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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
28 DEFINE_PER_CPU(int, eventfd_wake_count);
29
30 static DEFINE_IDA(eventfd_ida);
31
32 struct eventfd_ctx {
33 struct kref kref;
34 wait_queue_head_t wqh;
35 /*
36 * Every time that a write(2) is performed on an eventfd, the
37 * value of the __u64 being written is added to "count" and a
38 * wakeup is performed on "wqh". A read(2) will return the "count"
39 * value to userspace, and will reset "count" to zero. The kernel
40 * side eventfd_signal() also, adds to the "count" counter and
41 * issue a wakeup.
42 */
43 __u64 count;
44 unsigned int flags;
45 int id;
46 };
47
48 /**
49 * eventfd_signal - Adds @n to the eventfd counter.
50 * @ctx: [in] Pointer to the eventfd context.
51 * @n: [in] Value of the counter to be added to the eventfd internal counter.
52 * The value cannot be negative.
53 *
54 * This function is supposed to be called by the kernel in paths that do not
55 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
56 * value, and we signal this as overflow condition by returning a EPOLLERR
57 * to poll(2).
58 *
59 * Returns the amount by which the counter was incremented. This will be less
60 * than @n if the counter has overflowed.
61 */
62 __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
63 {
64 unsigned long flags;
65
66 /*
67 * Deadlock or stack overflow issues can happen if we recurse here
68 * through waitqueue wakeup handlers. If the caller users potentially
69 * nested waitqueues with custom wakeup handlers, then it should
70 * check eventfd_signal_count() before calling this function. If
71 * it returns true, the eventfd_signal() call should be deferred to a
72 * safe context.
73 */
74 if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
75 return 0;
76
77 spin_lock_irqsave(&ctx->wqh.lock, flags);
78 this_cpu_inc(eventfd_wake_count);
79 if (ULLONG_MAX - ctx->count < n)
80 n = ULLONG_MAX - ctx->count;
81 ctx->count += n;
82 if (waitqueue_active(&ctx->wqh))
83 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
84 this_cpu_dec(eventfd_wake_count);
85 spin_unlock_irqrestore(&ctx->wqh.lock, flags);
86
87 return n;
88 }
89 EXPORT_SYMBOL_GPL(eventfd_signal);
90
91 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
92 {
93 if (ctx->id >= 0)
94 ida_simple_remove(&eventfd_ida, ctx->id);
95 kfree(ctx);
96 }
97
98 static void eventfd_free(struct kref *kref)
99 {
100 struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
101
102 eventfd_free_ctx(ctx);
103 }
104
105 /**
106 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
107 * @ctx: [in] Pointer to eventfd context.
108 *
109 * The eventfd context reference must have been previously acquired either
110 * with eventfd_ctx_fdget() or eventfd_ctx_fileget().
111 */
112 void eventfd_ctx_put(struct eventfd_ctx *ctx)
113 {
114 kref_put(&ctx->kref, eventfd_free);
115 }
116 EXPORT_SYMBOL_GPL(eventfd_ctx_put);
117
118 static int eventfd_release(struct inode *inode, struct file *file)
119 {
120 struct eventfd_ctx *ctx = file->private_data;
121
122 wake_up_poll(&ctx->wqh, EPOLLHUP);
123 eventfd_ctx_put(ctx);
124 return 0;
125 }
126
127 static __poll_t eventfd_poll(struct file *file, poll_table *wait)
128 {
129 struct eventfd_ctx *ctx = file->private_data;
130 __poll_t events = 0;
131 u64 count;
132
133 poll_wait(file, &ctx->wqh, wait);
134
135 /*
136 * All writes to ctx->count occur within ctx->wqh.lock. This read
137 * can be done outside ctx->wqh.lock because we know that poll_wait
138 * takes that lock (through add_wait_queue) if our caller will sleep.
139 *
140 * The read _can_ therefore seep into add_wait_queue's critical
141 * section, but cannot move above it! add_wait_queue's spin_lock acts
142 * as an acquire barrier and ensures that the read be ordered properly
143 * against the writes. The following CAN happen and is safe:
144 *
145 * poll write
146 * ----------------- ------------
147 * lock ctx->wqh.lock (in poll_wait)
148 * count = ctx->count
149 * __add_wait_queue
150 * unlock ctx->wqh.lock
151 * lock ctx->qwh.lock
152 * ctx->count += n
153 * if (waitqueue_active)
154 * wake_up_locked_poll
155 * unlock ctx->qwh.lock
156 * eventfd_poll returns 0
157 *
158 * but the following, which would miss a wakeup, cannot happen:
159 *
160 * poll write
161 * ----------------- ------------
162 * count = ctx->count (INVALID!)
