Impressum refcount.h Sprache: unbekannt

/* SPDX-License-Identifier: GPL-2.0 */
/*
* Variant of atomic_t specialized for reference counts.
*
* The interface matches the atomic_t interface (to aid in porting) but only
* provides the few functions one should use for reference counting.
*
* Saturation semantics
* ====================
*
* refcount_t differs from atomic_t in that the counter saturates at
* REFCOUNT_SATURATED and will not move once there. This avoids wrapping the
* counter and causing 'spurious' use-after-free issues. In order to avoid the
* cost associated with introducing cmpxchg() loops into all of the saturating
* operations, we temporarily allow the counter to take on an unchecked value
* and then explicitly set it to REFCOUNT_SATURATED on detecting that underflow
* or overflow has occurred. Although this is racy when multiple threads
* access the refcount concurrently, by placing REFCOUNT_SATURATED roughly
* equidistant from 0 and INT_MAX we minimise the scope for error:
*
*                            INT_MAX     REFCOUNT_SATURATED   UINT_MAX
*   0                          (0x7fff_ffff)    (0xc000_0000)    (0xffff_ffff)
*   +--------------------------------+----------------+----------------+
*                                     <---------- bad value! ---------->
*
* (in a signed view of the world, the "bad value" range corresponds to
* a negative counter value).
*
* As an example, consider a refcount_inc() operation that causes the counter
* to overflow:
*
* int old = atomic_fetch_add_relaxed(r);
* // old is INT_MAX, refcount now INT_MIN (0x8000_0000)
* if (old < 0)
* atomic_set(r, REFCOUNT_SATURATED);
*
* If another thread also performs a refcount_inc() operation between the two
* atomic operations, then the count will continue to edge closer to 0. If it
* reaches a value of 1 before /any/ of the threads reset it to the saturated
* value, then a concurrent refcount_dec_and_test() may erroneously free the
* underlying object.
* Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
* 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
* With the current PID limit, if no batched refcounting operations are used and
* the attacker can't repeatedly trigger kernel oopses in the middle of refcount
* operations, this makes it impossible for a saturated refcount to leave the
* saturation range, even if it is possible for multiple uses of the same
* refcount to nest in the context of a single task:
*
*     (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
*     0x40000000 / 0x400000 = 0x100 = 256
*
* If hundreds of references are added/removed with a single refcounting
* operation, it may potentially be possible to leave the saturation range; but
* given the precise timing details involved with the round-robin scheduling of
* each thread manipulating the refcount and the need to hit the race multiple
* times in succession, there doesn't appear to be a practical avenue of attack
* even if using refcount_add() operations with larger increments.
*
* Memory ordering
* ===============
*
* Memory ordering rules are slightly relaxed wrt regular atomic_t functions
* and provide only what is strictly required for refcounts.
*
* The increments are fully relaxed; these will not provide ordering. The
* rationale is that whatever is used to obtain the object we're increasing the
* reference count on will provide the ordering. For locked data structures,
* its the lock acquire, for RCU/lockless data structures its the dependent
* load.
*
* Do note that inc_not_zero() provides a control dependency which will order
* future stores against the inc, this ensures we'll never modify the object
* if we did not in fact acquire a reference.
*
* The decrements will provide release order, such that all the prior loads and
* stores will be issued before, it also provides a control dependency, which
* will order us against the subsequent free().
*
* The control dependency is against the load of the cmpxchg (ll/sc) that
* succeeded. This means the stores aren't fully ordered, but this is fine
* because the 1->0 transition indicates no concurrency.
*
* Note that the allocator is responsible for ordering things between free()
* and alloc().
*
* The decrements dec_and_test() and sub_and_test() also provide acquire
* ordering on success.
*
* refcount_{add|inc}_not_zero_acquire() and refcount_set_release() provide
* acquire and release ordering for cases when the memory occupied by the
* object might be reused to store another object. This is important for the
* cases where secondary validation is required to detect such reuse, e.g.
* SLAB_TYPESAFE_BY_RCU. The secondary validation checks have to happen after
* the refcount is taken, hence acquire order is necessary. Similarly, when the
* object is initialized, all stores to its attributes should be visible before
* the refcount is set, otherwise a stale attribute value might be used by
* another task which succeeds in taking a refcount to the new object.
*/

#ifndef _LINUX_REFCOUNT_H
#define _LINUX_REFCOUNT_H

#include <linux/atomic.h>
#include <linux/bug.h>
#include <linux/compiler.h>
#include <linux/limits.h>
#include <linux/refcount_types.h>
#include <linux/spinlock_types.h>

struct mutex;

#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
#define REFCOUNT_MAX  INT_MAX
#define REFCOUNT_SATURATED (INT_MIN / 2)

enum refcount_saturation_type {
REFCOUNT_ADD_NOT_ZERO_OVF,
REFCOUNT_ADD_OVF,
REFCOUNT_ADD_UAF,
REFCOUNT_SUB_UAF,
REFCOUNT_DEC_LEAK,
};

void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);

