timerfd.c

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/*
 *  fs/timerfd.c
 *
 *  Copyright (C) 2007  Davide Libenzi <[email protected]>
 *
 *
 *  Thanks to Thomas Gleixner for code reviews and useful comments.
 *
 */

#include <linux/file.h>
#include <linux/poll.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/time.h>
#include <linux/hrtimer.h>
#include <linux/anon_inodes.h>
#include <linux/timerfd.h>
#include <linux/syscalls.h>
#include <linux/rcupdate.h>

struct timerfd_ctx {
    struct hrtimer tmr;
    ktime_t tintv;
    ktime_t moffs;
    wait_queue_head_t wqh;
    u64 ticks;
    int expired;
    int clockid;
    struct rcu_head rcu;
    struct list_head clist;
    bool might_cancel;
};

static LIST_HEAD(cancel_list);
static DEFINE_SPINLOCK(cancel_lock);

/*
 * This gets called when the timer event triggers. We set the "expired"
 * flag, but we do not re-arm the timer (in case it's necessary,
 * tintv.tv64 != 0) until the timer is accessed.
 */
static enum hrtimer_restart timerfd_tmrproc(struct hrtimer *htmr)
{
    struct timerfd_ctx *ctx = container_of(htmr, struct timerfd_ctx, tmr);
    unsigned long flags;

    spin_lock_irqsave(&ctx->wqh.lock, flags);
    ctx->expired = 1;
    ctx->ticks++;
    wake_up_locked(&ctx->wqh);
    spin_unlock_irqrestore(&ctx->wqh.lock, flags);

    return HRTIMER_NORESTART;
}

/*
 * Called when the clock was set to cancel the timers in the cancel
 * list. This will wake up processes waiting on these timers. The
 * wake-up requires ctx->ticks to be non zero, therefore we increment
 * it before calling wake_up_locked().
 */
void timerfd_clock_was_set(void)
{
    ktime_t moffs = ktime_get_monotonic_offset();
    struct timerfd_ctx *ctx;
    unsigned long flags;

    rcu_read_lock();
    list_for_each_entry_rcu(ctx, &cancel_list, clist) {
        if (!ctx->might_cancel)
            continue;
        spin_lock_irqsave(&ctx->wqh.lock, flags);
        if (ctx->moffs.tv64 != moffs.tv64) {
            ctx->moffs.tv64 = KTIME_MAX;
            ctx->ticks++;
            wake_up_locked(&ctx->wqh);
        }
        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
    }
    rcu_read_unlock();
}

static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
{
    if (ctx->might_cancel) {
        ctx->might_cancel = false;
        spin_lock(&cancel_lock);
        list_del_rcu(&ctx->clist);
        spin_unlock(&cancel_lock);
    }
}

static bool timerfd_canceled(struct timerfd_ctx *ctx)
{
    if (!ctx->might_cancel || ctx->moffs.tv64 != KTIME_MAX)
        return false;
    ctx->moffs = ktime_get_monotonic_offset();
    return true;
}

static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
{
    if (ctx->clockid == CLOCK_REALTIME && (flags & TFD_TIMER_ABSTIME) &&
        (flags & TFD_TIMER_CANCEL_ON_SET)) {
        if (!ctx->might_cancel) {
            ctx->might_cancel = true;
            spin_lock(&cancel_lock);
            list_add_rcu(&ctx->clist, &cancel_list);
            spin_unlock(&cancel_lock);
        }
    } else if (ctx->might_cancel) {
        timerfd_remove_cancel(ctx);
    }
}

static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
{
    ktime_t remaining;

    remaining = hrtimer_expires_remaining(&ctx->tmr);
    return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
}

static int timerfd_setup(struct timerfd_ctx *ctx, int flags,
             const struct itimerspec *ktmr)
{
    enum hrtimer_mode htmode;
    ktime_t texp;
    int clockid = ctx->clockid;

    htmode = (flags & TFD_TIMER_ABSTIME) ?
        HRTIMER_MODE_ABS: HRTIMER_MODE_REL;

    texp = timespec_to_ktime(ktmr->it_value);
    ctx->expired = 0;
    ctx->ticks = 0;
    ctx->tintv = timespec_to_ktime(ktmr->it_interval);
    hrtimer_init(&ctx->tmr, clockid, htmode);
    hrtimer_set_expires(&ctx->tmr, texp);
    ctx->tmr.function = timerfd_tmrproc;
    if (texp.tv64 != 0) {
        hrtimer_start(&ctx->tmr, texp, htmode);
        if (timerfd_canceled(ctx))
            return -ECANCELED;
    }
    return 0;
}

static int timerfd_release(struct inode *inode, struct file *file)
{
    struct timerfd_ctx *ctx = file->private_data;

    timerfd_remove_cancel(ctx);
    hrtimer_cancel(&ctx->tmr);
    kfree_rcu(ctx, rcu);
    return 0;
}

static unsigned int timerfd_poll(struct file *file, poll_table *wait)
{
    struct timerfd_ctx *ctx = file->private_data;
    unsigned int events = 0;
    unsigned long flags;

    poll_wait(file, &ctx->wqh, wait);

    spin_lock_irqsave(&ctx->wqh.lock, flags);
    if (ctx->ticks)
        events |= POLLIN;
    spin_unlock_irqrestore(&ctx->wqh.lock, flags);

    return events;
}

static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,
                loff_t *ppos)
{
    struct timerfd_ctx *ctx = file->private_data;
    ssize_t res;
    u64 ticks = 0;

    if (count < sizeof(ticks))
        return -EINVAL;
    spin_lock_irq(&ctx->wqh.lock);
    if (file->f_flags & O_NONBLOCK)
        res = -EAGAIN;
    else
        res = wait_event_interruptible_locked_irq(ctx->wqh, ctx->ticks);

