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369 lines
12 KiB
369 lines
12 KiB
/* SPDX-License-Identifier: GPL-2.0 */ |
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/* |
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* Variant of atomic_t specialized for reference counts. |
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* |
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* The interface matches the atomic_t interface (to aid in porting) but only |
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* provides the few functions one should use for reference counting. |
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* |
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* Saturation semantics |
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* ==================== |
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* |
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* refcount_t differs from atomic_t in that the counter saturates at |
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* REFCOUNT_SATURATED and will not move once there. This avoids wrapping the |
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* counter and causing 'spurious' use-after-free issues. In order to avoid the |
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* cost associated with introducing cmpxchg() loops into all of the saturating |
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* operations, we temporarily allow the counter to take on an unchecked value |
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* and then explicitly set it to REFCOUNT_SATURATED on detecting that underflow |
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* or overflow has occurred. Although this is racy when multiple threads |
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* access the refcount concurrently, by placing REFCOUNT_SATURATED roughly |
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* equidistant from 0 and INT_MAX we minimise the scope for error: |
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* |
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* INT_MAX REFCOUNT_SATURATED UINT_MAX |
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* 0 (0x7fff_ffff) (0xc000_0000) (0xffff_ffff) |
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* +--------------------------------+----------------+----------------+ |
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* <---------- bad value! ----------> |
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* |
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* (in a signed view of the world, the "bad value" range corresponds to |
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* a negative counter value). |
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* |
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* As an example, consider a refcount_inc() operation that causes the counter |
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* to overflow: |
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* |
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* int old = atomic_fetch_add_relaxed(r); |
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* // old is INT_MAX, refcount now INT_MIN (0x8000_0000) |
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* if (old < 0) |
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* atomic_set(r, REFCOUNT_SATURATED); |
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* |
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* If another thread also performs a refcount_inc() operation between the two |
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* atomic operations, then the count will continue to edge closer to 0. If it |
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* reaches a value of 1 before /any/ of the threads reset it to the saturated |
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* value, then a concurrent refcount_dec_and_test() may erroneously free the |
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* underlying object. |
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* Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently |
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* 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK). |
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* With the current PID limit, if no batched refcounting operations are used and |
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* the attacker can't repeatedly trigger kernel oopses in the middle of refcount |
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* operations, this makes it impossible for a saturated refcount to leave the |
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* saturation range, even if it is possible for multiple uses of the same |
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* refcount to nest in the context of a single task: |
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* |
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* (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT = |
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* 0x40000000 / 0x400000 = 0x100 = 256 |
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* |
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* If hundreds of references are added/removed with a single refcounting |
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* operation, it may potentially be possible to leave the saturation range; but |
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* given the precise timing details involved with the round-robin scheduling of |
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* each thread manipulating the refcount and the need to hit the race multiple |
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* times in succession, there doesn't appear to be a practical avenue of attack |
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* even if using refcount_add() operations with larger increments. |
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* |
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* Memory ordering |
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* =============== |
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* |
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* Memory ordering rules are slightly relaxed wrt regular atomic_t functions |
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* and provide only what is strictly required for refcounts. |
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* |
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* The increments are fully relaxed; these will not provide ordering. The |
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* rationale is that whatever is used to obtain the object we're increasing the |
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* reference count on will provide the ordering. For locked data structures, |
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* its the lock acquire, for RCU/lockless data structures its the dependent |
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* load. |
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* |
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* Do note that inc_not_zero() provides a control dependency which will order |
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* future stores against the inc, this ensures we'll never modify the object |
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* if we did not in fact acquire a reference. |
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* |
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* The decrements will provide release order, such that all the prior loads and |
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* stores will be issued before, it also provides a control dependency, which |
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* will order us against the subsequent free(). |
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* |
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* The control dependency is against the load of the cmpxchg (ll/sc) that |
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* succeeded. This means the stores aren't fully ordered, but this is fine |
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* because the 1->0 transition indicates no concurrency. |
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* |
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* Note that the allocator is responsible for ordering things between free() |
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* and alloc(). |
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* |
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* The decrements dec_and_test() and sub_and_test() also provide acquire |
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* ordering on success. |
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* |
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*/ |
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#ifndef _LINUX_REFCOUNT_H |
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#define _LINUX_REFCOUNT_H |
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#include <linux/atomic.h> |
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#include <linux/bug.h> |
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#include <linux/compiler.h> |
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#include <linux/limits.h> |
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#include <linux/spinlock_types.h> |
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struct mutex; |
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/** |
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* typedef refcount_t - variant of atomic_t specialized for reference counts |