/**
* refcount_set - set a refcount's value
* @r: the refcount
* @n: value to which the refcount will be set
*/
static inline void refcount_set(refcount_t *r, int n)
{
atomic_set(&r->refs, n);
}

/**
* refcount_set_release - set a refcount's value with release ordering
* @r: the refcount
* @n: value to which the refcount will be set
*
* This function should be used when memory occupied by the object might be
* reused to store another object -- consider SLAB_TYPESAFE_BY_RCU.
*
* Provides release memory ordering which will order previous memory operations
* against this store. This ensures all updates to this object are visible
* once the refcount is set and stale values from the object previously
* occupying this memory are overwritten with new ones.
*
* This function should be called only after new object is fully initialized.
* After this call the object should be considered visible to other tasks even
* if it was not yet added into an object collection normally used to discover
* it. This is because other tasks might have discovered the object previously
* occupying the same memory and after memory reuse they can succeed in taking
* refcount to the new object and start using it.
*/
static inline void refcount_set_release(refcount_t *r, int n)
{
atomic_set_release(&r->refs, n);
}

/**
* refcount_read - get a refcount's value
* @r: the refcount
*
* Return: the refcount's value
*/
static inline unsigned int refcount_read(const refcount_t *r)
{
return atomic_read(&r->refs);
}

static inline __must_check __signed_wrap
bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
{
int old = refcount_read(r);

do {
  if (!old)
   break;
} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));

if (oldp)
  *oldp = old;

if (unlikely(old < 0 || old + i < 0))
  refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);

return old;
}

/**
* refcount_add_not_zero - add a value to a refcount unless it is 0
* @i: the value to add to the refcount
* @r: the refcount
*
* Will saturate at REFCOUNT_SATURATED and WARN.
*
* Provides no memory ordering, it is assumed the caller has guaranteed the
* object memory to be stable (RCU, etc.). It does provide a control dependency
* and thereby orders future stores. See the comment on top.
*
* Use of this function is not recommended for the normal reference counting
* use case in which references are taken and released one at a time.  In these
* cases, refcount_inc(), or one of its variants, should instead be used to
* increment a reference count.
*
* Return: false if the passed refcount is 0, true otherwise
*/
static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
{
return __refcount_add_not_zero(i, r, NULL);
}

static inline __must_check __signed_wrap
bool __refcount_add_not_zero_limited_acquire(int i, refcount_t *r, int *oldp,
          int limit)
{
int old = refcount_read(r);

do {
  if (!old)
   break;

  if (i > limit - old) {
   if (oldp)
    *oldp = old;
   return false;
  }
} while (!atomic_try_cmpxchg_acquire(&r->refs, &old, old + i));

if (oldp)
  *oldp = old;

if (unlikely(old < 0 || old + i < 0))
  refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);

return old;
}

static inline __must_check bool
__refcount_inc_not_zero_limited_acquire(refcount_t *r, int *oldp, int limit)
{
return __refcount_add_not_zero_limited_acquire(1, r, oldp, limit);
}

static inline __must_check __signed_wrap
bool __refcount_add_not_zero_acquire(int i, refcount_t *r, int *oldp)
{
return __refcount_add_not_zero_limited_acquire(i, r, oldp, INT_MAX);
}

/**
* refcount_add_not_zero_acquire - add a value to a refcount with acquire ordering unless it is 0
*
* @i: the value to add to the refcount
* @r: the refcount
*
* Will saturate at REFCOUNT_SATURATED and WARN.
*
* This function should be used when memory occupied by the object might be
* reused to store another object -- consider SLAB_TYPESAFE_BY_RCU.
*
* Provides acquire memory ordering on success, it is assumed the caller has
* guaranteed the object memory to be stable (RCU, etc.). It does provide a
* control dependency and thereby orders future stores. See the comment on top.
*
* Use of this function is not recommended for the normal reference counting
* use case in which references are taken and released one at a time.  In these
* cases, refcount_inc_not_zero_acquire() should instead be used to increment a
* reference count.
*
* Return: false if the passed refcount is 0, true otherwise
*/
static inline __must_check bool refcount_add_not_zero_acquire(int i, refcount_t *r)
{
return __refcount_add_not_zero_acquire(i, r, NULL);
}

static inline __signed_wrap
void __refcount_add(int i, refcount_t *r, int *oldp)
{
int old = atomic_fetch_add_relaxed(i, &r->refs);

if (oldp)
  *oldp = old;

if (unlikely(!old))
  refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
else if (unlikely(old < 0 || old + i < 0))
  refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
}

