1 /* SPDX-License-Identifier: GPL-2.0 */
3 * Variant of atomic_t specialized for reference counts.
5 * The interface matches the atomic_t interface (to aid in porting) but only
6 * provides the few functions one should use for reference counting.
11 * refcount_t differs from atomic_t in that the counter saturates at
12 * REFCOUNT_SATURATED and will not move once there. This avoids wrapping the
13 * counter and causing 'spurious' use-after-free issues. In order to avoid the
14 * cost associated with introducing cmpxchg() loops into all of the saturating
15 * operations, we temporarily allow the counter to take on an unchecked value
16 * and then explicitly set it to REFCOUNT_SATURATED on detecting that underflow
17 * or overflow has occurred. Although this is racy when multiple threads
18 * access the refcount concurrently, by placing REFCOUNT_SATURATED roughly
19 * equidistant from 0 and INT_MAX we minimise the scope for error:
21 * INT_MAX REFCOUNT_SATURATED UINT_MAX
22 * 0 (0x7fff_ffff) (0xc000_0000) (0xffff_ffff)
23 * +--------------------------------+----------------+----------------+
24 * <---------- bad value! ---------->
26 * (in a signed view of the world, the "bad value" range corresponds to
27 * a negative counter value).
29 * As an example, consider a refcount_inc() operation that causes the counter
32 * int old = atomic_fetch_add_relaxed(r);
33 * // old is INT_MAX, refcount now INT_MIN (0x8000_0000)
35 * atomic_set(r, REFCOUNT_SATURATED);
37 * If another thread also performs a refcount_inc() operation between the two
38 * atomic operations, then the count will continue to edge closer to 0. If it
39 * reaches a value of 1 before /any/ of the threads reset it to the saturated
40 * value, then a concurrent refcount_dec_and_test() may erroneously free the
42 * Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
43 * 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
44 * With the current PID limit, if no batched refcounting operations are used and
45 * the attacker can't repeatedly trigger kernel oopses in the middle of refcount
46 * operations, this makes it impossible for a saturated refcount to leave the
47 * saturation range, even if it is possible for multiple uses of the same
48 * refcount to nest in the context of a single task:
50 * (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
51 * 0x40000000 / 0x400000 = 0x100 = 256
53 * If hundreds of references are added/removed with a single refcounting
54 * operation, it may potentially be possible to leave the saturation range; but
55 * given the precise timing details involved with the round-robin scheduling of
56 * each thread manipulating the refcount and the need to hit the race multiple
57 * times in succession, there doesn't appear to be a practical avenue of attack
58 * even if using refcount_add() operations with larger increments.
63 * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
64 * and provide only what is strictly required for refcounts.
66 * The increments are fully relaxed; these will not provide ordering. The
67 * rationale is that whatever is used to obtain the object we're increasing the
68 * reference count on will provide the ordering. For locked data structures,
69 * its the lock acquire, for RCU/lockless data structures its the dependent
72 * Do note that inc_not_zero() provides a control dependency which will order
73 * future stores against the inc, this ensures we'll never modify the object
74 * if we did not in fact acquire a reference.
76 * The decrements will provide release order, such that all the prior loads and
77 * stores will be issued before, it also provides a control dependency, which
78 * will order us against the subsequent free().
80 * The control dependency is against the load of the cmpxchg (ll/sc) that
81 * succeeded. This means the stores aren't fully ordered, but this is fine
82 * because the 1->0 transition indicates no concurrency.
84 * Note that the allocator is responsible for ordering things between free()
87 * The decrements dec_and_test() and sub_and_test() also provide acquire
88 * ordering on success.
92 #ifndef _LINUX_REFCOUNT_H
93 #define _LINUX_REFCOUNT_H
95 #include <linux/atomic.h>
96 #include <linux/bug.h>
97 #include <linux/compiler.h>
98 #include <linux/limits.h>
99 #include <linux/spinlock_types.h>
104 * struct refcount_t - variant of atomic_t specialized for reference counts
105 * @refs: atomic_t counter field
107 * The counter saturates at REFCOUNT_SATURATED and will not move once
108 * there. This avoids wrapping the counter and causing 'spurious'
109 * use-after-free bugs.
