linux/fs/timerfd.c
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   1/*
   2 *  fs/timerfd.c
   3 *
   4 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
   5 *
   6 *
   7 *  Thanks to Thomas Gleixner for code reviews and useful comments.
   8 *
   9 */
  10
  11#include <linux/file.h>
  12#include <linux/poll.h>
  13#include <linux/init.h>
  14#include <linux/fs.h>
  15#include <linux/sched.h>
  16#include <linux/kernel.h>
  17#include <linux/slab.h>
  18#include <linux/list.h>
  19#include <linux/spinlock.h>
  20#include <linux/time.h>
  21#include <linux/hrtimer.h>
  22#include <linux/anon_inodes.h>
  23#include <linux/timerfd.h>
  24#include <linux/syscalls.h>
  25#include <linux/compat.h>
  26#include <linux/rcupdate.h>
  27
  28struct timerfd_ctx {
  29        struct hrtimer tmr;
  30        ktime_t tintv;
  31        ktime_t moffs;
  32        wait_queue_head_t wqh;
  33        u64 ticks;
  34        int expired;
  35        int clockid;
  36        struct rcu_head rcu;
  37        struct list_head clist;
  38        bool might_cancel;
  39};
  40
  41static LIST_HEAD(cancel_list);
  42static DEFINE_SPINLOCK(cancel_lock);
  43
  44/*
  45 * This gets called when the timer event triggers. We set the "expired"
  46 * flag, but we do not re-arm the timer (in case it's necessary,
  47 * tintv.tv64 != 0) until the timer is accessed.
  48 */
  49static enum hrtimer_restart timerfd_tmrproc(struct hrtimer *htmr)
  50{
  51        struct timerfd_ctx *ctx = container_of(htmr, struct timerfd_ctx, tmr);
  52        unsigned long flags;
  53
  54        spin_lock_irqsave(&ctx->wqh.lock, flags);
  55        ctx->expired = 1;
  56        ctx->ticks++;
  57        wake_up_locked(&ctx->wqh);
  58        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
  59
  60        return HRTIMER_NORESTART;
  61}
  62
  63/*
  64 * Called when the clock was set to cancel the timers in the cancel
  65 * list. This will wake up processes waiting on these timers. The
  66 * wake-up requires ctx->ticks to be non zero, therefore we increment
  67 * it before calling wake_up_locked().
  68 */
  69void timerfd_clock_was_set(void)
  70{
  71        ktime_t moffs = ktime_get_monotonic_offset();
  72        struct timerfd_ctx *ctx;
  73        unsigned long flags;
  74
  75        rcu_read_lock();
  76        list_for_each_entry_rcu(ctx, &cancel_list, clist) {
  77                if (!ctx->might_cancel)
  78                        continue;
  79                spin_lock_irqsave(&ctx->wqh.lock, flags);
  80                if (ctx->moffs.tv64 != moffs.tv64) {
  81                        ctx->moffs.tv64 = KTIME_MAX;
  82                        ctx->ticks++;
  83                        wake_up_locked(&ctx->wqh);
  84                }
  85                spin_unlock_irqrestore(&ctx->wqh.lock, flags);
  86        }
  87        rcu_read_unlock();
  88}
  89
  90static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
  91{
  92        if (ctx->might_cancel) {
  93                ctx->might_cancel = false;
  94                spin_lock(&cancel_lock);
  95                list_del_rcu(&ctx->clist);
  96                spin_unlock(&cancel_lock);
  97        }
  98}
  99
 100static bool timerfd_canceled(struct timerfd_ctx *ctx)
 101{
 102        if (!ctx->might_cancel || ctx->moffs.tv64 != KTIME_MAX)
 103                return false;
 104        ctx->moffs = ktime_get_monotonic_offset();
 105        return true;
 106}
 107
 108static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
 109{
 110        if (ctx->clockid == CLOCK_REALTIME && (flags & TFD_TIMER_ABSTIME) &&
 111            (flags & TFD_TIMER_CANCEL_ON_SET)) {
 112                if (!ctx->might_cancel) {
 113                        ctx->might_cancel = true;
 114                        spin_lock(&cancel_lock);
 115                        list_add_rcu(&ctx->clist, &cancel_list);
 116                        spin_unlock(&cancel_lock);
 117                }
 118        } else if (ctx->might_cancel) {
 119                timerfd_remove_cancel(ctx);
 120        }
 121}
 122
 123static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
 124{
 125        ktime_t remaining;
 126
 127        remaining = hrtimer_expires_remaining(&ctx->tmr);
 128        return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
 129}
 130
 131static int timerfd_setup(struct timerfd_ctx *ctx, int flags,
 132                         const struct itimerspec *ktmr)
 133{
 134        enum hrtimer_mode htmode;
 135        ktime_t texp;
 136        int clockid = ctx->clockid;
 137
 138        htmode = (flags & TFD_TIMER_ABSTIME) ?
