1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * fs/eventfd.c
4 *
5 * Copyright (C) 2007 Davide Libenzi <davidel@xmailserver.org>
6 *
7 */
8
9#include <linux/file.h>
10#include <linux/poll.h>
11#include <linux/init.h>
12#include <linux/fs.h>
13#include <linux/sched/signal.h>
14#include <linux/kernel.h>
15#include <linux/slab.h>
16#include <linux/list.h>
17#include <linux/spinlock.h>
18#include <linux/anon_inodes.h>
19#include <linux/syscalls.h>
20#include <linux/export.h>
21#include <linux/kref.h>
22#include <linux/eventfd.h>
23#include <linux/proc_fs.h>
24#include <linux/seq_file.h>
25#include <linux/idr.h>
26#include <linux/uio.h>
27
28static DEFINE_IDA(eventfd_ida);
29
30struct eventfd_ctx {
31 struct kref kref;
32 wait_queue_head_t wqh;
33 /*
34 * Every time that a write(2) is performed on an eventfd, the
35 * value of the __u64 being written is added to "count" and a
36 * wakeup is performed on "wqh". If EFD_SEMAPHORE flag was not
37 * specified, a read(2) will return the "count" value to userspace,
38 * and will reset "count" to zero. The kernel side eventfd_signal()
39 * also, adds to the "count" counter and issue a wakeup.
40 */
41 __u64 count;
42 unsigned int flags;
43 int id;
44};
45
46__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
47{
48 unsigned long flags;
49
50 /*
51 * Deadlock or stack overflow issues can happen if we recurse here
52 * through waitqueue wakeup handlers. If the caller users potentially
53 * nested waitqueues with custom wakeup handlers, then it should
54 * check eventfd_signal_allowed() before calling this function. If
55 * it returns false, the eventfd_signal() call should be deferred to a
56 * safe context.
57 */
58 if (WARN_ON_ONCE(current->in_eventfd))
59 return 0;
60
61 spin_lock_irqsave(&ctx->wqh.lock, flags);
62 current->in_eventfd = 1;
63 if (ULLONG_MAX - ctx->count < n)
64 n = ULLONG_MAX - ctx->count;
65 ctx->count += n;
66 if (waitqueue_active(wq_head: &ctx->wqh))
67 wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask);
68 current->in_eventfd = 0;
69 spin_unlock_irqrestore(lock: &ctx->wqh.lock, flags);
70
71 return n;
72}
73
74/**
75 * eventfd_signal - Adds @n to the eventfd counter.
76 * @ctx: [in] Pointer to the eventfd context.
77 * @n: [in] Value of the counter to be added to the eventfd internal counter.
78 * The value cannot be negative.
79 *
80 * This function is supposed to be called by the kernel in paths that do not
81 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
82 * value, and we signal this as overflow condition by returning a EPOLLERR
83 * to poll(2).
84 *
85 * Returns the amount by which the counter was incremented. This will be less
86 * than @n if the counter has overflowed.
87 */
88__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
89{
90 return eventfd_signal_mask(ctx, n, mask: 0);
91}
92EXPORT_SYMBOL_GPL(eventfd_signal);
93
94static void eventfd_free_ctx(struct eventfd_ctx *ctx)
95{
96 if (ctx->id >= 0)
97 ida_simple_remove(&eventfd_ida, ctx->id);
98 kfree(objp: ctx);
99}
100
101static void eventfd_free(struct kref *kref)
102{
103 struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
104
105 eventfd_free_ctx(ctx);
106}
107
108/**
109 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
110 * @ctx: [in] Pointer to eventfd context.
111 *
112 * The eventfd context reference must have been previously acquired either
113 * with eventfd_ctx_fdget() or eventfd_ctx_fileget().
114 */
115void eventfd_ctx_put(struct eventfd_ctx *ctx)
116{
117 kref_put(kref: &ctx->kref, release: eventfd_free);
118}
119EXPORT_SYMBOL_GPL(eventfd_ctx_put);
120
121static int eventfd_release(struct inode *inode, struct file *file)
122{
123 struct eventfd_ctx *ctx = file->private_data;
124
125 wake_up_poll(&ctx->wqh, EPOLLHUP);
126 eventfd_ctx_put(ctx);
127 return 0;
128}
129
130static __poll_t eventfd_poll(struct file *file, poll_table *wait)
131{
132 struct eventfd_ctx *ctx = file->private_data;
133 __poll_t events = 0;
134 u64 count;
135
136 poll_wait(filp: file, wait_address: &ctx->wqh, p: wait);
137
138 /*
139 * All writes to ctx->count occur within ctx->wqh.lock. This read
140 * can be done outside ctx->wqh.lock because we know that poll_wait
141 * takes that lock (through add_wait_queue) if our caller will sleep.
