fs/eventfd.c

   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". 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  */
  56 void 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 }
  80 EXPORT_SYMBOL_GPL(eventfd_signal_mask);
  81
  82 static 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
  89 static 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  */
 103 void eventfd_ctx_put(struct eventfd_ctx *ctx)
 104 {
 105         kref_put(&ctx->kref, eventfd_free);
 106 }
 107 EXPORT_SYMBOL_GPL(eventfd_ctx_put);
 108
 109 static 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
 118 static __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
 176 void 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 }
 183 EXPORT_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  */
 198 int 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 }
 212 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
 213
 214 static 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
 247 static 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
 283 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
 284 {
 285         struct eventfd_ctx *ctx = f->private_data;
 286
 287         spin_lock_irq(&ctx->wqh.lock);
 288         seq_printf(m, "eventfd-count: %16llx\n",
 289                    (unsigned long long)ctx->count);
 290         spin_unlock_irq(&ctx->wqh.lock);
 291         seq_printf(m, "eventfd-id: %d\n", ctx->id);
 292         seq_printf(m, "eventfd-semaphore: %d\n",
 293                    !!(ctx->flags & EFD_SEMAPHORE));
 294 }
 295 #endif
 296
 297 static const struct file_operations eventfd_fops = {
 298 #ifdef CONFIG_PROC_FS
 299         .show_fdinfo    = eventfd_show_fdinfo,
 300 #endif
 301         .release        = eventfd_release,
 302         .poll           = eventfd_poll,
 303         .read_iter      = eventfd_read,
 304         .write          = eventfd_write,
 305         .llseek         = noop_llseek,
 306 };
 307
 308 /**
 309  * eventfd_fget - Acquire a reference of an eventfd file descriptor.
 310  * @fd: [in] Eventfd file descriptor.
 311  *
 312  * Returns a pointer to the eventfd file structure in case of success, or the
 313  * following error pointer:
 314  *
 315  * -EBADF    : Invalid @fd file descriptor.
 316  * -EINVAL   : The @fd file descriptor is not an eventfd file.
 317  */
 318 struct file *eventfd_fget(int fd)
 319 {
 320         struct file *file;
 321
 322         file = fget(fd);
 323         if (!file)
 324                 return ERR_PTR(-EBADF);
 325         if (file->f_op != &eventfd_fops) {
 326                 fput(file);
 327                 return ERR_PTR(-EINVAL);
 328         }
 329
 330         return file;
 331 }
 332 EXPORT_SYMBOL_GPL(eventfd_fget);
 333
 334 /**
 335  * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
 336  * @fd: [in] Eventfd file descriptor.
 337  *
 338  * Returns a pointer to the internal eventfd context, otherwise the error
 339  * pointers returned by the following functions:
 340  *
 341  * eventfd_fget
 342  */
 343 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
 344 {
 345         struct eventfd_ctx *ctx;
 346         struct fd f = fdget(fd);
 347         if (!f.file)
 348                 return ERR_PTR(-EBADF);
 349         ctx = eventfd_ctx_fileget(f.file);
 350         fdput(f);
 351         return ctx;
 352 }
 353 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
 354
 355 /**
 356  * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
 357  * @file: [in] Eventfd file pointer.
 358  *
 359  * Returns a pointer to the internal eventfd context, otherwise the error
 360  * pointer:
 361  *
 362  * -EINVAL   : The @fd file descriptor is not an eventfd file.
 363  */
 364 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
 365 {
 366         struct eventfd_ctx *ctx;
 367
 368         if (file->f_op != &eventfd_fops)
 369                 return ERR_PTR(-EINVAL);
 370
 371         ctx = file->private_data;
 372         kref_get(&ctx->kref);
 373         return ctx;
 374 }
 375 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
 376
 377 static int do_eventfd(unsigned int count, int flags)
 378 {
 379         struct eventfd_ctx *ctx;
 380         struct file *file;
 381         int fd;
 382
 383         /* Check the EFD_* constants for consistency.  */
 384         BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
 385         BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
 386
 387         if (flags & ~EFD_FLAGS_SET)
 388                 return -EINVAL;
 389
 390         ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
 391         if (!ctx)
 392                 return -ENOMEM;
 393
 394         kref_init(&ctx->kref);
 395         init_waitqueue_head(&ctx->wqh);
 396         ctx->count = count;
 397         ctx->flags = flags;
 398         ctx->id = ida_alloc(&eventfd_ida, GFP_KERNEL);
 399
 400         flags &= EFD_SHARED_FCNTL_FLAGS;
 401         flags |= O_RDWR;
 402         fd = get_unused_fd_flags(flags);
 403         if (fd < 0)
 404                 goto err;
 405
 406         file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, flags);
 407         if (IS_ERR(file)) {
 408                 put_unused_fd(fd);
 409                 fd = PTR_ERR(file);
 410                 goto err;
 411         }
 412
 413         file->f_mode |= FMODE_NOWAIT;
 414         fd_install(fd, file);
 415         return fd;
 416 err:
 417         eventfd_free_ctx(ctx);
 418         return fd;
 419 }
 420
 421 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
 422 {
 423         return do_eventfd(count, flags);
 424 }
 425
 426 SYSCALL_DEFINE1(eventfd, unsigned int, count)
 427 {
 428         return do_eventfd(count, 0);
 429 }
 430