+++ /dev/null
-/* 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>
-
-struct mutex;
-
-/**
- * typedef 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,
-};
-
-/**
- * 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);
-}
-
-static inline __must_check 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_acquire(&r->refs, &old, old + i));
-
- if (oldp)
- *oldp = old;
-
- 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 void __refcount_add(int i, refcount_t *r, int *oldp)
-{
- int old = atomic_add_return(i, &r->refs);
-
- if (oldp)
- *oldp = 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)
-{
- __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 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 bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
-{
- int old = atomic_sub_return_release(i, &r->refs);
-
- if (oldp)
- *oldp = old;
-
- if (old == i) {
- smp_acquire__after_ctrl_dep();
- return true;
- }
-
- 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_sub_return_release(1, &r->refs);
-
- if (oldp)
- *oldp = old;
-}
-
-/**
- * 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 */
projects / bcachefs-tools-debian / blobdiff