1 /*
2 * fs/eventfd.c
3 *
4 * Copyright (C) 2007 Davide Libenzi <davidel@xmailserver.org>
5 *
6 */
7
8 #include <linux/file.h>
9 #include <linux/poll.h>
10 #include <linux/init.h>
11 #include <linux/fs.h>
12 #include <linux/sched/signal.h>
13 #include <linux/kernel.h>
14 #include <linux/slab.h>
15 #include <linux/list.h>
16 #include <linux/spinlock.h>
17 #include <linux/anon_inodes.h>
18 #include <linux/syscalls.h>
19 #include <linux/export.h>
20 #include <linux/kref.h>
21 #include <linux/eventfd.h>
22 #include <linux/proc_fs.h>
23 #include <linux/seq_file.h>
24
25 struct eventfd_ctx {
26 struct kref kref;
27 wait_queue_head_t wqh;
28 /*
29 * Every time that a write(2) is performed on an eventfd, the
30 * value of the __u64 being written is added to "count" and a
31 * wakeup is performed on "wqh". A read(2) will return the "count"
32 * value to userspace, and will reset "count" to zero. The kernel
33 * side eventfd_signal() also, adds to the "count" counter and
34 * issue a wakeup.
35 */
36 __u64 count;
37 unsigned int flags;
38 };
39
40 /**
41 * eventfd_signal - Adds @n to the eventfd counter.
42 * @ctx: [in] Pointer to the eventfd context.
43 * @n: [in] Value of the counter to be added to the eventfd internal counter.
44 * The value cannot be negative.
45 *
46 * This function is supposed to be called by the kernel in paths that do not
47 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
48 * value, and we signal this as overflow condition by returning a EPOLLERR
49 * to poll(2).
50 *
51 * Returns the amount by which the counter was incremented. This will be less
52 * than @n if the counter has overflowed.
53 */
eventfd_signal(struct eventfd_ctx * ctx,__u64 n)54 __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
55 {
56 unsigned long flags;
57
58 spin_lock_irqsave(&ctx->wqh.lock, flags);
59 if (ULLONG_MAX - ctx->count < n)
60 n = ULLONG_MAX - ctx->count;
61 ctx->count += n;
62 if (waitqueue_active(&ctx->wqh))
63 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
64 spin_unlock_irqrestore(&ctx->wqh.lock, flags);
65
66 return n;
67 }
68 EXPORT_SYMBOL_GPL(eventfd_signal);
69
eventfd_free_ctx(struct eventfd_ctx * ctx)70 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
71 {
72 kfree(ctx);
73 }
74
eventfd_free(struct kref * kref)75 static void eventfd_free(struct kref *kref)
76 {
77 struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
78
79 eventfd_free_ctx(ctx);
80 }
81
82 /**
83 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
84 * @ctx: [in] Pointer to eventfd context.
85 *
86 * The eventfd context reference must have been previously acquired either
87 * with eventfd_ctx_fdget() or eventfd_ctx_fileget().
88 */
eventfd_ctx_put(struct eventfd_ctx * ctx)89 void eventfd_ctx_put(struct eventfd_ctx *ctx)
90 {
91 kref_put(&ctx->kref, eventfd_free);
92 }
93 EXPORT_SYMBOL_GPL(eventfd_ctx_put);
94
eventfd_release(struct inode * inode,struct file * file)95 static int eventfd_release(struct inode *inode, struct file *file)
96 {
97 struct eventfd_ctx *ctx = file->private_data;
98
99 wake_up_poll(&ctx->wqh, EPOLLHUP);
100 eventfd_ctx_put(ctx);
101 return 0;
102 }
103
eventfd_poll(struct file * file,poll_table * wait)104 static __poll_t eventfd_poll(struct file *file, poll_table *wait)
105 {
106 struct eventfd_ctx *ctx = file->private_data;
107 __poll_t events = 0;
108 u64 count;
109
110 poll_wait(file, &ctx->wqh, wait);
111
112 /*
113 * All writes to ctx->count occur within ctx->wqh.lock. This read
114 * can be done outside ctx->wqh.lock because we know that poll_wait
115 * takes that lock (through add_wait_queue) if our caller will sleep.
116 *
117 * The read _can_ therefore seep into add_wait_queue's critical
118 * section, but cannot move above it! add_wait_queue's spin_lock acts
119 * as an acquire barrier and ensures that the read be ordered properly
120 * against the writes. The following CAN happen and is safe:
121 *
122 * poll write
123 * ----------------- ------------
124 * lock ctx->wqh.lock (in poll_wait)
125 * count = ctx->count
126 * __add_wait_queue
127 * unlock ctx->wqh.lock
128 * lock ctx->qwh.lock
129 * ctx->count += n
130 * if (waitqueue_active)
131 * wake_up_locked_poll
132 * unlock ctx->qwh.lock
133 * eventfd_poll returns 0
134 *
135 * but the following, which would miss a wakeup, cannot happen:
136 *
137 * poll write
138 * ----------------- ------------
139 * count = ctx->count (INVALID!)
