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_signal))
73 		return 0;
74 
75 	spin_lock_irqsave(&ctx->wqh.lock, flags);
76 	current->in_eventfd_signal = 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_signal = 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 	if (waitqueue_active(&ctx->wqh))
257 		wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
258 	spin_unlock_irq(&ctx->wqh.lock);
259 	if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
260 		return -EFAULT;
261 
262 	return sizeof(ucnt);
263 }
264 
eventfd_write(struct file * file,const char __user * buf,size_t count,loff_t * ppos)265 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
266 			     loff_t *ppos)
267 {
268 	struct eventfd_ctx *ctx = file->private_data;
269 	ssize_t res;
270 	__u64 ucnt;
271 	DECLARE_WAITQUEUE(wait, current);
272 
273 	if (count < sizeof(ucnt))
274 		return -EINVAL;
275 	if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
276 		return -EFAULT;
277 	if (ucnt == ULLONG_MAX)
278 		return -EINVAL;
279 	spin_lock_irq(&ctx->wqh.lock);
280 	res = -EAGAIN;
281 	if (ULLONG_MAX - ctx->count > ucnt)
282 		res = sizeof(ucnt);
283 	else if (!(file->f_flags & O_NONBLOCK)) {
284 		__add_wait_queue(&ctx->wqh, &wait);
285 		for (res = 0;;) {
286 			set_current_state(TASK_INTERRUPTIBLE);
287 			if (ULLONG_MAX - ctx->count > ucnt) {
288 				res = sizeof(ucnt);
289 				break;
290 			}
291 			if (signal_pending(current)) {
292 				res = -ERESTARTSYS;
293 				break;
294 			}
295 			spin_unlock_irq(&ctx->wqh.lock);
296 			schedule();
297 			spin_lock_irq(&ctx->wqh.lock);
298 		}
299 		__remove_wait_queue(&ctx->wqh, &wait);
300 		__set_current_state(TASK_RUNNING);
301 	}
302 	if (likely(res > 0)) {
303 		ctx->count += ucnt;
304 		if (waitqueue_active(&ctx->wqh))
305 			wake_up_locked_poll(&ctx->wqh, EPOLLIN);
306 	}
307 	spin_unlock_irq(&ctx->wqh.lock);
308 
309 	return res;
310 }
311 
312 #ifdef CONFIG_PROC_FS
eventfd_show_fdinfo(struct seq_file * m,struct file * f)313 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
314 {
315 	struct eventfd_ctx *ctx = f->private_data;
316 
317 	spin_lock_irq(&ctx->wqh.lock);
318 	seq_printf(m, "eventfd-count: %16llx\n",
319 		   (unsigned long long)ctx->count);
320 	spin_unlock_irq(&ctx->wqh.lock);
321 	seq_printf(m, "eventfd-id: %d\n", ctx->id);
322 }
323 #endif
324 
325 static const struct file_operations eventfd_fops = {
326 #ifdef CONFIG_PROC_FS
327 	.show_fdinfo	= eventfd_show_fdinfo,
328 #endif
329 	.release	= eventfd_release,
330 	.poll		= eventfd_poll,
331 	.read_iter	= eventfd_read,
332 	.write		= eventfd_write,
333 	.llseek		= noop_llseek,
334 };
335 
336 /**
337  * eventfd_fget - Acquire a reference of an eventfd file descriptor.
338  * @fd: [in] Eventfd file descriptor.
339  *
340  * Returns a pointer to the eventfd file structure in case of success, or the
341  * following error pointer:
342  *
343  * -EBADF    : Invalid @fd file descriptor.
344  * -EINVAL   : The @fd file descriptor is not an eventfd file.
345  */
eventfd_fget(int fd)346 struct file *eventfd_fget(int fd)
347 {
348 	struct file *file;
349 
350 	file = fget(fd);
351 	if (!file)
352 		return ERR_PTR(-EBADF);
353 	if (file->f_op != &eventfd_fops) {
354 		fput(file);
355 		return ERR_PTR(-EINVAL);
356 	}
357 
358 	return file;
359 }
360 EXPORT_SYMBOL_GPL(eventfd_fget);
361 
362 /**
363  * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
364  * @fd: [in] Eventfd file descriptor.
365  *
366  * Returns a pointer to the internal eventfd context, otherwise the error
367  * pointers returned by the following functions:
368  *
369  * eventfd_fget
370  */
eventfd_ctx_fdget(int fd)371 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
372 {
373 	struct eventfd_ctx *ctx;
374 	struct fd f = fdget(fd);
375 	if (!f.file)
376 		return ERR_PTR(-EBADF);
377 	ctx = eventfd_ctx_fileget(f.file);
378 	fdput(f);
379 	return ctx;
380 }
381 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
382 
383 /**
384  * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
385  * @file: [in] Eventfd file pointer.
386  *
387  * Returns a pointer to the internal eventfd context, otherwise the error
388  * pointer:
389  *
390  * -EINVAL   : The @fd file descriptor is not an eventfd file.
391  */
eventfd_ctx_fileget(struct file * file)392 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
393 {
394 	struct eventfd_ctx *ctx;
395 
396 	if (file->f_op != &eventfd_fops)
397 		return ERR_PTR(-EINVAL);
398 
399 	ctx = file->private_data;
400 	kref_get(&ctx->kref);
401 	return ctx;
402 }
403 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
404 
do_eventfd(unsigned int count,int flags)405 static int do_eventfd(unsigned int count, int flags)
406 {
407 	struct eventfd_ctx *ctx;
408 	struct file *file;
409 	int fd;
410 
411 	/* Check the EFD_* constants for consistency.  */
412 	BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
413 	BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
414 
415 	if (flags & ~EFD_FLAGS_SET)
416 		return -EINVAL;
417 
418 	ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
419 	if (!ctx)
420 		return -ENOMEM;
421 
422 	kref_init(&ctx->kref);
423 	init_waitqueue_head(&ctx->wqh);
424 	ctx->count = count;
425 	ctx->flags = flags;
426 	ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
427 
428 	flags &= EFD_SHARED_FCNTL_FLAGS;
429 	flags |= O_RDWR;
430 	fd = get_unused_fd_flags(flags);
431 	if (fd < 0)
432 		goto err;
433 
434 	file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, flags);
435 	if (IS_ERR(file)) {
436 		put_unused_fd(fd);
437 		fd = PTR_ERR(file);
438 		goto err;
439 	}
440 
441 	file->f_mode |= FMODE_NOWAIT;
442 	fd_install(fd, file);
443 	return fd;
444 err:
445 	eventfd_free_ctx(ctx);
446 	return fd;
447 }
448 
SYSCALL_DEFINE2(eventfd2,unsigned int,count,int,flags)449 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
450 {
451 	return do_eventfd(count, flags);
452 }
453 
SYSCALL_DEFINE1(eventfd,unsigned int,count)454 SYSCALL_DEFINE1(eventfd, unsigned int, count)
455 {
456 	return do_eventfd(count, 0);
457 }
458 
459