1/* SPDX-License-Identifier: GPL-2.0-only */
2#ifndef _LINUX_FILE_REF_H
3#define _LINUX_FILE_REF_H
4
5#include <linux/atomic.h>
6#include <linux/preempt.h>
7#include <linux/types.h>
8
9/*
10 * file_ref is a reference count implementation specifically for use by
11 * files. It takes inspiration from rcuref but differs in key aspects
12 * such as support for SLAB_TYPESAFE_BY_RCU type caches.
13 *
14 * FILE_REF_ONEREF FILE_REF_MAXREF
15 * 0x0000000000000000UL 0x7FFFFFFFFFFFFFFFUL
16 * <-------------------valid ------------------->
17 *
18 * FILE_REF_SATURATED
19 * 0x8000000000000000UL 0xA000000000000000UL 0xBFFFFFFFFFFFFFFFUL
20 * <-----------------------saturation zone---------------------->
21 *
22 * FILE_REF_RELEASED FILE_REF_DEAD
23 * 0xC000000000000000UL 0xE000000000000000UL
24 * <-------------------dead zone------------------->
25 *
26 * FILE_REF_NOREF
27 * 0xFFFFFFFFFFFFFFFFUL
28 */
29
30#ifdef CONFIG_64BIT
31#define FILE_REF_ONEREF 0x0000000000000000UL
32#define FILE_REF_MAXREF 0x7FFFFFFFFFFFFFFFUL
33#define FILE_REF_SATURATED 0xA000000000000000UL
34#define FILE_REF_RELEASED 0xC000000000000000UL
35#define FILE_REF_DEAD 0xE000000000000000UL
36#define FILE_REF_NOREF 0xFFFFFFFFFFFFFFFFUL
37#else
38#define FILE_REF_ONEREF 0x00000000U
39#define FILE_REF_MAXREF 0x7FFFFFFFU
40#define FILE_REF_SATURATED 0xA0000000U
41#define FILE_REF_RELEASED 0xC0000000U
42#define FILE_REF_DEAD 0xE0000000U
43#define FILE_REF_NOREF 0xFFFFFFFFU
44#endif
45
46typedef struct {
47#ifdef CONFIG_64BIT
48 atomic64_t refcnt;
49#else
50 atomic_t refcnt;
51#endif
52} file_ref_t;
53
54/**
55 * file_ref_init - Initialize a file reference count
56 * @ref: Pointer to the reference count
57 * @cnt: The initial reference count typically '1'
58 */
59static inline void file_ref_init(file_ref_t *ref, unsigned long cnt)
60{
61 atomic_long_set(v: &ref->refcnt, i: cnt - 1);
62}
63
64bool __file_ref_put(file_ref_t *ref, unsigned long cnt);
65
66/**
67 * file_ref_get - Acquire one reference on a file
68 * @ref: Pointer to the reference count
69 *
70 * Similar to atomic_inc_not_zero() but saturates at FILE_REF_MAXREF.
71 *
72 * Provides full memory ordering.
73 *
74 * Return: False if the attempt to acquire a reference failed. This happens
75 * when the last reference has been put already. True if a reference
76 * was successfully acquired
77 */
78static __always_inline __must_check bool file_ref_get(file_ref_t *ref)
79{
80 /*
81 * Unconditionally increase the reference count with full
82 * ordering. The saturation and dead zones provide enough
83 * tolerance for this.
84 *
85 * If this indicates negative the file in question the fail can
86 * be freed and immediately reused due to SLAB_TYPSAFE_BY_RCU.
87 * Hence, unconditionally altering the file reference count to
88 * e.g., reset the file reference count back to the middle of
89 * the deadzone risk end up marking someone else's file as dead
90 * behind their back.
91 *
92 * It would be possible to do a careful:
93 *
94 * cnt = atomic_long_inc_return();
95 * if (likely(cnt >= 0))
96 * return true;
97 *
98 * and then something like:
99 *
100 * if (cnt >= FILE_REF_RELEASE)
101 * atomic_long_try_cmpxchg(&ref->refcnt, &cnt, FILE_REF_DEAD),
102 *
103 * to set the value back to the middle of the deadzone. But it's
104 * practically impossible to go from FILE_REF_DEAD to
105 * FILE_REF_ONEREF. It would need 2305843009213693952/2^61
106 * file_ref_get()s to resurrect such a dead file.