140 * lock ctx->qwh.lock
141 * ctx->count += n
142 * **waitqueue_active is false**
143 * **no wake_up_locked_poll!**
144 * unlock ctx->qwh.lock
145 * lock ctx->wqh.lock (in poll_wait)
146 * __add_wait_queue
147 * unlock ctx->wqh.lock
148 * eventfd_poll returns 0
149 */
150 count = READ_ONCE(ctx->count);
151
152 if (count > 0)
153 events |= EPOLLIN;
154 if (count == ULLONG_MAX)
155 events |= EPOLLERR;
156 if (ULLONG_MAX - 1 > count)
157 events |= EPOLLOUT;
158
159 return events;
160 }
161
eventfd_ctx_do_read(struct eventfd_ctx * ctx,__u64 * cnt)162 static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
163 {
164 *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
165 ctx->count -= *cnt;
166 }
167
168 /**
169 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
170 * @ctx: [in] Pointer to eventfd context.
171 * @wait: [in] Wait queue to be removed.
172 * @cnt: [out] Pointer to the 64-bit counter value.
173 *
174 * Returns %0 if successful, or the following error codes:
175 *
176 * -EAGAIN : The operation would have blocked.
177 *
178 * This is used to atomically remove a wait queue entry from the eventfd wait
179 * queue head, and read/reset the counter value.
180 */
eventfd_ctx_remove_wait_queue(struct eventfd_ctx * ctx,wait_queue_entry_t * wait,__u64 * cnt)181 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
182 __u64 *cnt)
183 {
184 unsigned long flags;
185
186 spin_lock_irqsave(&ctx->wqh.lock, flags);
187 eventfd_ctx_do_read(ctx, cnt);
188 __remove_wait_queue(&ctx->wqh, wait);
189 if (*cnt != 0 && waitqueue_active(&ctx->wqh))
190 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
191 spin_unlock_irqrestore(&ctx->wqh.lock, flags);
192
193 return *cnt != 0 ? 0 : -EAGAIN;
194 }
195 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
196
eventfd_read(struct file * file,char __user * buf,size_t count,loff_t * ppos)197 static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
198 loff_t *ppos)
199 {
200 struct eventfd_ctx *ctx = file->private_data;
201 ssize_t res;
202 __u64 ucnt = 0;
203 DECLARE_WAITQUEUE(wait, current);
204
205 if (count < sizeof(ucnt))
206 return -EINVAL;
207
208 spin_lock_irq(&ctx->wqh.lock);
209 res = -EAGAIN;
210 if (ctx->count > 0)
211 res = sizeof(ucnt);
212 else if (!(file->f_flags & O_NONBLOCK)) {
213 __add_wait_queue(&ctx->wqh, &wait);
214 for (;;) {
215 set_current_state(TASK_INTERRUPTIBLE);
216 if (ctx->count > 0) {
217 res = sizeof(ucnt);
218 break;
219 }
220 if (signal_pending(current)) {
221 res = -ERESTARTSYS;
222 break;
223 }
224 spin_unlock_irq(&ctx->wqh.lock);
225 schedule();
226 spin_lock_irq(&ctx->wqh.lock);
227 }
228 __remove_wait_queue(&ctx->wqh, &wait);
229 __set_current_state(TASK_RUNNING);
230 }
231 if (likely(res > 0)) {
232 eventfd_ctx_do_read(ctx, &ucnt);
233 if (waitqueue_active(&ctx->wqh))
234 wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
235 }
236 spin_unlock_irq(&ctx->wqh.lock);
237
238 if (res > 0 && put_user(ucnt, (__u64 __user *)buf))
239 return -EFAULT;
240
241 return res;
242 }
243
eventfd_write(struct file * file,const char __user * buf,size_t count,loff_t * ppos)244 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
245 loff_t *ppos)
246 {
247 struct eventfd_ctx *ctx = file->private_data;
248 ssize_t res;
249 __u64 ucnt;
250 DECLARE_WAITQUEUE(wait, current);
251
252 if (count < sizeof(ucnt))
253 return -EINVAL;
254 if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
255 return -EFAULT;
256 if (ucnt == ULLONG_MAX)
257 return -EINVAL;
258 spin_lock_irq(&ctx->wqh.lock);
259 res = -EAGAIN;
260 if (ULLONG_MAX - ctx->count > ucnt)
261 res = sizeof(ucnt);
262 else if (!(file->f_flags & O_NONBLOCK)) {
263 __add_wait_queue(&ctx->wqh, &wait);
264 for (res = 0;;) {
265 set_current_state(TASK_INTERRUPTIBLE);
266 if (ULLONG_MAX - ctx->count > ucnt) {
267 res = sizeof(ucnt);
268 break;
269 }
270 if (signal_pending(current)) {
271 res = -ERESTARTSYS;
272 break;