    /*
     * If clock has changed, we do not care about the
     * ticks and we do not rearm the timer. Userspace must
     * reevaluate anyway.
     */
    if (timerfd_canceled(ctx)) {
        ctx->ticks = 0;
        ctx->expired = 0;
        res = -ECANCELED;
    }

    if (ctx->ticks) {
        ticks = ctx->ticks;

        if (ctx->expired && ctx->tintv.tv64) {
            /*
             * If tintv.tv64 != 0, this is a periodic timer that
             * needs to be re-armed. We avoid doing it in the timer
             * callback to avoid DoS attacks specifying a very
             * short timer period.
             */
            ticks += hrtimer_forward_now(&ctx->tmr,
                             ctx->tintv) - 1;
            hrtimer_restart(&ctx->tmr);
        }
        ctx->expired = 0;
        ctx->ticks = 0;
    }
    spin_unlock_irq(&ctx->wqh.lock);
    if (ticks)
        res = put_user(ticks, (u64 __user *) buf) ? -EFAULT: sizeof(ticks);
    return res;
}

static const struct file_operations timerfd_fops = {
    .release    = timerfd_release,
    .poll       = timerfd_poll,
    .read       = timerfd_read,
    .llseek     = noop_llseek,
};

static int timerfd_fget(int fd, struct fd *p)
{
    struct fd f = fdget(fd);
    if (!f.file)
        return -EBADF;
    if (f.file->f_op != &timerfd_fops) {
        fdput(f);
        return -EINVAL;
    }
    *p = f;
    return 0;
}

SYSCALL_DEFINE2(timerfd_create, int, clockid, int, flags)
{
    int ufd;
    struct timerfd_ctx *ctx;

    /* Check the TFD_* constants for consistency.  */
    BUILD_BUG_ON(TFD_CLOEXEC != O_CLOEXEC);
    BUILD_BUG_ON(TFD_NONBLOCK != O_NONBLOCK);

    if ((flags & ~TFD_CREATE_FLAGS) ||
        (clockid != CLOCK_MONOTONIC &&
         clockid != CLOCK_REALTIME))
        return -EINVAL;

    ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
    if (!ctx)
        return -ENOMEM;

    init_waitqueue_head(&ctx->wqh);
    ctx->clockid = clockid;
    hrtimer_init(&ctx->tmr, clockid, HRTIMER_MODE_ABS);
    ctx->moffs = ktime_get_monotonic_offset();

    ufd = anon_inode_getfd("[timerfd]", &timerfd_fops, ctx,
                   O_RDWR | (flags & TFD_SHARED_FCNTL_FLAGS));
    if (ufd < 0)
        kfree(ctx);

    return ufd;
}

SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
        const struct itimerspec __user *, utmr,
        struct itimerspec __user *, otmr)
{
    struct fd f;
    struct timerfd_ctx *ctx;
    struct itimerspec ktmr, kotmr;
    int ret;

    if (copy_from_user(&ktmr, utmr, sizeof(ktmr)))
        return -EFAULT;

    if ((flags & ~TFD_SETTIME_FLAGS) ||
        !timespec_valid(&ktmr.it_value) ||
        !timespec_valid(&ktmr.it_interval))
        return -EINVAL;

    ret = timerfd_fget(ufd, &f);
    if (ret)
        return ret;
    ctx = f.file->private_data;

    timerfd_setup_cancel(ctx, flags);

    /*
     * We need to stop the existing timer before reprogramming
     * it to the new values.
     */
    for (;;) {
        spin_lock_irq(&ctx->wqh.lock);
        if (hrtimer_try_to_cancel(&ctx->tmr) >= 0)
            break;
        spin_unlock_irq(&ctx->wqh.lock);
        cpu_relax();
    }

    /*
     * If the timer is expired and it's periodic, we need to advance it
     * because the caller may want to know the previous expiration time.
     * We do not update "ticks" and "expired" since the timer will be
     * re-programmed again in the following timerfd_setup() call.
     */
    if (ctx->expired && ctx->tintv.tv64)
        hrtimer_forward_now(&ctx->tmr, ctx->tintv);

    kotmr.it_value = ktime_to_timespec(timerfd_get_remaining(ctx));
    kotmr.it_interval = ktime_to_timespec(ctx->tintv);

    /*
     * Re-program the timer to the new value ...
     */
    ret = timerfd_setup(ctx, flags, &ktmr);

    spin_unlock_irq(&ctx->wqh.lock);
    fdput(f);
    if (otmr && copy_to_user(otmr, &kotmr, sizeof(kotmr)))
        return -EFAULT;

    return ret;
}

SYSCALL_DEFINE2(timerfd_gettime, int, ufd, struct itimerspec __user *, otmr)
{
    struct fd f;
    struct timerfd_ctx *ctx;
    struct itimerspec kotmr;
    int ret = timerfd_fget(ufd, &f);
    if (ret)
        return ret;
    ctx = f.file->private_data;

    spin_lock_irq(&ctx->wqh.lock);
    if (ctx->expired && ctx->tintv.tv64) {
        ctx->expired = 0;
        ctx->ticks +=
            hrtimer_forward_now(&ctx->tmr, ctx->tintv) - 1;
        hrtimer_restart(&ctx->tmr);
    }
    kotmr.it_value = ktime_to_timespec(timerfd_get_remaining(ctx));
    kotmr.it_interval = ktime_to_timespec(ctx->tintv);
    spin_unlock_irq(&ctx->wqh.lock);
    fdput(f);

    return copy_to_user(otmr, &kotmr, sizeof(kotmr)) ? -EFAULT: 0;
}