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* @refs: atomic_t counter field |
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* |
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* The counter saturates at REFCOUNT_SATURATED and will not move once |
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* there. This avoids wrapping the counter and causing 'spurious' |
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* use-after-free bugs. |
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*/ |
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typedef struct refcount_struct { |
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atomic_t refs; |
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} refcount_t; |
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#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), } |
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#define REFCOUNT_MAX INT_MAX |
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#define REFCOUNT_SATURATED (INT_MIN / 2) |
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enum refcount_saturation_type { |
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REFCOUNT_ADD_NOT_ZERO_OVF, |
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REFCOUNT_ADD_OVF, |
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REFCOUNT_ADD_UAF, |
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REFCOUNT_SUB_UAF, |
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REFCOUNT_DEC_LEAK, |
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}; |
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void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t); |
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/** |
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* refcount_set - set a refcount's value |
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* @r: the refcount |
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* @n: value to which the refcount will be set |
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*/ |
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static inline void refcount_set(refcount_t *r, int n) |
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{ |
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atomic_set(&r->refs, n); |
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} |
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/** |
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* refcount_read - get a refcount's value |
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* @r: the refcount |
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* |
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* Return: the refcount's value |
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*/ |
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static inline unsigned int refcount_read(const refcount_t *r) |
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{ |
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return atomic_read(&r->refs); |
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} |
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static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp) |
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{ |
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int old = refcount_read(r); |
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do { |
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if (!old) |
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break; |
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} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i)); |
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if (oldp) |
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*oldp = old; |
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if (unlikely(old < 0 || old + i < 0)) |
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refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF); |
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return old; |
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} |
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/** |
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* refcount_add_not_zero - add a value to a refcount unless it is 0 |
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* @i: the value to add to the refcount |
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* @r: the refcount |
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* |
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* Will saturate at REFCOUNT_SATURATED and WARN. |
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* |
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* Provides no memory ordering, it is assumed the caller has guaranteed the |
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* object memory to be stable (RCU, etc.). It does provide a control dependency |
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* and thereby orders future stores. See the comment on top. |
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* |
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* Use of this function is not recommended for the normal reference counting |
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* use case in which references are taken and released one at a time. In these |
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* cases, refcount_inc(), or one of its variants, should instead be used to |
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* increment a reference count. |
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* |
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* Return: false if the passed refcount is 0, true otherwise |
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*/ |
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static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r) |
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{ |
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return __refcount_add_not_zero(i, r, NULL); |
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} |
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static inline void __refcount_add(int i, refcount_t *r, int *oldp) |
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{ |
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int old = atomic_fetch_add_relaxed(i, &r->refs); |
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if (oldp) |
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*oldp = old; |
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if (unlikely(!old)) |
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refcount_warn_saturate(r, REFCOUNT_ADD_UAF); |
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else if (unlikely(old < 0 || old + i < 0)) |
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refcount_warn_saturate(r, REFCOUNT_ADD_OVF); |
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} |
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/** |
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* refcount_add - add a value to a refcount |
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* @i: the value to add to the refcount |
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* @r: the refcount |
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* |
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* Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED and WARN. |
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* |
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* Provides no memory ordering, it is assumed the caller has guaranteed the |
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* object memory to be stable (RCU, etc.). It does provide a control dependency |
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* and thereby orders future stores. See the comment on top. |
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* |
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* Use of this function is not recommended for the normal reference counting |
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* use case in which references are taken and released one at a time. In these |
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* cases, refcount_inc(), or one of its variants, should instead be used to |
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* increment a reference count. |
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*/ |
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static inline void refcount_add(int i, refcount_t *r) |
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{ |
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__refcount_add(i, r, NULL); |
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} |
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static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp) |
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{ |
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return __refcount_add_not_zero(1, r, oldp); |
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} |
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/** |
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* refcount_inc_not_zero - increment a refcount unless it is 0 |
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* @r: the refcount to increment |
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* |
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* Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED |
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* and WARN. |
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* |
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* Provides no memory ordering, it is assumed the caller has guaranteed the |
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* object memory to be stable (RCU, etc.). It does provide a control dependency |
