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