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  */
eventfd_signal(struct eventfd_ctx * ctx,__u64 n)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 
eventfd_free_ctx(struct eventfd_ctx * ctx)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 
eventfd_free(struct kref * kref)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  */
eventfd_ctx_put(struct eventfd_ctx * ctx)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 
eventfd_release(struct inode * inode,struct file * file)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 
eventfd_poll(struct file * file,poll_table * wait)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 
eventfd_ctx_do_read(struct eventfd_ctx * ctx,__u64 * cnt)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  */
eventfd_ctx_remove_wait_queue(struct eventfd_ctx * ctx,wait_queue_entry_t * wait,__u64 * cnt)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 
eventfd_read(struct kiocb * iocb,struct iov_iter * to)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 
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 		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
eventfd_show_fdinfo(struct seq_file * m,struct file * f)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  */
eventfd_fget(int fd)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  */
eventfd_ctx_fdget(int fd)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  */
eventfd_ctx_fileget(struct file * file)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 
do_eventfd(unsigned int count,int flags)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 
SYSCALL_DEFINE2(eventfd2,unsigned int,count,int,flags)451 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
452 {
453 	return do_eventfd(count, flags);
454 }
455 
SYSCALL_DEFINE1(eventfd,unsigned int,count)456 SYSCALL_DEFINE1(eventfd, unsigned int, count)
457 {
458 	return do_eventfd(count, 0);
459 }
460 
461