163 * lock ctx->qwh.lock
164 * ctx->count += n
165 * **waitqueue_active is false**
166 * **no wake_up_locked_poll!**
167 * unlock ctx->qwh.lock
168 * lock ctx->wqh.lock (in poll_wait)
169 * __add_wait_queue
170 * unlock ctx->wqh.lock
171 * eventfd_poll returns 0
172 */
173 count = READ_ONCE(ctx->count);
174
175 if (count > 0)
176 events |= EPOLLIN;
177 if (count == ULLONG_MAX)
178 events |= EPOLLERR;
179 if (ULLONG_MAX - 1 > count)
180 events |= EPOLLOUT;
181
182 return events;
183 }
184
185 void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
186 {
187 lockdep_assert_held(&ctx->wqh.lock);
188
189 *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
190 ctx->count -= *cnt;
191 }
192 EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
193
194 /**
195 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
196 * @ctx: [in] Pointer to eventfd context.
197 * @wait: [in] Wait queue to be removed.
198 * @cnt: [out] Pointer to the 64-bit counter value.
199 *
200 * Returns %0 if successful, or the following error codes:
201 *
202 * -EAGAIN : The operation would have blocked.
203 *
204 * This is used to atomically remove a wait queue entry from the eventfd wait
205 * queue head, and read/reset the counter value.
206 */
207 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
208 __u64 *cnt)
209 {
210 unsigned long flags;
211
212 spin_lock_irqsave(&ctx->wqh.lock, flags);
213 eventfd_ctx_do_read(ctx, cnt);
214 __remove_wait_queue(&ctx->wqh, wait);
215 if (*cnt != 0 && waitqueue_active(&ctx->wqh))
216 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
217 spin_unlock_irqrestore(&ctx->wqh.lock, flags);
218
219 return *cnt != 0 ? 0 : -EAGAIN;
220 }
221 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
222
223 static ssize_t eventfd_read(struct kiocb *iocb, struct iov_iter *to)
224 {
225 struct file *file = iocb->ki_filp;
226 struct eventfd_ctx *ctx = file->private_data;
227 __u64 ucnt = 0;
228 DECLARE_WAITQUEUE(wait, current);
229
230 if (iov_iter_count(to) < sizeof(ucnt))
231 return -EINVAL;
232 spin_lock_irq(&ctx->wqh.lock);
233 if (!ctx->count) {
234 if ((file->f_flags & O_NONBLOCK) ||
235 (iocb->ki_flags & IOCB_NOWAIT)) {
236 spin_unlock_irq(&ctx->wqh.lock);
237 return -EAGAIN;
238 }
239 __add_wait_queue(&ctx->wqh, &wait);
240 for (;;) {
241 set_current_state(TASK_INTERRUPTIBLE);
242 if (ctx->count)
243 break;
244 if (signal_pending(current)) {
245 __remove_wait_queue(&ctx->wqh, &wait);
246 __set_current_state(TASK_RUNNING);
247 spin_unlock_irq(&ctx->wqh.lock);
248 return -ERESTARTSYS;
249 }
250 spin_unlock_irq(&ctx->wqh.lock);
251 schedule();
252 spin_lock_irq(&ctx->wqh.lock);
253 }
254 __remove_wait_queue(&ctx->wqh, &wait);
255 __set_current_state(TASK_RUNNING);
256 }
257 eventfd_ctx_do_read(ctx, &ucnt);
258 if (waitqueue_active(&ctx->wqh))
259 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
260 spin_unlock_irq(&ctx->wqh.lock);
261 if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
262 return -EFAULT;
263
264 return sizeof(ucnt);
265 }
266
267 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
268 loff_t *ppos)
269 {
270 struct eventfd_ctx *ctx = file->private_data;
271 ssize_t res;
272 __u64 ucnt;
273 DECLARE_WAITQUEUE(wait, current);
274
275 if (count < sizeof(ucnt))
276 return -EINVAL;
277 if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
278 return -EFAULT;
279 if (ucnt == ULLONG_MAX)
280 return -EINVAL;
281 spin_lock_irq(&ctx->wqh.lock);
282 res = -EAGAIN;
283 if (ULLONG_MAX - ctx->count > ucnt)
284 res = sizeof(ucnt);
285 else if (!(file->f_flags & O_NONBLOCK)) {
286 __add_wait_queue(&ctx->wqh, &wait);
287 for (res = 0;;) {
288 set_current_state(TASK_INTERRUPTIBLE);
289 if (ULLONG_MAX - ctx->count > ucnt) {
290 res = sizeof(ucnt);
291 break;
292 }
293 if (signal_pending(current)) {
294 res = -ERESTARTSYS;
295 break;
296 }
297 spin_unlock_irq(&ctx->wqh.lock);
298 schedule();
299 spin_lock_irq(&ctx->wqh.lock);
300 }
301 __remove_wait_queue(&ctx->wqh, &wait);
