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2024-05-10 093a6c67005148ae32a5c9e4553491b9f5c2457b 
 kernel/include/linux/refcount.h
....@@ -1,9 +1,101 @@
11 /* SPDX-License-Identifier: GPL-2.0 */
2
+/*
3
+ * Variant of atomic_t specialized for reference counts.
4
+ *
5
+ * 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
9
+ * ====================
10
+ *
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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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+ */
91
+
292 #ifndef _LINUX_REFCOUNT_H
393 #define _LINUX_REFCOUNT_H
494 
595 #include <linux/atomic.h>
96
+#include <linux/bug.h>
697 #include <linux/compiler.h>
98
+#include <linux/limits.h>
799 #include <linux/spinlock_types.h>
8100 
9101 struct mutex;
....@@ -12,7 +104,7 @@
12104  * struct refcount_t - variant of atomic_t specialized for reference counts
13105  * @refs: atomic_t counter field
14106  *
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- * The counter saturates at UINT_MAX and will not move once
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+ * The counter saturates at REFCOUNT_SATURATED and will not move once
16108  * there. This avoids wrapping the counter and causing 'spurious'
17109  * use-after-free bugs.
18110  */
....@@ -21,13 +113,25 @@
21113 } refcount_t;
22114 
23115 #define REFCOUNT_INIT(n)	{ .refs = ATOMIC_INIT(n), }
116
+#define REFCOUNT_MAX		INT_MAX
117
+#define REFCOUNT_SATURATED	(INT_MIN / 2)
118
+
119
+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,
125
+};
126
+
127
+void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);
24128 
25129 /**
26130  * refcount_set - set a refcount's value
27131  * @r: the refcount
28132  * @n: value to which the refcount will be set
29133  */
30
-static inline void refcount_set(refcount_t *r, unsigned int n)
134
+static inline void refcount_set(refcount_t *r, int n)
31135 {
32136 	atomic_set(&r->refs, n);
33137 }
....@@ -43,70 +147,217 @@
43147 	return atomic_read(&r->refs);
44148 }
45149 
46
-extern __must_check bool refcount_add_not_zero_checked(unsigned int i, refcount_t *r);
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-extern void refcount_add_checked(unsigned int i, refcount_t *r);
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-
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-extern __must_check bool refcount_inc_not_zero_checked(refcount_t *r);
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-extern void refcount_inc_checked(refcount_t *r);
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-
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-extern __must_check bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r);
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-
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-extern __must_check bool refcount_dec_and_test_checked(refcount_t *r);
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-extern void refcount_dec_checked(refcount_t *r);
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-
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-#ifdef CONFIG_REFCOUNT_FULL
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-
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-#define refcount_add_not_zero	refcount_add_not_zero_checked
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-#define refcount_add		refcount_add_checked
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-
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-#define refcount_inc_not_zero	refcount_inc_not_zero_checked
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-#define refcount_inc		refcount_inc_checked
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-
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-#define refcount_sub_and_test	refcount_sub_and_test_checked
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-
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-#define refcount_dec_and_test	refcount_dec_and_test_checked
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-#define refcount_dec		refcount_dec_checked
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-
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-#else
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-# ifdef CONFIG_ARCH_HAS_REFCOUNT
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-#  include <asm/refcount.h>
73
-# else
74
-static inline __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r)
150
+static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
75151 {
76
-	return atomic_add_unless(&r->refs, i, 0);
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+	int old = refcount_read(r);
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+
154
+	do {
155
+		if (!old)
156
+			break;
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+	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
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+
159
+	if (oldp)
160
+		*oldp = old;
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+
162
+	if (unlikely(old < 0 || old + i < 0))
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+		refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
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+
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+	return old;
77166 }
78167 
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-static inline void refcount_add(unsigned int i, refcount_t *r)
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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)
80187 {
81
-	atomic_add(i, &r->refs);
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+	return __refcount_add_not_zero(i, r, NULL);
82189 }
83190 
191
+static inline void __refcount_add(int i, refcount_t *r, int *oldp)
192
+{
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+	int old = atomic_fetch_add_relaxed(i, &r->refs);
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+
195
+	if (oldp)
196
+		*oldp = old;
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+
198
+	if (unlikely(!old))
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+		refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
200
+	else if (unlikely(old < 0 || old + i < 0))
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+		refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
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+}
203
+
204
+/**
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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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+
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+static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
226
+{
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+	return __refcount_add_not_zero(1, r, oldp);
228
+}
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+
230
+/**
231
+ * 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
242
+ */
84243 static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
85244 {
86
-	return atomic_add_unless(&r->refs, 1, 0);
245
+	return __refcount_inc_not_zero(r, NULL);
87246 }
88247 
248
+static inline void __refcount_inc(refcount_t *r, int *oldp)
249
+{
250
+	__refcount_add(1, r, oldp);
251
+}
252
+
253
+/**
254
+ * refcount_inc - increment a refcount
255
+ * @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.