111 typedef struct refcount_struct {
115 #define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
116 #define REFCOUNT_MAX INT_MAX
117 #define REFCOUNT_SATURATED (INT_MIN / 2)
119 enum refcount_saturation_type {
120 REFCOUNT_ADD_NOT_ZERO_OVF,
127 void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);
130 * refcount_set - set a refcount's value
132 * @n: value to which the refcount will be set
134 static inline void refcount_set(refcount_t *r, int n)
136 atomic_set(&r->refs, n);
140 * refcount_read - get a refcount's value
143 * Return: the refcount's value
145 static inline unsigned int refcount_read(const refcount_t *r)
147 return atomic_read(&r->refs);
151 * refcount_add_not_zero - add a value to a refcount unless it is 0
152 * @i: the value to add to the refcount
155 * Will saturate at REFCOUNT_SATURATED and WARN.
157 * Provides no memory ordering, it is assumed the caller has guaranteed the
158 * object memory to be stable (RCU, etc.). It does provide a control dependency
159 * and thereby orders future stores. See the comment on top.
161 * Use of this function is not recommended for the normal reference counting
162 * use case in which references are taken and released one at a time. In these
163 * cases, refcount_inc(), or one of its variants, should instead be used to
164 * increment a reference count.
166 * Return: false if the passed refcount is 0, true otherwise
168 static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
170 int old = refcount_read(r);
175 } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
177 if (unlikely(old < 0 || old + i < 0))
178 refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
184 * refcount_add - add a value to a refcount
185 * @i: the value to add to the refcount
188 * Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED and WARN.
190 * Provides no memory ordering, it is assumed the caller has guaranteed the
191 * object memory to be stable (RCU, etc.). It does provide a control dependency
192 * and thereby orders future stores. See the comment on top.
194 * Use of this function is not recommended for the normal reference counting
195 * use case in which references are taken and released one at a time. In these
196 * cases, refcount_inc(), or one of its variants, should instead be used to
197 * increment a reference count.
199 static inline void refcount_add(int i, refcount_t *r)
201 int old = atomic_fetch_add_relaxed(i, &r->refs);
204 refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
205 else if (unlikely(old < 0 || old + i < 0))
206 refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
210 * refcount_inc_not_zero - increment a refcount unless it is 0
211 * @r: the refcount to increment
213 * Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
216 * Provides no memory ordering, it is assumed the caller has guaranteed the
217 * object memory to be stable (RCU, etc.). It does provide a control dependency
218 * and thereby orders future stores. See the comment on top.
220 * Return: true if the increment was successful, false otherwise
222 static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
224 return refcount_add_not_zero(1, r);
228 * refcount_inc - increment a refcount
229 * @r: the refcount to increment
231 * Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
233 * Provides no memory ordering, it is assumed the caller already has a
234 * reference on the object.
236 * Will WARN if the refcount is 0, as this represents a possible use-after-free
239 static inline void refcount_inc(refcount_t *r)
245 * refcount_sub_and_test - subtract from a refcount and test if it is 0
246 * @i: amount to subtract from the refcount
249 * Similar to atomic_dec_and_test(), but it will WARN, return false and
250 * ultimately leak on underflow and will fail to decrement when saturated
251 * at REFCOUNT_SATURATED.
253 * Provides release memory ordering, such that prior loads and stores are done
254 * before, and provides an acquire ordering on success such that free()
257 * Use of this function is not recommended for the normal reference counting
258 * use case in which references are taken and released one at a time. In these
259 * cases, refcount_dec(), or one of its variants, should instead be used to
260 * decrement a reference count.
262 * Return: true if the resulting refcount is 0, false otherwise
264 static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
266 int old = atomic_fetch_sub_release(i, &r->refs);
269 smp_acquire__after_ctrl_dep();
273 if (unlikely(old < 0 || old - i < 0))
274 refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
280 * refcount_dec_and_test - decrement a refcount and test if it is 0
283 * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
284 * decrement when saturated at REFCOUNT_SATURATED.
286 * Provides release memory ordering, such that prior loads and stores are done
287 * before, and provides an acquire ordering on success such that free()
290 * Return: true if the resulting refcount is 0, false otherwise
292 static inline __must_check bool refcount_dec_and_test(refcount_t *r)
294 return refcount_sub_and_test(1, r);
298 * refcount_dec - decrement a refcount
301 * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
302 * when saturated at REFCOUNT_SATURATED.
304 * Provides release memory ordering, such that prior loads and stores are done
307 static inline void refcount_dec(refcount_t *r)
309 if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
310 refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
313 extern __must_check bool refcount_dec_if_one(refcount_t *r);
314 extern __must_check bool refcount_dec_not_one(refcount_t *r);
315 extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);
316 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
317 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
319 unsigned long *flags);
320 #endif /* _LINUX_REFCOUNT_H */