refcount_t: Introduce a special purpose refcount type

#ifndef _LINUX_REFCOUNT_H

#define _LINUX_REFCOUNT_H

/*

 * 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.

 *

 * It differs in that the counter saturates at UINT_MAX and will not move once

 * there. This avoids wrapping the counter and causing 'spurious'

 * use-after-free issues.

 *

 * 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().

 *

 */

#include <linux/atomic.h>

#include <linux/bug.h>

#include <linux/mutex.h>

#include <linux/spinlock.h>

#ifdef CONFIG_DEBUG_REFCOUNT

#define REFCOUNT_WARN(cond, str) WARN_ON(cond)

#define __refcount_check	__must_check

#else

#define REFCOUNT_WARN(cond, str) (void)(cond)

#define __refcount_check

#endif

typedef struct refcount_struct {

	atomic_t refs;

} refcount_t;

#define REFCOUNT_INIT(n)	{ .refs = ATOMIC_INIT(n), }

static inline void refcount_set(refcount_t *r, unsigned int n)

{

	atomic_set(&r->refs, n);

}

static inline unsigned int refcount_read(const refcount_t *r)

{

	return atomic_read(&r->refs);

}

static inline __refcount_check

bool refcount_add_not_zero(unsigned int i, refcount_t *r)

{

	unsigned int old, new, val = atomic_read(&r->refs);

	for (;;) {

		if (!val)

			return false;

		if (unlikely(val == UINT_MAX))

			return true;

		new = val + i;

		if (new < val)

			new = UINT_MAX;

		old = atomic_cmpxchg_relaxed(&r->refs, val, new);

		if (old == val)

			break;

		val = old;

	}

	REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

	return true;

}

static inline void refcount_add(unsigned int i, refcount_t *r)

{

	REFCOUNT_WARN(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");

}

/*

 * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX 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.

 */

static inline __refcount_check

bool refcount_inc_not_zero(refcount_t *r)

{

	unsigned int old, new, val = atomic_read(&r->refs);

	for (;;) {

		new = val + 1;

		if (!val)

			return false;

		if (unlikely(!new))

			return true;

		old = atomic_cmpxchg_relaxed(&r->refs, val, new);

		if (old == val)

			break;

		val = old;

	}

	REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

	return true;

}

/*

 * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.

 *

 * Provides no memory ordering, it is assumed the caller already has a

 * reference on the object, will WARN when this is not so.

 */

static inline void refcount_inc(refcount_t *r)

{

	REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");

}

/*

 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to

 * decrement when saturated at UINT_MAX.

 *

 * Provides release memory ordering, such that prior loads and stores are done

 * before, and provides a control dependency such that free() must come after.

 * See the comment on top.

 */

static inline __refcount_check

bool refcount_sub_and_test(unsigned int i, refcount_t *r)

{

	unsigned int old, new, val = atomic_read(&r->refs);

	for (;;) {

		if (unlikely(val == UINT_MAX))

			return false;

		new = val - i;

		if (new > val) {

			REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");

			return false;

		}

		old = atomic_cmpxchg_release(&r->refs, val, new);

		if (old == val)

			break;

		val = old;

	}

	return !new;

}

static inline __refcount_check

bool refcount_dec_and_test(refcount_t *r)

{

	return refcount_sub_and_test(1, r);

}

/*

 * Similar to atomic_dec(), it will WARN on underflow and fail to decrement

 * when saturated at UINT_MAX.

 *

 * Provides release memory ordering, such that prior loads and stores are done

 * before.

 */

static inline

void refcount_dec(refcount_t *r)

{

	REFCOUNT_WARN(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");

}

/*

 * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the

 * success thereof.

 *

 * Like all decrement operations, it provides release memory order and provides

 * a control dependency.

 *

 * It can be used like a try-delete operator; this explicit case is provided

 * and not cmpxchg in generic, because that would allow implementing unsafe

 * operations.

 */

static inline __refcount_check

bool refcount_dec_if_one(refcount_t *r)

{

	return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;

}

/*

 * No atomic_t counterpart, it decrements unless the value is 1, in which case

 * it will return false.

 *

 * Was often done like: atomic_add_unless(&var, -1, 1)

 */

static inline __refcount_check

bool refcount_dec_not_one(refcount_t *r)

{

	unsigned int old, new, val = atomic_read(&r->refs);

	for (;;) {

		if (unlikely(val == UINT_MAX))

			return true;

		if (val == 1)

			return false;

		new = val - 1;

		if (new > val) {

			REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");

			return true;

		}

		old = atomic_cmpxchg_release(&r->refs, val, new);

		if (old == val)

			break;

		val = old;

	}

	return true;

}

/*

 * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail

 * to decrement when saturated at UINT_MAX.

 *

 * Provides release memory ordering, such that prior loads and stores are done

 * before, and provides a control dependency such that free() must come after.

 * See the comment on top.

 */

static inline __refcount_check

bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)

{

	if (refcount_dec_not_one(r))

		return false;

	mutex_lock(lock);

	if (!refcount_dec_and_test(r)) {

		mutex_unlock(lock);

		return false;

	}

	return true;

}

/*

 * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to

 * decrement when saturated at UINT_MAX.

 *

 * Provides release memory ordering, such that prior loads and stores are done

 * before, and provides a control dependency such that free() must come after.

 * See the comment on top.

 */

static inline __refcount_check

bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)

{

	if (refcount_dec_not_one(r))

		return false;

	spin_lock(lock);

	if (!refcount_dec_and_test(r)) {

		spin_unlock(lock);

		return false;

	}

	return true;

}

#endif /* _LINUX_REFCOUNT_H */

source "lib/Kconfig.kasan"

config DEBUG_REFCOUNT

	bool "Verbose refcount checks"

	help

	  Say Y here if you want reference counters (refcount_t and kref) to

	  generate WARNs on dubious usage. Without this refcount_t will still

	  be a saturating counter and avoid Use-After-Free by turning it into

	  a resource leak Denial-Of-Service.

	  Use of this option will increase kernel text size but will alert the

	  admin of potential abuse.

	  If in doubt, say "N".

endmenu # "Memory Debugging"

config ARCH_HAS_KCOV