261
+ *
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+ * Will WARN if the refcount is 0, as this represents a possible use-after-free
263
+ * condition.
264
+ */
89265 static inline void refcount_inc(refcount_t *r)
90266 {
91
-	atomic_inc(&r->refs);
267
+	__refcount_inc(r, NULL);
92268 }
93269 
94
-static inline __must_check bool refcount_sub_and_test(unsigned int i, refcount_t *r)
270
+static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
95271 {
96
-	return atomic_sub_and_test(i, &r->refs);
272
+	int old = atomic_fetch_sub_release(i, &r->refs);
273
+
274
+	if (oldp)
275
+		*oldp = old;
276
+
277
+	if (old == i) {
278
+		smp_acquire__after_ctrl_dep();
279
+		return true;
280
+	}
281
+
282
+	if (unlikely(old < 0 || old - i < 0))
283
+		refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
284
+
285
+	return false;
97286 }
98287 
288
+/**
289
+ * refcount_sub_and_test - subtract from a refcount and test if it is 0
290
+ * @i: amount to subtract from the refcount
291
+ * @r: the refcount
292
+ *
293
+ * Similar to atomic_dec_and_test(), but it will WARN, return false and
294
+ * ultimately leak on underflow and will fail to decrement when saturated
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+ * at REFCOUNT_SATURATED.
296
+ *
297
+ * Provides release memory ordering, such that prior loads and stores are done
298
+ * before, and provides an acquire ordering on success such that free()
299
+ * must come after.
300
+ *
301
+ * Use of this function is not recommended for the normal reference counting
302
+ * use case in which references are taken and released one at a time.  In these
303
+ * cases, refcount_dec(), or one of its variants, should instead be used to
304
+ * decrement a reference count.
305
+ *
306
+ * Return: true if the resulting refcount is 0, false otherwise
307
+ */
308
+static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
309
+{
310
+	return __refcount_sub_and_test(i, r, NULL);
311
+}
312
+
313
+static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
314
+{
315
+	return __refcount_sub_and_test(1, r, oldp);
316
+}
317
+
318
+/**
319
+ * refcount_dec_and_test - decrement a refcount and test if it is 0
320
+ * @r: the refcount
321
+ *
322
+ * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
323
+ * decrement when saturated at REFCOUNT_SATURATED.
324
+ *
325
+ * Provides release memory ordering, such that prior loads and stores are done
326
+ * before, and provides an acquire ordering on success such that free()
327
+ * must come after.
328
+ *
329
+ * Return: true if the resulting refcount is 0, false otherwise
330
+ */
99331 static inline __must_check bool refcount_dec_and_test(refcount_t *r)
100332 {
101
-	return atomic_dec_and_test(&r->refs);
333
+	return __refcount_dec_and_test(r, NULL);
102334 }
103335 
336
+static inline void __refcount_dec(refcount_t *r, int *oldp)
337
+{
338
+	int old = atomic_fetch_sub_release(1, &r->refs);
339
+
340
+	if (oldp)
341
+		*oldp = old;
342
+
343
+	if (unlikely(old <= 1))
344
+		refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
345
+}
346
+
347
+/**
348
+ * refcount_dec - decrement a refcount
349
+ * @r: the refcount
350
+ *
351
+ * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
352
+ * when saturated at REFCOUNT_SATURATED.
353
+ *
354
+ * Provides release memory ordering, such that prior loads and stores are done
355
+ * before.
356
+ */
104357 static inline void refcount_dec(refcount_t *r)
105358 {
106
-	atomic_dec(&r->refs);
359
+	__refcount_dec(r, NULL);
107360 }
108
-# endif /* !CONFIG_ARCH_HAS_REFCOUNT */
109
-#endif /* CONFIG_REFCOUNT_FULL */
110361 
111362 extern __must_check bool refcount_dec_if_one(refcount_t *r);
112363 extern __must_check bool refcount_dec_not_one(refcount_t *r);