107 */
108 return !atomic_long_add_negative(i: 1, v: &ref->refcnt);
109}
110
111/**
112 * file_ref_inc - Acquire one reference on a file
113 * @ref: Pointer to the reference count
114 *
115 * Acquire an additional reference on a file. Warns if the caller didn't
116 * already hold a reference.
117 */
118static __always_inline void file_ref_inc(file_ref_t *ref)
119{
120 long prior = atomic_long_fetch_inc_relaxed(v: &ref->refcnt);
121 WARN_ONCE(prior < 0, "file_ref_inc() on a released file reference");
122}
123
124/**
125 * file_ref_put -- Release a file reference
126 * @ref: Pointer to the reference count
127 *
128 * Provides release memory ordering, such that prior loads and stores
129 * are done before, and provides an acquire ordering on success such
130 * that free() must come after.
131 *
132 * Return: True if this was the last reference with no future references
133 * possible. This signals the caller that it can safely release
134 * the object which is protected by the reference counter.
135 * False if there are still active references or the put() raced
136 * with a concurrent get()/put() pair. Caller is not allowed to
137 * release the protected object.
138 */
139static __always_inline __must_check bool file_ref_put(file_ref_t *ref)
140{
141 long cnt;
142
143 /*
144 * While files are SLAB_TYPESAFE_BY_RCU and thus file_ref_put()
145 * calls don't risk UAFs when a file is recyclyed, it is still
146 * vulnerable to UAFs caused by freeing the whole slab page once
147 * it becomes unused. Prevent file_ref_put() from being
148 * preempted protects against this.
149 */
150 guard(preempt)();
151 /*
152 * Unconditionally decrease the reference count. The saturation
153 * and dead zones provide enough tolerance for this. If this
154 * fails then we need to handle the last reference drop and
155 * cases inside the saturation and dead zones.
156 */
157 cnt = atomic_long_dec_return(v: &ref->refcnt);
158 if (cnt >= 0)
159 return false;
160 return __file_ref_put(ref, cnt);
161}
162
163/**
164 * file_ref_put_close - drop a reference expecting it would transition to FILE_REF_NOREF
165 * @ref: Pointer to the reference count
166 *
167 * Semantically it is equivalent to calling file_ref_put(), but it trades lower
168 * performance in face of other CPUs also modifying the refcount for higher
169 * performance when this happens to be the last reference.
170 *
171 * For the last reference file_ref_put() issues 2 atomics. One to drop the
172 * reference and another to transition it to FILE_REF_DEAD. This routine does
173 * the work in one step, but in order to do it has to pre-read the variable which
174 * decreases scalability.
175 *
176 * Use with close() et al, stick to file_ref_put() by default.
177 */
178static __always_inline __must_check bool file_ref_put_close(file_ref_t *ref)
179{
180 long old;
181
182 old = atomic_long_read(v: &ref->refcnt);
183 if (likely(old == FILE_REF_ONEREF)) {
184 if (likely(atomic_long_try_cmpxchg(&ref->refcnt, &old, FILE_REF_DEAD)))
185 return true;
186 }
187 return file_ref_put(ref);
188}
189
190/**
191 * file_ref_read - Read the number of file references
192 * @ref: Pointer to the reference count
193 *
194 * Return: The number of held references (0 ... N)
195 */
196static inline unsigned long file_ref_read(file_ref_t *ref)
197{
198 unsigned long c = atomic_long_read(v: &ref->refcnt);
199
200 /* Return 0 if within the DEAD zone. */
201 return c >= FILE_REF_RELEASED ? 0 : c + 1;
202}
203
204/*
205 * __file_ref_read_raw - Return the value stored in ref->refcnt
206 * @ref: Pointer to the reference count
207 *
208 * Return: The raw value found in the counter
209 *
210 * A hack for file_needs_f_pos_lock(), you probably want to use
211 * file_ref_read() instead.
212 */
213static inline unsigned long __file_ref_read_raw(file_ref_t *ref)
214{
215 return atomic_long_read(v: &ref->refcnt);
216}
217
218#endif
219

source code of linux/include/linux/file_ref.h

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