 139                HRTIMER_MODE_ABS: HRTIMER_MODE_REL;
 140
 141        texp = timespec_to_ktime(ktmr->it_value);
 142        ctx->expired = 0;
 143        ctx->ticks = 0;
 144        ctx->tintv = timespec_to_ktime(ktmr->it_interval);
 145        hrtimer_init(&ctx->tmr, clockid, htmode);
 146        hrtimer_set_expires(&ctx->tmr, texp);
 147        ctx->tmr.function = timerfd_tmrproc;
 148        if (texp.tv64 != 0) {
 149                hrtimer_start(&ctx->tmr, texp, htmode);
 150                if (timerfd_canceled(ctx))
 151                        return -ECANCELED;
 152        }
 153        return 0;
 154}
 155
 156static int timerfd_release(struct inode *inode, struct file *file)
 157{
 158        struct timerfd_ctx *ctx = file->private_data;
 159
 160        timerfd_remove_cancel(ctx);
 161        hrtimer_cancel(&ctx->tmr);
 162        kfree_rcu(ctx, rcu);
 163        return 0;
 164}
 165
 166static unsigned int timerfd_poll(struct file *file, poll_table *wait)
 167{
 168        struct timerfd_ctx *ctx = file->private_data;
 169        unsigned int events = 0;
 170        unsigned long flags;
 171
 172        poll_wait(file, &ctx->wqh, wait);
 173
 174        spin_lock_irqsave(&ctx->wqh.lock, flags);
 175        if (ctx->ticks)
 176                events |= POLLIN;
 177        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 178
 179        return events;
 180}
 181
 182static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,
 183                            loff_t *ppos)
 184{
 185        struct timerfd_ctx *ctx = file->private_data;
 186        ssize_t res;
 187        u64 ticks = 0;
 188
 189        if (count < sizeof(ticks))
 190                return -EINVAL;
 191        spin_lock_irq(&ctx->wqh.lock);
 192        if (file->f_flags & O_NONBLOCK)
 193                res = -EAGAIN;
 194        else
 195                res = wait_event_interruptible_locked_irq(ctx->wqh, ctx->ticks);
 196
 197        /*
 198         * If clock has changed, we do not care about the
 199         * ticks and we do not rearm the timer. Userspace must
 200         * reevaluate anyway.
 201         */
 202        if (timerfd_canceled(ctx)) {
 203                ctx->ticks = 0;
 204                ctx->expired = 0;
 205                res = -ECANCELED;
 206        }
 207
 208        if (ctx->ticks) {
 209                ticks = ctx->ticks;
 210
 211                if (ctx->expired && ctx->tintv.tv64) {
 212                        /*
 213                         * If tintv.tv64 != 0, this is a periodic timer that
 214                         * needs to be re-armed. We avoid doing it in the timer
 215                         * callback to avoid DoS attacks specifying a very
 216                         * short timer period.
 217                         */
 218                        ticks += hrtimer_forward_now(&ctx->tmr,
 219                                                     ctx->tintv) - 1;
 220                        hrtimer_restart(&ctx->tmr);
 221                }
 222                ctx->expired = 0;
 223                ctx->ticks = 0;
 224        }
 225        spin_unlock_irq(&ctx->wqh.lock);
 226        if (ticks)
 227                res = put_user(ticks, (u64 __user *) buf) ? -EFAULT: sizeof(ticks);
 228        return res;
 229}
 230
 231static const struct file_operations timerfd_fops = {
 232        .release        = timerfd_release,
 233        .poll           = timerfd_poll,
 234        .read           = timerfd_read,
 235        .llseek         = noop_llseek,
 236};
 237
 238static int timerfd_fget(int fd, struct fd *p)
 239{
 240        struct fd f = fdget(fd);
 241        if (!f.file)
 242                return -EBADF;
 243        if (f.file->f_op != &timerfd_fops) {
 244                fdput(f);
 245                return -EINVAL;
 246        }
 247        *p = f;
 248        return 0;
 249}
 250
 251SYSCALL_DEFINE2(timerfd_create, int, clockid, int, flags)
 252{
 253        int ufd;
 254        struct timerfd_ctx *ctx;
 255
 256        /* Check the TFD_* constants for consistency.  */
 257        BUILD_BUG_ON(TFD_CLOEXEC != O_CLOEXEC);
 258        BUILD_BUG_ON(TFD_NONBLOCK != O_NONBLOCK);
 259
 260        if ((flags & ~TFD_CREATE_FLAGS) ||
 261            (clockid != CLOCK_MONOTONIC &&
 262             clockid != CLOCK_REALTIME))
 263                return -EINVAL;
 264
 265        ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 266        if (!ctx)
 267                return -ENOMEM;
 268
 269        init_waitqueue_head(&ctx->wqh);
 270        ctx->clockid = clockid;
 271        hrtimer_init(&ctx->tmr, clockid, HRTIMER_MODE_ABS);
 272        ctx->moffs = ktime_get_monotonic_offset();
 273
 274        ufd = anon_inode_getfd("[timerfd]", &timerfd_fops, ctx,
 275                               O_RDWR | (flags & TFD_SHARED_FCNTL_FLAGS));
 276        if (ufd < 0)
 277                kfree(ctx);
 278
 279        return ufd;
 280}
 281
 282static int do_timerfd_settime(int ufd, int flags, 
 283                const struct itimerspec *new,
 284                struct itimerspec *old)
 285{
 286        struct fd f;
 287        struct timerfd_ctx *ctx;
 288        int ret;
 289
 290        if ((flags & ~TFD_SETTIME_FLAGS) ||
 291            !timespec_valid(&new->it_value) ||
 292            !timespec_valid(&new->it_interval))
 293                return -EINVAL;
 294
 295        ret = timerfd_fget(ufd, &f);
 296        if (ret)
 297                return ret;
 298        ctx = f.file->private_data;
 299
 300        timerfd_setup_cancel(ctx, flags);
 301
 302        /*
 303         * We need to stop the existing timer before reprogramming
 304         * it to the new values.
