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". A read(2) will return the "count"
37 * value to userspace, and will reset "count" to zero. The kernel
38 * side eventfd_signal() also, adds to the "count" counter and
39 * issue a wakeup.
40 */
41 __u64 count;
42 unsigned int flags;
43 int id;
44 };
45
46 /**
47 * eventfd_signal - Adds @n to the eventfd counter.
48 * @ctx: [in] Pointer to the eventfd context.
49 * @n: [in] Value of the counter to be added to the eventfd internal counter.
50 * The value cannot be negative.
51 *
52 * This function is supposed to be called by the kernel in paths that do not
53 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
54 * value, and we signal this as overflow condition by returning a EPOLLERR
55 * to poll(2).
56 *
57 * Returns the amount by which the counter was incremented. This will be less
58 * than @n if the counter has overflowed.
59 */
eventfd_signal(struct eventfd_ctx * ctx,__u64 n)60 __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
61 {
62 unsigned long flags;
63
64 /*
65 * Deadlock or stack overflow issues can happen if we recurse here
66 * through waitqueue wakeup handlers. If the caller users potentially
67 * nested waitqueues with custom wakeup handlers, then it should
68 * check eventfd_signal_allowed() before calling this function. If
69 * it returns false, the eventfd_signal() call should be deferred to a
70 * safe context.
71 */
72 if (WARN_ON_ONCE(current->in_eventfd))
73 return 0;
74
75 spin_lock_irqsave(&ctx->wqh.lock, flags);
76 current->in_eventfd = 1;
77 if (ULLONG_MAX - ctx->count < n)
78 n = ULLONG_MAX - ctx->count;
79 ctx->count += n;
80 if (waitqueue_active(&ctx->wqh))
81 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
82 current->in_eventfd = 0;
83 spin_unlock_irqrestore(&ctx->wqh.lock, flags);
84
85 return n;
86 }
87 EXPORT_SYMBOL_GPL(eventfd_signal);
88
eventfd_free_ctx(struct eventfd_ctx * ctx)89 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
90 {
91 if (ctx->id >= 0)
92 ida_simple_remove(&eventfd_ida, ctx->id);
93 kfree(ctx);
94 }
95
eventfd_free(struct kref * kref)96 static void eventfd_free(struct kref *kref)
97 {
98 struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
99
100 eventfd_free_ctx(ctx);
101 }
102
103 /**
104 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
105 * @ctx: [in] Pointer to eventfd context.
106 *
107 * The eventfd context reference must have been previously acquired either
108 * with eventfd_ctx_fdget() or eventfd_ctx_fileget().
109 */
eventfd_ctx_put(struct eventfd_ctx * ctx)110 void eventfd_ctx_put(struct eventfd_ctx *ctx)
111 {
112 kref_put(&ctx->kref, eventfd_free);
113 }
114 EXPORT_SYMBOL_GPL(eventfd_ctx_put);
115
eventfd_release(struct inode * inode,struct file * file)116 static int eventfd_release(struct inode *inode, struct file *file)
117 {
118 struct eventfd_ctx *ctx = file->private_data;
119
120 wake_up_poll(&ctx->wqh, EPOLLHUP);
121 eventfd_ctx_put(ctx);
122 return 0;
123 }
124
eventfd_poll(struct file * file,poll_table * wait)125 static __poll_t eventfd_poll(struct file *file, poll_table *wait)
126 {
127 struct eventfd_ctx *ctx = file->private_data;
128 __poll_t events = 0;
129 u64 count;
130
131 poll_wait(file, &ctx->wqh, wait);
132
133 /*
134 * All writes to ctx->count occur within ctx->wqh.lock. This read
135 * can be done outside ctx->wqh.lock because we know that poll_wait
136 * takes that lock (through add_wait_queue) if our caller will sleep.
137 *
138 * The read _can_ therefore seep into add_wait_queue's critical
139 * section, but cannot move above it! add_wait_queue's spin_lock acts
140 * as an acquire barrier and ensures that the read be ordered properly
141 * against the writes. The following CAN happen and is safe:
142 *
143 * poll write
144 * ----------------- ------------
145 * lock ctx->wqh.lock (in poll_wait)
146 * count = ctx->count
147 * __add_wait_queue
148 * unlock ctx->wqh.lock
149 * lock ctx->qwh.lock
150 * ctx->count += n
151 * if (waitqueue_active)
152 * wake_up_locked_poll
153 * unlock ctx->qwh.lock
154 * eventfd_poll returns 0
155 *
156 * but the following, which would miss a wakeup, cannot happen:
157 *
158 * poll write
159 * ----------------- ------------
160 * count = ctx->count (INVALID!)
