308 lines
10 KiB
C
308 lines
10 KiB
C
/* 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. Given the precise timing details involved with the
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* round-robin scheduling of each thread manipulating the refcount and the need
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* to hit the race multiple times in succession, there doesn't appear to be a
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* practical avenue of attack even if using refcount_add() operations with
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* 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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* struct 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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/**
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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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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 (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 - 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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int old = atomic_fetch_add_relaxed(i, &r->refs);
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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_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_add_not_zero(1, r);
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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_add(1, r);
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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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int old = atomic_fetch_sub_release(i, &r->refs);
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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_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_sub_and_test(1, r);
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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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if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
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refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
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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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