 305         */
 306        for (;;) {
 307                spin_lock_irq(&ctx->wqh.lock);
 308                if (hrtimer_try_to_cancel(&ctx->tmr) >= 0)
 309                        break;
 310                spin_unlock_irq(&ctx->wqh.lock);
 311                cpu_relax();
 312        }
 313
 314        /*
 315         * If the timer is expired and it's periodic, we need to advance it
 316         * because the caller may want to know the previous expiration time.
 317         * We do not update "ticks" and "expired" since the timer will be
 318         * re-programmed again in the following timerfd_setup() call.
 319         */
 320        if (ctx->expired && ctx->tintv.tv64)
 321                hrtimer_forward_now(&ctx->tmr, ctx->tintv);
 322
 323        old->it_value = ktime_to_timespec(timerfd_get_remaining(ctx));
 324        old->it_interval = ktime_to_timespec(ctx->tintv);
 325
 326        /*
 327         * Re-program the timer to the new value ...
 328         */
 329        ret = timerfd_setup(ctx, flags, new);
 330
 331        spin_unlock_irq(&ctx->wqh.lock);
 332        fdput(f);
 333        return ret;
 334}
 335
 336static int do_timerfd_gettime(int ufd, struct itimerspec *t)
 337{
 338        struct fd f;
 339        struct timerfd_ctx *ctx;
 340        int ret = timerfd_fget(ufd, &f);
 341        if (ret)
 342                return ret;
 343        ctx = f.file->private_data;
 344
 345        spin_lock_irq(&ctx->wqh.lock);
 346        if (ctx->expired && ctx->tintv.tv64) {
 347                ctx->expired = 0;
 348                ctx->ticks +=
 349                        hrtimer_forward_now(&ctx->tmr, ctx->tintv) - 1;
 350                hrtimer_restart(&ctx->tmr);
 351        }
 352        t->it_value = ktime_to_timespec(timerfd_get_remaining(ctx));
 353        t->it_interval = ktime_to_timespec(ctx->tintv);
 354        spin_unlock_irq(&ctx->wqh.lock);
 355        fdput(f);
 356        return 0;
 357}
 358
 359SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
 360                const struct itimerspec __user *, utmr,
 361                struct itimerspec __user *, otmr)
 362{
 363        struct itimerspec new, old;
 364        int ret;
 365
 366        if (copy_from_user(&new, utmr, sizeof(new)))
 367                return -EFAULT;
 368        ret = do_timerfd_settime(ufd, flags, &new, &old);
 369        if (ret)
 370                return ret;
 371        if (otmr && copy_to_user(otmr, &old, sizeof(old)))
 372                return -EFAULT;
 373
 374        return ret;
 375}
 376
 377SYSCALL_DEFINE2(timerfd_gettime, int, ufd, struct itimerspec __user *, otmr)
 378{
 379        struct itimerspec kotmr;
 380        int ret = do_timerfd_gettime(ufd, &kotmr);
 381        if (ret)
 382                return ret;
 383        return copy_to_user(otmr, &kotmr, sizeof(kotmr)) ? -EFAULT: 0;
 384}
 385
 386#ifdef CONFIG_COMPAT
 387COMPAT_SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
 388                const struct compat_itimerspec __user *, utmr,
 389                struct compat_itimerspec __user *, otmr)
 390{
 391        struct itimerspec new, old;
 392        int ret;
 393
 394        if (get_compat_itimerspec(&new, utmr))
 395                return -EFAULT;
 396        ret = do_timerfd_settime(ufd, flags, &new, &old);
 397        if (ret)
 398                return ret;
 399        if (otmr && put_compat_itimerspec(otmr, &old))
 400                return -EFAULT;
 401        return ret;
 402}
 403
 404COMPAT_SYSCALL_DEFINE2(timerfd_gettime, int, ufd,
 405                struct compat_itimerspec __user *, otmr)
 406{
 407        struct itimerspec kotmr;
 408        int ret = do_timerfd_gettime(ufd, &kotmr);
 409        if (ret)
 410                return ret;
 411        return put_compat_itimerspec(otmr, &kotmr) ? -EFAULT: 0;
 412}
 413#endif
 414
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