161 * lock ctx->qwh.lock
162 * ctx->count += n
163 * **waitqueue_active is false**
164 * **no wake_up_locked_poll!**
165 * unlock ctx->qwh.lock
166 * lock ctx->wqh.lock (in poll_wait)
167 * __add_wait_queue
168 * unlock ctx->wqh.lock
169 * eventfd_poll returns 0
170 */
171 count = READ_ONCE(ctx->count);
172
173 if (count > 0)
174 events |= EPOLLIN;
175 if (count == ULLONG_MAX)
176 events |= EPOLLERR;
177 if (ULLONG_MAX - 1 > count)
178 events |= EPOLLOUT;
179
180 return events;
181 }
182
eventfd_ctx_do_read(struct eventfd_ctx * ctx,__u64 * cnt)183 void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
184 {
185 lockdep_assert_held(&ctx->wqh.lock);
186
187 *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
188 ctx->count -= *cnt;
189 }
190 EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
191
192 /**
193 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
194 * @ctx: [in] Pointer to eventfd context.
195 * @wait: [in] Wait queue to be removed.
196 * @cnt: [out] Pointer to the 64-bit counter value.
197 *
198 * Returns %0 if successful, or the following error codes:
199 *
200 * -EAGAIN : The operation would have blocked.
201 *
202 * This is used to atomically remove a wait queue entry from the eventfd wait
203 * queue head, and read/reset the counter value.
204 */
eventfd_ctx_remove_wait_queue(struct eventfd_ctx * ctx,wait_queue_entry_t * wait,__u64 * cnt)205 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
206 __u64 *cnt)
207 {
208 unsigned long flags;
209
210 spin_lock_irqsave(&ctx->wqh.lock, flags);
211 eventfd_ctx_do_read(ctx, cnt);
212 __remove_wait_queue(&ctx->wqh, wait);
213 if (*cnt != 0 && waitqueue_active(&ctx->wqh))
214 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
215 spin_unlock_irqrestore(&ctx->wqh.lock, flags);
216
217 return *cnt != 0 ? 0 : -EAGAIN;
218 }
219 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
220
eventfd_read(struct kiocb * iocb,struct iov_iter * to)221 static ssize_t eventfd_read(struct kiocb *iocb, struct iov_iter *to)
222 {
223 struct file *file = iocb->ki_filp;
224 struct eventfd_ctx *ctx = file->private_data;
225 __u64 ucnt = 0;
226 DECLARE_WAITQUEUE(wait, current);
227
228 if (iov_iter_count(to) < sizeof(ucnt))
229 return -EINVAL;
230 spin_lock_irq(&ctx->wqh.lock);
231 if (!ctx->count) {
232 if ((file->f_flags & O_NONBLOCK) ||
233 (iocb->ki_flags & IOCB_NOWAIT)) {
234 spin_unlock_irq(&ctx->wqh.lock);
235 return -EAGAIN;
236 }
237 __add_wait_queue(&ctx->wqh, &wait);
238 for (;;) {
239 set_current_state(TASK_INTERRUPTIBLE);
240 if (ctx->count)
241 break;
242 if (signal_pending(current)) {
243 __remove_wait_queue(&ctx->wqh, &wait);
244 __set_current_state(TASK_RUNNING);
245 spin_unlock_irq(&ctx->wqh.lock);
246 return -ERESTARTSYS;
247 }
248 spin_unlock_irq(&ctx->wqh.lock);
249 schedule();
250 spin_lock_irq(&ctx->wqh.lock);
251 }
252 __remove_wait_queue(&ctx->wqh, &wait);
253 __set_current_state(TASK_RUNNING);
254 }
255 eventfd_ctx_do_read(ctx, &ucnt);
256 current->in_eventfd = 1;
257 if (waitqueue_active(&ctx->wqh))
258 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
259 current->in_eventfd = 0;
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
eventfd_write(struct file * file,const char __user * buf,size_t count,loff_t * ppos)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 current->in_eventfd = 1;
307 if (waitqueue_active(&ctx->wqh))
308 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
309 current->in_eventfd = 0;
310 }
311 spin_unlock_irq(&ctx->wqh.lock);
312
313 return res;
314 }
315
316 #ifdef CONFIG_PROC_FS
eventfd_show_fdinfo(struct seq_file * m,struct file * f)317 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
318 {
319 struct eventfd_ctx *ctx = f->private_data;
320
321 spin_lock_irq(&ctx->wqh.lock);
322 seq_printf(m, "eventfd-count: %16llx\n",
323 (unsigned long long)ctx->count);
324 spin_unlock_irq(&ctx->wqh.lock);
325 seq_printf(m, "eventfd-id: %d\n", ctx->id);
326 }
327 #endif
328
329 static const struct file_operations eventfd_fops = {
330 #ifdef CONFIG_PROC_FS
331 .show_fdinfo = eventfd_show_fdinfo,
332 #endif
333 .release = eventfd_release,
334 .poll = eventfd_poll,
335 .read_iter = eventfd_read,
336 .write = eventfd_write,
337 .llseek = noop_llseek,
338 };
339
340 /**
341 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
342 * @fd: [in] Eventfd file descriptor.