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* and thereby orders future stores. See the comment on top. |
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* |
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* Return: true if the increment was successful, false otherwise |
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*/ |
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static inline __must_check bool refcount_inc_not_zero(refcount_t *r) |
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{ |
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return __refcount_inc_not_zero(r, NULL); |
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} |
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static inline void __refcount_inc(refcount_t *r, int *oldp) |
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{ |
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__refcount_add(1, r, oldp); |
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} |
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/** |
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* refcount_inc - increment a refcount |
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* @r: the refcount to increment |
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* |
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* Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN. |
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* |
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* Provides no memory ordering, it is assumed the caller already has a |
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* reference on the object. |
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* |
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* Will WARN if the refcount is 0, as this represents a possible use-after-free |
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* condition. |
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*/ |
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static inline void refcount_inc(refcount_t *r) |
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{ |
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__refcount_inc(r, NULL); |
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} |
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static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp) |
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{ |
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int old = atomic_fetch_sub_release(i, &r->refs); |
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if (oldp) |
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*oldp = old; |
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if (old == i) { |
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smp_acquire__after_ctrl_dep(); |
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return true; |
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} |
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if (unlikely(old < 0 || old - i < 0)) |
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refcount_warn_saturate(r, REFCOUNT_SUB_UAF); |
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return false; |
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} |
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/** |
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* refcount_sub_and_test - subtract from a refcount and test if it is 0 |
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* @i: amount to subtract from the refcount |
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* @r: the refcount |
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* |
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* Similar to atomic_dec_and_test(), but it will WARN, return false and |
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* ultimately leak on underflow and will fail to decrement when saturated |
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* at REFCOUNT_SATURATED. |
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* |
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* Provides release memory ordering, such that prior loads and stores are done |
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* before, and provides an acquire ordering on success such that free() |
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* must come after. |
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* |
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* Use of this function is not recommended for the normal reference counting |
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* use case in which references are taken and released one at a time. In these |
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* cases, refcount_dec(), or one of its variants, should instead be used to |
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* decrement a reference count. |
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* |
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* Return: true if the resulting refcount is 0, false otherwise |
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*/ |
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static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r) |
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{ |
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return __refcount_sub_and_test(i, r, NULL); |
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} |
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static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp) |
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{ |
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return __refcount_sub_and_test(1, r, oldp); |
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} |
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/** |
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* refcount_dec_and_test - decrement a refcount and test if it is 0 |
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* @r: the refcount |
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* |
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* Similar to atomic_dec_and_test(), it will WARN on underflow and fail to |
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* decrement when saturated at REFCOUNT_SATURATED. |
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* |
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* Provides release memory ordering, such that prior loads and stores are done |
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* before, and provides an acquire ordering on success such that free() |
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* must come after. |
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* |
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* Return: true if the resulting refcount is 0, false otherwise |
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*/ |
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static inline __must_check bool refcount_dec_and_test(refcount_t *r) |
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{ |
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return __refcount_dec_and_test(r, NULL); |
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} |
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static inline void __refcount_dec(refcount_t *r, int *oldp) |
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{ |
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int old = atomic_fetch_sub_release(1, &r->refs); |
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if (oldp) |
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*oldp = old; |
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if (unlikely(old <= 1)) |
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refcount_warn_saturate(r, REFCOUNT_DEC_LEAK); |
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} |
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/** |
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* refcount_dec - decrement a refcount |
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* @r: the refcount |
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* |
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* Similar to atomic_dec(), it will WARN on underflow and fail to decrement |
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* when saturated at REFCOUNT_SATURATED. |
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* |
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* Provides release memory ordering, such that prior loads and stores are done |
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* before. |
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*/ |
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static inline void refcount_dec(refcount_t *r) |
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{ |
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__refcount_dec(r, NULL); |
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} |
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extern __must_check bool refcount_dec_if_one(refcount_t *r); |
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extern __must_check bool refcount_dec_not_one(refcount_t *r); |
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extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock); |
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extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock); |
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extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r, |
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spinlock_t *lock, |
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unsigned long *flags); |
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#endif /* _LINUX_REFCOUNT_H */
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