout->tv_sec | timeout->tv_nsec)) {
  slack = select_estimate_accuracy(timeout);
  to = &expires;
  *to = timespec64_to_ktime(*timeout);
} else if (timeout) {
  /*
* Avoid the unnecessary trip to the wait queue loop, if the
* caller specified a non blocking operation.
*/
  timed_out = 1;
}

/*
* This call is racy: We may or may not see events that are being added
* to the ready list under the lock (e.g., in IRQ callbacks). For cases
* with a non-zero timeout, this thread will check the ready list under
* lock and will add to the wait queue.  For cases with a zero
* timeout, the user by definition should not care and will have to
* recheck again.
*/
eavail = ep_events_available(ep);

while (1) {
  if (eavail) {
   res = ep_try_send_events(ep, events, maxevents);
   if (res)
    return res;
  }

  if (timed_out)
   return 0;

  eavail = ep_busy_loop(ep);
  if (eavail)
   continue;

  if (signal_pending(current))
   return -EINTR;

  /*
* Internally init_wait() uses autoremove_wake_function(),
* thus wait entry is removed from the wait queue on each
* wakeup. Why it is important? In case of several waiters
* each new wakeup will hit the next waiter, giving it the
* chance to harvest new event. Otherwise wakeup can be
* lost. This is also good performance-wise, because on
* normal wakeup path no need to call __remove_wait_queue()
* explicitly, thus ep->lock is not taken, which halts the
* event delivery.
*
* In fact, we now use an even more aggressive function that
* unconditionally removes, because we don't reuse the wait
* entry between loop iterations. This lets us also avoid the
* performance issue if a process is killed, causing all of its
* threads to wake up without being removed normally.
*/
  init_wait(&wait);
  wait.func = ep_autoremove_wake_function;

  spin_lock_irq(&ep->lock);
  /*
* Barrierless variant, waitqueue_active() is called under
* the same lock on wakeup ep_poll_callback() side, so it
* is safe to avoid an explicit barrier.
*/
  __set_current_state(TASK_INTERRUPTIBLE);

  /*
* Do the final check under the lock. ep_start/done_scan()
* plays with two lists (->rdllist and ->ovflist) and there
* is always a race when both lists are empty for short
* period of time although events are pending, so lock is
* important.
*/
  eavail = ep_events_available(ep);
  if (!eavail)
   __add_wait_queue_exclusive(&ep->wq, &wait);

  spin_unlock_irq(&ep->lock);

  if (!eavail)
   timed_out = !ep_schedule_timeout(to) ||
    !schedule_hrtimeout_range(to, slack,
         HRTIMER_MODE_ABS);
  __set_current_state(TASK_RUNNING);

  /*
* We were woken up, thus go and try to harvest some events.
* If timed out and still on the wait queue, recheck eavail
* carefully under lock, below.
*/
  eavail = 1;

  if (!list_empty_careful(&wait.entry)) {
   spin_lock_irq(&ep->lock);
   /*
* If the thread timed out and is not on the wait queue,
* it means that the thread was woken up after its
* timeout expired before it could reacquire the lock.
* Thus, when wait.entry is empty, it needs to harvest
* events.
*/
   if (timed_out)
    eavail = list_empty(&wait.entry);
   __remove_wait_queue(&ep->wq, &wait);
   spin_unlock_irq(&ep->lock);
  }
}
}

/**
* ep_loop_check_proc - verify that adding an epoll file @ep inside another
*                      epoll file does not create closed loops, and
*                      determine the depth of the subtree starting at @ep
*
* @ep: the &struct eventpoll to be currently checked.
* @depth: Current depth of the path being checked.
*
* Return: depth of the subtree, or INT_MAX if we found a loop or went too deep.
*/
static int ep_loop_check_proc(struct eventpoll *ep, int depth)
{
int result = 0;
struct rb_node *rbp;
struct epitem *epi;

if (ep->gen == loop_check_gen)
  return ep->loop_check_depth;

mutex_lock_nested(&ep->mtx, depth + 1);
ep->gen = loop_check_gen;
for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
  epi = rb_entry(rbp, struct epitem, rbn);
  if (unlikely(is_file_epoll(epi->ffd.file))) {
   struct eventpoll *ep_tovisit;
   ep_tovisit = epi->ffd.file->private_data;
   if (ep_tovisit == inserting_into || depth > EP_MAX_NESTS)
    result = INT_MAX;
   else
    result = max(result, ep_loop_check_proc(ep_tovisit, depth + 1) + 1);
   if (result > EP_MAX_NESTS)
    break;
  } else {
   /*
* If we've reached a file that is not associated with
* an ep, then we need to check if the newly added
* links are going to add too many wakeup paths. We do
* this by adding it to the tfile_check_list, if it's
* not already there, and calling reverse_path_check()
* during ep_insert().
*/
   list_file(epi->ffd.file);
  }
}
ep->loop_check_depth = result;
mutex_unlock(&ep->mtx);

