diff options
Diffstat (limited to 'lib/refcount.c')
| -rw-r--r-- | lib/refcount.c | 255 |
1 files changed, 27 insertions, 228 deletions
diff --git a/lib/refcount.c b/lib/refcount.c index 6e904af0fb3e..ebac8b7d15a7 100644 --- a/lib/refcount.c +++ b/lib/refcount.c @@ -1,41 +1,6 @@ // 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. - * - * 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(). - * - * The decrements dec_and_test() and sub_and_test() also provide acquire - * ordering on success. - * + * Out-of-line refcount functions. */ #include <linux/mutex.h> @@ -43,199 +8,33 @@ #include <linux/spinlock.h> #include <linux/bug.h> -/** - * refcount_add_not_zero_checked - add a value to a refcount unless it is 0 - * @i: the value to add to the refcount - * @r: the refcount - * - * 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. - * - * 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 - */ -bool refcount_add_not_zero_checked(unsigned int i, refcount_t *r) -{ - unsigned int new, val = atomic_read(&r->refs); - - do { - if (!val) - return false; - - if (unlikely(val == UINT_MAX)) - return true; - - new = val + i; - if (new < val) - new = UINT_MAX; - - } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); - - WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); - - return true; -} -EXPORT_SYMBOL(refcount_add_not_zero_checked); - -/** - * refcount_add_checked - 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 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. - * - * 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. - */ -void refcount_add_checked(unsigned int i, refcount_t *r) -{ - WARN_ONCE(!refcount_add_not_zero_checked(i, r), "refcount_t: addition on 0; use-after-free.\n"); -} -EXPORT_SYMBOL(refcount_add_checked); - -/** - * refcount_inc_not_zero_checked - increment a refcount unless it is 0 - * @r: the refcount to increment - * - * Similar to atomic_inc_not_zero(), but 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. - * - * Return: true if the increment was successful, false otherwise - */ -bool refcount_inc_not_zero_checked(refcount_t *r) -{ - unsigned int new, val = atomic_read(&r->refs); - - do { - new = val + 1; - - if (!val) - return false; - - if (unlikely(!new)) - return true; - - } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); +#define REFCOUNT_WARN(str) WARN_ONCE(1, "refcount_t: " str ".\n") - WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); - - return true; -} -EXPORT_SYMBOL(refcount_inc_not_zero_checked); - -/** - * refcount_inc_checked - increment a refcount - * @r: the refcount to increment - * - * Similar to atomic_inc(), but 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 if the refcount is 0, as this represents a possible use-after-free - * condition. - */ -void refcount_inc_checked(refcount_t *r) +void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t) { - WARN_ONCE(!refcount_inc_not_zero_checked(r), "refcount_t: increment on 0; use-after-free.\n"); -} -EXPORT_SYMBOL(refcount_inc_checked); - -/** - * refcount_sub_and_test_checked - 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 UINT_MAX. - * - * 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 - */ -bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r) -{ - unsigned int new, val = atomic_read(&r->refs); - - do { - if (unlikely(val == UINT_MAX)) - return false; - - new = val - i; - if (new > val) { - WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n"); - return false; - } - - } while (!atomic_try_cmpxchg_release(&r->refs, &val, new)); - - if (!new) { - smp_acquire__after_ctrl_dep(); - return true; + refcount_set(r, REFCOUNT_SATURATED); + + switch (t) { + case REFCOUNT_ADD_NOT_ZERO_OVF: + REFCOUNT_WARN("saturated; leaking memory"); + break; + case REFCOUNT_ADD_OVF: + REFCOUNT_WARN("saturated; leaking memory"); + break; + case REFCOUNT_ADD_UAF: + REFCOUNT_WARN("addition on 0; use-after-free"); + break; + case REFCOUNT_SUB_UAF: + REFCOUNT_WARN("underflow; use-after-free"); + break; + case REFCOUNT_DEC_LEAK: + REFCOUNT_WARN("decrement hit 0; leaking memory"); + break; + default: + REFCOUNT_WARN("unknown saturation event!?"); } - return false; - -} -EXPORT_SYMBOL(refcount_sub_and_test_checked); - -/** - * refcount_dec_and_test_checked - 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 UINT_MAX. - * - * 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 - */ -bool refcount_dec_and_test_checked(refcount_t *r) -{ - return refcount_sub_and_test_checked(1, r); -} -EXPORT_SYMBOL(refcount_dec_and_test_checked); - -/** - * refcount_dec_checked - decrement a refcount - * @r: the refcount - * - * 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. - */ -void refcount_dec_checked(refcount_t *r) -{ - WARN_ONCE(refcount_dec_and_test_checked(r), "refcount_t: decrement hit 0; leaking memory.\n"); } -EXPORT_SYMBOL(refcount_dec_checked); +EXPORT_SYMBOL(refcount_warn_saturate); /** * refcount_dec_if_one - decrement a refcount if it is 1 @@ -277,7 +76,7 @@ bool refcount_dec_not_one(refcount_t *r) unsigned int new, val = atomic_read(&r->refs); do { - if (unlikely(val == UINT_MAX)) + if (unlikely(val == REFCOUNT_SATURATED)) return true; if (val == 1) @@ -302,7 +101,7 @@ EXPORT_SYMBOL(refcount_dec_not_one); * @lock: the mutex to be locked * * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail - * to decrement when saturated at UINT_MAX. + * to decrement when saturated at REFCOUNT_SATURATED. * * Provides release memory ordering, such that prior loads and stores are done * before, and provides a control dependency such that free() must come after. @@ -333,7 +132,7 @@ EXPORT_SYMBOL(refcount_dec_and_mutex_lock); * @lock: the spinlock to be locked * * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to - * decrement when saturated at UINT_MAX. + * decrement when saturated at REFCOUNT_SATURATED. * * Provides release memory ordering, such that prior loads and stores are done * before, and provides a control dependency such that free() must come after. |