142 *
143 * The read _can_ therefore seep into add_wait_queue's critical
144 * section, but cannot move above it! add_wait_queue's spin_lock acts
145 * as an acquire barrier and ensures that the read be ordered properly
146 * against the writes. The following CAN happen and is safe:
147 *
148 * poll write
149 * ----------------- ------------
150 * lock ctx->wqh.lock (in poll_wait)
151 * count = ctx->count
152 * __add_wait_queue
153 * unlock ctx->wqh.lock
154 * lock ctx->qwh.lock
155 * ctx->count += n
156 * if (waitqueue_active)
157 * wake_up_locked_poll
158 * unlock ctx->qwh.lock
159 * eventfd_poll returns 0
160 *
161 * but the following, which would miss a wakeup, cannot happen:
162 *
163 * poll write
164 * ----------------- ------------
165 * count = ctx->count (INVALID!)
166 * lock ctx->qwh.lock
167 * ctx->count += n
168 * **waitqueue_active is false**
169 * **no wake_up_locked_poll!**
170 * unlock ctx->qwh.lock
171 * lock ctx->wqh.lock (in poll_wait)
172 * __add_wait_queue
173 * unlock ctx->wqh.lock
174 * eventfd_poll returns 0
175 */
176 count = READ_ONCE(ctx->count);
177
178 if (count > 0)
179 events |= EPOLLIN;
180 if (count == ULLONG_MAX)
181 events |= EPOLLERR;
182 if (ULLONG_MAX - 1 > count)
183 events |= EPOLLOUT;
184
185 return events;
186}
187
188void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
189{
190 lockdep_assert_held(&ctx->wqh.lock);
191
192 *cnt = ((ctx->flags & EFD_SEMAPHORE) && ctx->count) ? 1 : ctx->count;
193 ctx->count -= *cnt;
194}
195EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
196
197/**
198 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
199 * @ctx: [in] Pointer to eventfd context.
200 * @wait: [in] Wait queue to be removed.
201 * @cnt: [out] Pointer to the 64-bit counter value.
202 *
203 * Returns %0 if successful, or the following error codes:
204 *
205 * -EAGAIN : The operation would have blocked.
206 *
207 * This is used to atomically remove a wait queue entry from the eventfd wait
208 * queue head, and read/reset the counter value.
209 */
210int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
211 __u64 *cnt)
212{
213 unsigned long flags;
214
215 spin_lock_irqsave(&ctx->wqh.lock, flags);
216 eventfd_ctx_do_read(ctx, cnt);
217 __remove_wait_queue(wq_head: &ctx->wqh, wq_entry: wait);
218 if (*cnt != 0 && waitqueue_active(wq_head: &ctx->wqh))
219 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
220 spin_unlock_irqrestore(lock: &ctx->wqh.lock, flags);
221
222 return *cnt != 0 ? 0 : -EAGAIN;
223}
224EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
225
226static ssize_t eventfd_read(struct kiocb *iocb, struct iov_iter *to)
227{
228 struct file *file = iocb->ki_filp;
229 struct eventfd_ctx *ctx = file->private_data;
230 __u64 ucnt = 0;
231
232 if (iov_iter_count(i: to) < sizeof(ucnt))
233 return -EINVAL;
234 spin_lock_irq(lock: &ctx->wqh.lock);
235 if (!ctx->count) {
236 if ((file->f_flags & O_NONBLOCK) ||
237 (iocb->ki_flags & IOCB_NOWAIT)) {
238 spin_unlock_irq(lock: &ctx->wqh.lock);
239 return -EAGAIN;
240 }
241
242 if (wait_event_interruptible_locked_irq(ctx->wqh, ctx->count)) {
243 spin_unlock_irq(lock: &ctx->wqh.lock);
244 return -ERESTARTSYS;
245 }
246 }
247 eventfd_ctx_do_read(ctx, &ucnt);
248 current->in_eventfd = 1;
249 if (waitqueue_active(wq_head: &ctx->wqh))
250 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
251 current->in_eventfd = 0;
252 spin_unlock_irq(lock: &ctx->wqh.lock);
253 if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
254 return -EFAULT;
255
256 return sizeof(ucnt);
257}
258
259static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
260 loff_t *ppos)
261{
262 struct eventfd_ctx *ctx = file->private_data;
263 ssize_t res;
264 __u64 ucnt;
265
266 if (count < sizeof(ucnt))
267 return -EINVAL;
268 if (copy_from_user(to: &ucnt, from: buf, n: sizeof(ucnt)))
269 return -EFAULT;
270 if (ucnt == ULLONG_MAX)
271 return -EINVAL;
272 spin_lock_irq(lock: &ctx->wqh.lock);
273 res = -EAGAIN;
274 if (ULLONG_MAX - ctx->count > ucnt)
275 res = sizeof(ucnt);
276 else if (!(file->f_flags & O_NONBLOCK)) {
277 res = wait_event_interruptible_locked_irq(ctx->wqh,
278 ULLONG_MAX - ctx->count > ucnt);
279 if (!res)
280 res = sizeof(ucnt);
281 }
282 if (likely(res > 0)) {
283 ctx->count += ucnt;
284 current->in_eventfd = 1;
285 if (waitqueue_active(wq_head: &ctx->wqh))
286 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
287 current->in_eventfd = 0;
288 }
289 spin_unlock_irq(lock: &ctx->wqh.lock);
290
291 return res;
292}
293
294#ifdef CONFIG_PROC_FS
295static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
296{
297 struct eventfd_ctx *ctx = f->private_data;
298
299 spin_lock_irq(lock: &ctx->wqh.lock);
300 seq_printf(m, fmt: "eventfd-count: %16llx\n",
301 (unsigned long long)ctx->count);
302 spin_unlock_irq(lock: &ctx->wqh.lock);
303 seq_printf(m, fmt: "eventfd-id: %d\n", ctx->id);
304 seq_printf(m, fmt: "eventfd-semaphore: %d\n",
305 !!(ctx->flags & EFD_SEMAPHORE));
306}
307#endif
308
309static const struct file_operations eventfd_fops = {
310#ifdef CONFIG_PROC_FS
311 .show_fdinfo = eventfd_show_fdinfo,
312#endif
313 .release = eventfd_release,
314 .poll = eventfd_poll,
315 .read_iter = eventfd_read,
316 .write = eventfd_write,
317 .llseek = noop_llseek,
318};
319
320/**
321 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
322 * @fd: [in] Eventfd file descriptor.
