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/**
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 */
56void 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}
80EXPORT_SYMBOL_GPL(eventfd_signal_mask);
81
82static 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
89static 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 */
103void eventfd_ctx_put(struct eventfd_ctx *ctx)
104{
105	kref_put(&ctx->kref, eventfd_free);
106}
107EXPORT_SYMBOL_GPL(eventfd_ctx_put);
108
109static 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
118static __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
176void 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}
183EXPORT_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 */
198int 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}
212EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
213
214static 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
247static 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
283static 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
302static 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 */
323struct 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}
337EXPORT_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 */
348struct 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}
358EXPORT_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 */
369struct 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}
380EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
381
382static 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;
422err:
423	eventfd_free_ctx(ctx);
424	return fd;
425}
426
427SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
428{
429	return do_eventfd(count, flags);
430}
431
432SYSCALL_DEFINE1(eventfd, unsigned int, count)
433{
434	return do_eventfd(count, 0);
435}
436
437