273 }
274 spin_unlock_irq(&ctx->wqh.lock);
275 schedule();
276 spin_lock_irq(&ctx->wqh.lock);
277 }
278 __remove_wait_queue(&ctx->wqh, &wait);
279 __set_current_state(TASK_RUNNING);
280 }
281 if (likely(res > 0)) {
282 ctx->count += ucnt;
283 if (waitqueue_active(&ctx->wqh))
284 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
285 }
286 spin_unlock_irq(&ctx->wqh.lock);
287
288 return res;
289 }
290
291 #ifdef CONFIG_PROC_FS
eventfd_show_fdinfo(struct seq_file * m,struct file * f)292 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
293 {
294 struct eventfd_ctx *ctx = f->private_data;
295
296 spin_lock_irq(&ctx->wqh.lock);
297 seq_printf(m, "eventfd-count: %16llx\n",
298 (unsigned long long)ctx->count);
299 spin_unlock_irq(&ctx->wqh.lock);
300 }
301 #endif
302
303 static const struct file_operations eventfd_fops = {
304 #ifdef CONFIG_PROC_FS
305 .show_fdinfo = eventfd_show_fdinfo,
306 #endif
307 .release = eventfd_release,
308 .poll = eventfd_poll,
309 .read = eventfd_read,
310 .write = eventfd_write,
311 .llseek = noop_llseek,
312 };
313
314 /**
315 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
316 * @fd: [in] Eventfd file descriptor.
317 *
318 * Returns a pointer to the eventfd file structure in case of success, or the
319 * following error pointer:
320 *
321 * -EBADF : Invalid @fd file descriptor.
322 * -EINVAL : The @fd file descriptor is not an eventfd file.
323 */
eventfd_fget(int fd)324 struct file *eventfd_fget(int fd)
325 {
326 struct file *file;
327
328 file = fget(fd);
329 if (!file)
330 return ERR_PTR(-EBADF);
331 if (file->f_op != &eventfd_fops) {
332 fput(file);
333 return ERR_PTR(-EINVAL);
334 }
335
336 return file;
337 }
338 EXPORT_SYMBOL_GPL(eventfd_fget);
339
340 /**
341 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
342 * @fd: [in] Eventfd file descriptor.
343 *
344 * Returns a pointer to the internal eventfd context, otherwise the error
345 * pointers returned by the following functions:
346 *
347 * eventfd_fget
348 */
eventfd_ctx_fdget(int fd)349 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
350 {
351 struct eventfd_ctx *ctx;
352 struct fd f = fdget(fd);
353 if (!f.file)
354 return ERR_PTR(-EBADF);
355 ctx = eventfd_ctx_fileget(f.file);
356 fdput(f);
357 return ctx;
358 }
359 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
360
361 /**
362 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
363 * @file: [in] Eventfd file pointer.
364 *
365 * Returns a pointer to the internal eventfd context, otherwise the error
366 * pointer:
367 *
368 * -EINVAL : The @fd file descriptor is not an eventfd file.
369 */
eventfd_ctx_fileget(struct file * file)370 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
371 {
372 struct eventfd_ctx *ctx;
373
374 if (file->f_op != &eventfd_fops)
375 return ERR_PTR(-EINVAL);
376
377 ctx = file->private_data;
378 kref_get(&ctx->kref);
379 return ctx;
380 }
381 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
382
do_eventfd(unsigned int count,int flags)383 static int do_eventfd(unsigned int count, int flags)
384 {
385 struct eventfd_ctx *ctx;
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
392 if (flags & ~EFD_FLAGS_SET)
393 return -EINVAL;
394
395 ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
396 if (!ctx)
397 return -ENOMEM;
398
399 kref_init(&ctx->kref);
400 init_waitqueue_head(&ctx->wqh);
401 ctx->count = count;
402 ctx->flags = flags;
403
404 fd = anon_inode_getfd("[eventfd]", &eventfd_fops, ctx,
405 O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
406 if (fd < 0)
407 eventfd_free_ctx(ctx);
408
409 return fd;
410 }
411
SYSCALL_DEFINE2(eventfd2,unsigned int,count,int,flags)412 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
413 {
414 return do_eventfd(count, flags);
415 }
416
SYSCALL_DEFINE1(eventfd,unsigned int,count)417 SYSCALL_DEFINE1(eventfd, unsigned int, count)
418 {
419 return do_eventfd(count, 0);
420 }
421
422