302 __set_current_state(TASK_RUNNING);
303 }
304 if (likely(res > 0)) {
305 ctx->count += ucnt;
306 if (waitqueue_active(&ctx->wqh))
307 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
308 }
309 spin_unlock_irq(&ctx->wqh.lock);
310
311 return res;
312 }
313
314 #ifdef CONFIG_PROC_FS
315 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
316 {
317 struct eventfd_ctx *ctx = f->private_data;
318
319 spin_lock_irq(&ctx->wqh.lock);
320 seq_printf(m, "eventfd-count: %16llx\n",
321 (unsigned long long)ctx->count);
322 spin_unlock_irq(&ctx->wqh.lock);
323 seq_printf(m, "eventfd-id: %d\n", ctx->id);
324 }
325 #endif
326
327 static const struct file_operations eventfd_fops = {
328 #ifdef CONFIG_PROC_FS
329 .show_fdinfo = eventfd_show_fdinfo,
330 #endif
331 .release = eventfd_release,
332 .poll = eventfd_poll,
333 .read_iter = eventfd_read,
334 .write = eventfd_write,
335 .llseek = noop_llseek,
336 };
337
338 /**
339 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
340 * @fd: [in] Eventfd file descriptor.
341 *
342 * Returns a pointer to the eventfd file structure in case of success, or the
343 * following error pointer:
344 *
345 * -EBADF : Invalid @fd file descriptor.
346 * -EINVAL : The @fd file descriptor is not an eventfd file.
347 */
348 struct file *eventfd_fget(int fd)
349 {
350 struct file *file;
351
352 file = fget(fd);
353 if (!file)
354 return ERR_PTR(-EBADF);
355 if (file->f_op != &eventfd_fops) {
356 fput(file);
357 return ERR_PTR(-EINVAL);
358 }
359
360 return file;
361 }
362 EXPORT_SYMBOL_GPL(eventfd_fget);
363
364 /**
365 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
366 * @fd: [in] Eventfd file descriptor.
367 *
368 * Returns a pointer to the internal eventfd context, otherwise the error
369 * pointers returned by the following functions:
370 *
371 * eventfd_fget
372 */
373 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
374 {
375 struct eventfd_ctx *ctx;
376 struct fd f = fdget(fd);
377 if (!f.file)
378 return ERR_PTR(-EBADF);
379 ctx = eventfd_ctx_fileget(f.file);
380 fdput(f);
381 return ctx;
382 }
383 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
384
385 /**
386 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
387 * @file: [in] Eventfd file pointer.
388 *
389 * Returns a pointer to the internal eventfd context, otherwise the error
390 * pointer:
391 *
392 * -EINVAL : The @fd file descriptor is not an eventfd file.
393 */
394 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
395 {
396 struct eventfd_ctx *ctx;
397
398 if (file->f_op != &eventfd_fops)
399 return ERR_PTR(-EINVAL);
400
401 ctx = file->private_data;
402 kref_get(&ctx->kref);
403 return ctx;
404 }
405 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
406
407 static int do_eventfd(unsigned int count, int flags)
408 {
409 struct eventfd_ctx *ctx;
410 struct file *file;
411 int fd;
412
413 /* Check the EFD_* constants for consistency. */
414 BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
415 BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
416
417 if (flags & ~EFD_FLAGS_SET)
418 return -EINVAL;
419
420 ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
421 if (!ctx)
422 return -ENOMEM;
423
424 kref_init(&ctx->kref);
425 init_waitqueue_head(&ctx->wqh);
426 ctx->count = count;
427 ctx->flags = flags;
428 ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
429
430 flags &= EFD_SHARED_FCNTL_FLAGS;
431 flags |= O_RDWR;
432 fd = get_unused_fd_flags(flags);
433 if (fd < 0)
434 goto err;
435
436 file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, flags);
437 if (IS_ERR(file)) {
438 put_unused_fd(fd);
439 fd = PTR_ERR(file);
440 goto err;
441 }
442
443 file->f_mode |= FMODE_NOWAIT;
444 fd_install(fd, file);
445 return fd;
446 err:
447 eventfd_free_ctx(ctx);
448 return fd;
449 }
450
451 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
452 {
453 return do_eventfd(count, flags);
454 }
455
456 SYSCALL_DEFINE1(eventfd, unsigned int, count)
457 {
458 return do_eventfd(count, 0);
459 }
460