/**
* refcount_add - add a value to a refcount
* @i: the value to add to the refcount
* @r: the refcount
*
* Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED and WARN.
*
* Provides no memory ordering, it is assumed the caller has guaranteed the
* object memory to be stable (RCU, etc.). It does provide a control dependency
* and thereby orders future stores. See the comment on top.
*
* Use of this function is not recommended for the normal reference counting
* use case in which references are taken and released one at a time.  In these
* cases, refcount_inc(), or one of its variants, should instead be used to
* increment a reference count.
*/
static inline void refcount_add(int i, refcount_t *r)
{
__refcount_add(i, r, NULL);
}

static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
{
return __refcount_add_not_zero(1, r, oldp);
}

/**
* refcount_inc_not_zero - increment a refcount unless it is 0
* @r: the refcount to increment
*
* Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
* and WARN.
*
* Provides no memory ordering, it is assumed the caller has guaranteed the
* object memory to be stable (RCU, etc.). It does provide a control dependency
* and thereby orders future stores. See the comment on top.
*
* Return: true if the increment was successful, false otherwise
*/
static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
{
return __refcount_inc_not_zero(r, NULL);
}

static inline __must_check bool __refcount_inc_not_zero_acquire(refcount_t *r, int *oldp)
{
return __refcount_add_not_zero_acquire(1, r, oldp);
}

/**
* refcount_inc_not_zero_acquire - increment a refcount with acquire ordering unless it is 0
* @r: the refcount to increment
*
* Similar to refcount_inc_not_zero(), but provides acquire memory ordering on
* success.
*
* This function should be used when memory occupied by the object might be
* reused to store another object -- consider SLAB_TYPESAFE_BY_RCU.
*
* Provides acquire memory ordering on success, it is assumed the caller has
* guaranteed the object memory to be stable (RCU, etc.). It does provide a
* control dependency and thereby orders future stores. See the comment on top.
*
* Return: true if the increment was successful, false otherwise
*/
static inline __must_check bool refcount_inc_not_zero_acquire(refcount_t *r)
{
return __refcount_inc_not_zero_acquire(r, NULL);
}

static inline void __refcount_inc(refcount_t *r, int *oldp)
{
__refcount_add(1, r, oldp);
}

/**
* refcount_inc - increment a refcount
* @r: the refcount to increment
*
* Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
*
* Provides no memory ordering, it is assumed the caller already has a
* reference on the object.
*
* Will WARN if the refcount is 0, as this represents a possible use-after-free
* condition.
*/
static inline void refcount_inc(refcount_t *r)
{
__refcount_inc(r, NULL);
}

static inline __must_check __signed_wrap
bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
{
int old = atomic_fetch_sub_release(i, &r->refs);

if (oldp)
  *oldp = old;

if (old > 0 && old == i) {
  smp_acquire__after_ctrl_dep();
  return true;
}

if (unlikely(old <= 0 || old - i < 0))
  refcount_warn_saturate(r, REFCOUNT_SUB_UAF);

return false;
}

/**
* refcount_sub_and_test - subtract from a refcount and test if it is 0
* @i: amount to subtract from the refcount
* @r: the refcount
*
* Similar to atomic_dec_and_test(), but it will WARN, return false and
* ultimately leak on underflow and will fail to decrement when saturated
* at REFCOUNT_SATURATED.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides an acquire ordering on success such that free()
* must come after.
*
* Use of this function is not recommended for the normal reference counting
* use case in which references are taken and released one at a time.  In these
* cases, refcount_dec(), or one of its variants, should instead be used to
* decrement a reference count.
*
* Return: true if the resulting refcount is 0, false otherwise
*/
static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
{
return __refcount_sub_and_test(i, r, NULL);
}

static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
{
return __refcount_sub_and_test(1, r, oldp);
}

/**
* refcount_dec_and_test - decrement a refcount and test if it is 0
* @r: the refcount
*
* Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
* decrement when saturated at REFCOUNT_SATURATED.
*
* Provides release memory ordering, such that prior loads and stores are done
* before, and provides an acquire ordering on success such that free()
* must come after.
*
* Return: true if the resulting refcount is 0, false otherwise
*/
static inline __must_check bool refcount_dec_and_test(refcount_t *r)
{
return __refcount_dec_and_test(r, NULL);
}

static inline void __refcount_dec(refcount_t *r, int *oldp)
{
int old = atomic_fetch_sub_release(1, &r->refs);

if (oldp)
  *oldp = old;

if (unlikely(old <= 1))
  refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
}

/**
* refcount_dec - decrement a refcount
* @r: the refcount
*
* Similar to atomic_dec(), it will WARN on underflow and fail to decrement
* when saturated at REFCOUNT_SATURATED.
*
* Provides release memory ordering, such that prior loads and stores are done
* before.
*/
static inline void refcount_dec(refcount_t *r)
{
__refcount_dec(r, NULL);
}

extern __must_check bool refcount_dec_if_one(refcount_t *r);
extern __must_check bool refcount_dec_not_one(refcount_t *r);
extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock);
extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock);
extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
             spinlock_t *lock,
             unsigned long *flags) __cond_acquires(lock);
#endif /* _LINUX_REFCOUNT_H */

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Impressum refcount.h Sprache: unbekannt

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