return result;
}

/* ep_get_upwards_depth_proc - determine depth of @ep when traversed upwards */
static int ep_get_upwards_depth_proc(struct eventpoll *ep, int depth)
{
int result = 0;
struct epitem *epi;

if (ep->gen == loop_check_gen)
  return ep->loop_check_depth;
hlist_for_each_entry_rcu(epi, &ep->refs, fllink)
  result = max(result, ep_get_upwards_depth_proc(epi->ep, depth + 1) + 1);
ep->gen = loop_check_gen;
ep->loop_check_depth = result;
return result;
}

/**
* ep_loop_check - Performs a check to verify that adding an epoll file (@to)
*                 into another epoll file (represented by @ep) does not create
*                 closed loops or too deep chains.
*
* @ep: Pointer to the epoll we are inserting into.
* @to: Pointer to the epoll to be inserted.
*
* Return: %zero if adding the epoll @to inside the epoll @from
* does not violate the constraints, or %-1 otherwise.
*/
static int ep_loop_check(struct eventpoll *ep, struct eventpoll *to)
{
int depth, upwards_depth;

inserting_into = ep;
/*
* Check how deep down we can get from @to, and whether it is possible
* to loop up to @ep.
*/
depth = ep_loop_check_proc(to, 0);
if (depth > EP_MAX_NESTS)
  return -1;
/* Check how far up we can go from @ep. */
rcu_read_lock();
upwards_depth = ep_get_upwards_depth_proc(ep, 0);
rcu_read_unlock();

return (depth+1+upwards_depth > EP_MAX_NESTS) ? -1 : 0;
}

static void clear_tfile_check_list(void)
{
rcu_read_lock();
while (tfile_check_list != EP_UNACTIVE_PTR) {
  struct epitems_head *head = tfile_check_list;
  tfile_check_list = head->next;
  unlist_file(head);
}
rcu_read_unlock();
}

/*
* Open an eventpoll file descriptor.
*/
static int do_epoll_create(int flags)
{
int error, fd;
struct eventpoll *ep = NULL;
struct file *file;

/* Check the EPOLL_* constant for consistency.  */
BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);

if (flags & ~EPOLL_CLOEXEC)
  return -EINVAL;
/*
* Create the internal data structure ("struct eventpoll").
*/
error = ep_alloc(&ep);
if (error < 0)
  return error;
/*
* Creates all the items needed to setup an eventpoll file. That is,
* a file structure and a free file descriptor.
*/
fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
if (fd < 0) {
  error = fd;
  goto out_free_ep;
}
file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
     O_RDWR | (flags & O_CLOEXEC));
if (IS_ERR(file)) {
  error = PTR_ERR(file);
  goto out_free_fd;
}
ep->file = file;
fd_install(fd, file);
return fd;

out_free_fd:
put_unused_fd(fd);
out_free_ep:
ep_clear_and_put(ep);
return error;
}

SYSCALL_DEFINE1(epoll_create1, int, flags)
{
return do_epoll_create(flags);
}

SYSCALL_DEFINE1(epoll_create, int, size)
{
if (size <= 0)
  return -EINVAL;

return do_epoll_create(0);
}

#ifdef CONFIG_PM_SLEEP
static inline void ep_take_care_of_epollwakeup(struct epoll_event *epev)
{
if ((epev->events & EPOLLWAKEUP) && !capable(CAP_BLOCK_SUSPEND))
  epev->events &= ~EPOLLWAKEUP;
}
#else
static inline void ep_take_care_of_epollwakeup(struct epoll_event *epev)
{
epev->events &= ~EPOLLWAKEUP;
}
#endif

static inline int epoll_mutex_lock(struct mutex *mutex, int depth,
       bool nonblock)
{
if (!nonblock) {
  mutex_lock_nested(mutex, depth);
  return 0;
}
if (mutex_trylock(mutex))
  return 0;
return -EAGAIN;
}

int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
   bool nonblock)
{
int error;
int full_check = 0;
struct eventpoll *ep;
struct epitem *epi;
struct eventpoll *tep = NULL;

CLASS(fd, f)(epfd);
if (fd_empty(f))
  return -EBADF;

/* Get the "struct file *" for the target file */
CLASS(fd, tf)(fd);
if (fd_empty(tf))
  return -EBADF;

/* The target file descriptor must support poll */
if (!file_can_poll(fd_file(tf)))
  return -EPERM;

/* Check if EPOLLWAKEUP is allowed */
if (ep_op_has_event(op))
  ep_take_care_of_epollwakeup(epds);