323 *
324 * Returns a pointer to the eventfd file structure in case of success, or the
325 * following error pointer:
326 *
327 * -EBADF : Invalid @fd file descriptor.
328 * -EINVAL : The @fd file descriptor is not an eventfd file.
329 */
330struct file *eventfd_fget(int fd)
331{
332 struct file *file;
333
334 file = fget(fd);
335 if (!file)
336 return ERR_PTR(error: -EBADF);
337 if (file->f_op != &eventfd_fops) {
338 fput(file);
339 return ERR_PTR(error: -EINVAL);
340 }
341
342 return file;
343}
344EXPORT_SYMBOL_GPL(eventfd_fget);
345
346/**
347 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
348 * @fd: [in] Eventfd file descriptor.
349 *
350 * Returns a pointer to the internal eventfd context, otherwise the error
351 * pointers returned by the following functions:
352 *
353 * eventfd_fget
354 */
355struct eventfd_ctx *eventfd_ctx_fdget(int fd)
356{
357 struct eventfd_ctx *ctx;
358 struct fd f = fdget(fd);
359 if (!f.file)
360 return ERR_PTR(error: -EBADF);
361 ctx = eventfd_ctx_fileget(file: f.file);
362 fdput(fd: f);
363 return ctx;
364}
365EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
366
367/**
368 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
369 * @file: [in] Eventfd file pointer.
370 *
371 * Returns a pointer to the internal eventfd context, otherwise the error
372 * pointer:
373 *
374 * -EINVAL : The @fd file descriptor is not an eventfd file.
375 */
376struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
377{
378 struct eventfd_ctx *ctx;
379
380 if (file->f_op != &eventfd_fops)
381 return ERR_PTR(error: -EINVAL);
382
383 ctx = file->private_data;
384 kref_get(kref: &ctx->kref);
385 return ctx;
386}
387EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
388
389static int do_eventfd(unsigned int count, int flags)
390{
391 struct eventfd_ctx *ctx;
392 struct file *file;
393 int fd;
394
395 /* Check the EFD_* constants for consistency. */
396 BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
397 BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
398
399 if (flags & ~EFD_FLAGS_SET)
400 return -EINVAL;
401
402 ctx = kmalloc(size: sizeof(*ctx), GFP_KERNEL);
403 if (!ctx)
404 return -ENOMEM;
405
406 kref_init(kref: &ctx->kref);
407 init_waitqueue_head(&ctx->wqh);
408 ctx->count = count;
409 ctx->flags = flags;
410 ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
411
412 flags &= EFD_SHARED_FCNTL_FLAGS;
413 flags |= O_RDWR;
414 fd = get_unused_fd_flags(flags);
415 if (fd < 0)
416 goto err;
417
418 file = anon_inode_getfile(name: "[eventfd]", fops: &eventfd_fops, priv: ctx, flags);
419 if (IS_ERR(ptr: file)) {
420 put_unused_fd(fd);
421 fd = PTR_ERR(ptr: file);
422 goto err;
423 }
424
425 file->f_mode |= FMODE_NOWAIT;
426 fd_install(fd, file);
427 return fd;
428err:
429 eventfd_free_ctx(ctx);
430 return fd;
431}
432
433SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
434{
435 return do_eventfd(count, flags);
436}
437
438SYSCALL_DEFINE1(eventfd, unsigned int, count)
439{
440 return do_eventfd(count, flags: 0);
441}
442
443

source code of linux/fs/eventfd.c