343 *
344 * Returns a pointer to the eventfd file structure in case of success, or the
345 * following error pointer:
346 *
347 * -EBADF : Invalid @fd file descriptor.
348 * -EINVAL : The @fd file descriptor is not an eventfd file.
349 */
eventfd_fget(int fd)350 struct file *eventfd_fget(int fd)
351 {
352 struct file *file;
353
354 file = fget(fd);
355 if (!file)
356 return ERR_PTR(-EBADF);
357 if (file->f_op != &eventfd_fops) {
358 fput(file);
359 return ERR_PTR(-EINVAL);
360 }
361
362 return file;
363 }
364 EXPORT_SYMBOL_GPL(eventfd_fget);
365
366 /**
367 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
368 * @fd: [in] Eventfd file descriptor.
369 *
370 * Returns a pointer to the internal eventfd context, otherwise the error
371 * pointers returned by the following functions:
372 *
373 * eventfd_fget
374 */
eventfd_ctx_fdget(int fd)375 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
376 {
377 struct eventfd_ctx *ctx;
378 struct fd f = fdget(fd);
379 if (!f.file)
380 return ERR_PTR(-EBADF);
381 ctx = eventfd_ctx_fileget(f.file);
382 fdput(f);
383 return ctx;
384 }
385 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
386
387 /**
388 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
389 * @file: [in] Eventfd file pointer.
390 *
391 * Returns a pointer to the internal eventfd context, otherwise the error
392 * pointer:
393 *
394 * -EINVAL : The @fd file descriptor is not an eventfd file.
395 */
eventfd_ctx_fileget(struct file * file)396 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
397 {
398 struct eventfd_ctx *ctx;
399
400 if (file->f_op != &eventfd_fops)
401 return ERR_PTR(-EINVAL);
402
403 ctx = file->private_data;
404 kref_get(&ctx->kref);
405 return ctx;
406 }
407 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
408
do_eventfd(unsigned int count,int flags)409 static int do_eventfd(unsigned int count, int flags)
410 {
411 struct eventfd_ctx *ctx;
412 struct file *file;
413 int fd;
414
415 /* Check the EFD_* constants for consistency. */
416 BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
417 BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
418
419 if (flags & ~EFD_FLAGS_SET)
420 return -EINVAL;
421
422 ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
423 if (!ctx)
424 return -ENOMEM;
425
426 kref_init(&ctx->kref);
427 init_waitqueue_head(&ctx->wqh);
428 ctx->count = count;
429 ctx->flags = flags;
430 ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
431
432 flags &= EFD_SHARED_FCNTL_FLAGS;
433 flags |= O_RDWR;
434 fd = get_unused_fd_flags(flags);
435 if (fd < 0)
436 goto err;
437
438 file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, flags);
439 if (IS_ERR(file)) {
440 put_unused_fd(fd);
441 fd = PTR_ERR(file);
442 goto err;
443 }
444
445 file->f_mode |= FMODE_NOWAIT;
446 fd_install(fd, file);
447 return fd;
448 err:
449 eventfd_free_ctx(ctx);
450 return fd;
451 }
452
SYSCALL_DEFINE2(eventfd2,unsigned int,count,int,flags)453 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
454 {
455 return do_eventfd(count, flags);
456 }
457
SYSCALL_DEFINE1(eventfd,unsigned int,count)458 SYSCALL_DEFINE1(eventfd, unsigned int, count)
459 {
460 return do_eventfd(count, 0);
461 }
462
463