/*
* We have to check that the file structure underneath the file descriptor
* the user passed to us _is_ an eventpoll file. And also we do not permit
* adding an epoll file descriptor inside itself.
*/
error = -EINVAL;
if (fd_file(f) == fd_file(tf) || !is_file_epoll(fd_file(f)))
  goto error_tgt_fput;

/*
* epoll adds to the wakeup queue at EPOLL_CTL_ADD time only,
* so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.
* Also, we do not currently supported nested exclusive wakeups.
*/
if (ep_op_has_event(op) && (epds->events & EPOLLEXCLUSIVE)) {
  if (op == EPOLL_CTL_MOD)
   goto error_tgt_fput;
  if (op == EPOLL_CTL_ADD && (is_file_epoll(fd_file(tf)) ||
    (epds->events & ~EPOLLEXCLUSIVE_OK_BITS)))
   goto error_tgt_fput;
}

/*
* At this point it is safe to assume that the "private_data" contains
* our own data structure.
*/
ep = fd_file(f)->private_data;

/*
* When we insert an epoll file descriptor inside another epoll file
* descriptor, there is the chance of creating closed loops, which are
* better be handled here, than in more critical paths. While we are
* checking for loops we also determine the list of files reachable
* and hang them on the tfile_check_list, so we can check that we
* haven't created too many possible wakeup paths.
*
* We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
* the epoll file descriptor is attaching directly to a wakeup source,
* unless the epoll file descriptor is nested. The purpose of taking the
* 'epnested_mutex' on add is to prevent complex toplogies such as loops and
* deep wakeup paths from forming in parallel through multiple
* EPOLL_CTL_ADD operations.
*/
error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
if (error)
  goto error_tgt_fput;
if (op == EPOLL_CTL_ADD) {
  if (READ_ONCE(fd_file(f)->f_ep) || ep->gen == loop_check_gen ||
      is_file_epoll(fd_file(tf))) {
   mutex_unlock(&ep->mtx);
   error = epoll_mutex_lock(&epnested_mutex, 0, nonblock);
   if (error)
    goto error_tgt_fput;
   loop_check_gen++;
   full_check = 1;
   if (is_file_epoll(fd_file(tf))) {
    tep = fd_file(tf)->private_data;
    error = -ELOOP;
    if (ep_loop_check(ep, tep) != 0)
     goto error_tgt_fput;
   }
   error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
   if (error)
    goto error_tgt_fput;
  }
}

/*
* Try to lookup the file inside our RB tree. Since we grabbed "mtx"
* above, we can be sure to be able to use the item looked up by
* ep_find() till we release the mutex.
*/
epi = ep_find(ep, fd_file(tf), fd);

error = -EINVAL;
switch (op) {
case EPOLL_CTL_ADD:
  if (!epi) {
   epds->events |= EPOLLERR | EPOLLHUP;
   error = ep_insert(ep, epds, fd_file(tf), fd, full_check);
  } else
   error = -EEXIST;
  break;
case EPOLL_CTL_DEL:
  if (epi) {
   /*
* The eventpoll itself is still alive: the refcount
* can't go to zero here.
*/
   ep_remove_safe(ep, epi);
   error = 0;
  } else {
   error = -ENOENT;
  }
  break;
case EPOLL_CTL_MOD:
  if (epi) {
   if (!(epi->event.events & EPOLLEXCLUSIVE)) {
    epds->events |= EPOLLERR | EPOLLHUP;
    error = ep_modify(ep, epi, epds);
   }
  } else
   error = -ENOENT;
  break;
}
mutex_unlock(&ep->mtx);

error_tgt_fput:
if (full_check) {
  clear_tfile_check_list();
  loop_check_gen++;
  mutex_unlock(&epnested_mutex);
}
return error;
}

/*
* The following function implements the controller interface for
* the eventpoll file that enables the insertion/removal/change of
* file descriptors inside the interest set.
*/
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
  struct epoll_event __user *, event)
{
struct epoll_event epds;

if (ep_op_has_event(op) &&
     copy_from_user(&epds, event, sizeof(struct epoll_event)))
  return -EFAULT;

return do_epoll_ctl(epfd, op, fd, &epds, false);
}

static int ep_check_params(struct file *file, struct epoll_event __user *evs,
      int maxevents)
{
/* The maximum number of event must be greater than zero */
if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
  return -EINVAL;

/* Verify that the area passed by the user is writeable */
if (!access_ok(evs, maxevents * sizeof(struct epoll_event)))
  return -EFAULT;

/*
* We have to check that the file structure underneath the fd
* the user passed to us _is_ an eventpoll file.
*/
if (!is_file_epoll(file))
  return -EINVAL;

return 0;
}

--> --------------------

--> maximum size reached

--> --------------------

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