include/linux/refcount.h - SHIFTPHONES/kernel/shift/mainline - Gitiles

 /* 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.
  *
  */

 #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/spinlock_types.h>

 struct mutex;

 /**
  * struct refcount_t - variant of atomic_t specialized for reference counts
  * @refs: atomic_t counter field
  *
  * The counter saturates at REFCOUNT_SATURATED and will not move once
  * there. This avoids wrapping the counter and causing 'spurious'
  * use-after-free bugs.
  */
 typedef struct refcount_struct {
 	atomic_t refs;
 } refcount_t;

 #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_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);
 }

 /**
  * 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)
 {
 	int old = refcount_read(r);

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

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

 	return old;
 }

 /**
  * 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)
 {
 	int old = atomic_fetch_add_relaxed(i, &r->refs);

 	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_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_add_not_zero(1, r);
 }

 /**
  * 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_add(1, r);
 }

 /**
  * 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)
 {
 	int old = atomic_fetch_sub_release(i, &r->refs);

 	if (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_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_sub_and_test(1, r);
 }

 /**
  * 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)
 {
 	if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
 		refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }

 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);
 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
 						       spinlock_t *lock,
 						       unsigned long *flags);
 #endif /* _LINUX_REFCOUNT_H */
	/* 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.
	*
	*/

	#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/spinlock_types.h>

	struct mutex;

	/**
	* struct refcount_t - variant of atomic_t specialized for reference counts
	* @refs: atomic_t counter field
	*
	* The counter saturates at REFCOUNT_SATURATED and will not move once
	* there. This avoids wrapping the counter and causing 'spurious'
	* use-after-free bugs.
	*/
	typedef struct refcount_struct {
	atomic_t refs;
	} refcount_t;

	#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_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);
	}

	/**
	* 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)
	{
	int old = refcount_read(r);

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

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

	return old;
	}

	/**
	* 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)
	{
	int old = atomic_fetch_add_relaxed(i, &r->refs);

	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_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_add_not_zero(1, r);
	}

	/**
	* 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_add(1, r);
	}

	/**
	* 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)
	{
	int old = atomic_fetch_sub_release(i, &r->refs);

	if (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_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_sub_and_test(1, r);
	}

	/**
	* 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)
	{
	if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
	refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
	}

	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);
	extern __must_check bool refcount_dec_and_lock(refcount_t r, spinlock_t lock);
	extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
	spinlock_t *lock,
	unsigned long *flags);
	#endif /* _LINUX_REFCOUNT_H */