/*
 *  fs/eventfd.c
 *
 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
 *
 */

#include <linux/file.h>
#include <linux/poll.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/sched/signal.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/spinlock.h>
#include <linux/anon_inodes.h>
#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/kref.h>
#include <linux/eventfd.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>

struct eventfd_ctx {
        struct kref kref;
        wait_queue_head_t wqh;
        /*
         * Every time that a write(2) is performed on an eventfd, the
         * value of the __u64 being written is added to "count" and a
         * wakeup is performed on "wqh". A read(2) will return the "count"
         * value to userspace, and will reset "count" to zero. The kernel
         * side eventfd_signal() also, adds to the "count" counter and
         * issue a wakeup.
         */
        __u64 count;
        unsigned int flags;
};

/**
 * eventfd_signal - Adds @n to the eventfd counter.
 * @ctx: [in] Pointer to the eventfd context.
 * @n: [in] Value of the counter to be added to the eventfd internal counter.
 *          The value cannot be negative.
 *
 * This function is supposed to be called by the kernel in paths that do not
 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
 * value, and we signal this as overflow condition by returning a EPOLLERR
 * to poll(2).
 *
 * Returns the amount by which the counter was incremented.  This will be less
 * than @n if the counter has overflowed.
 */
__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
{
        unsigned long flags;

        spin_lock_irqsave(&ctx->wqh.lock, flags);
        if (ULLONG_MAX - ctx->count < n)
                n = ULLONG_MAX - ctx->count;
        ctx->count += n;
        if (waitqueue_active(&ctx->wqh))
                wake_up_locked_poll(&ctx->wqh, EPOLLIN);
        spin_unlock_irqrestore(&ctx->wqh.lock, flags);

        return n;
}
EXPORT_SYMBOL_GPL(eventfd_signal);

static void eventfd_free_ctx(struct eventfd_ctx *ctx)
{
        kfree(ctx);
}

static void eventfd_free(struct kref *kref)
{
        struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);

        eventfd_free_ctx(ctx);
}

/**
 * eventfd_ctx_put - Releases a reference to the internal eventfd context.
 * @ctx: [in] Pointer to eventfd context.
 *
 * The eventfd context reference must have been previously acquired either
 * with eventfd_ctx_fdget() or eventfd_ctx_fileget().
 */
void eventfd_ctx_put(struct eventfd_ctx *ctx)
{
        kref_put(&ctx->kref, eventfd_free);
}
EXPORT_SYMBOL_GPL(eventfd_ctx_put);

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

        wake_up_poll(&ctx->wqh, EPOLLHUP);
        eventfd_ctx_put(ctx);
        return 0;
}

static __poll_t eventfd_poll(struct file *file, poll_table *wait)
{
        struct eventfd_ctx *ctx = file->private_data;
        __poll_t events = 0;
        u64 count;

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

        /*
         * All writes to ctx->count occur within ctx->wqh.lock.  This read
         * can be done outside ctx->wqh.lock because we know that poll_wait
         * takes that lock (through add_wait_queue) if our caller will sleep.
         *
         * The read _can_ therefore seep into add_wait_queue's critical
         * section, but cannot move above it!  add_wait_queue's spin_lock acts
         * as an acquire barrier and ensures that the read be ordered properly
         * against the writes.  The following CAN happen and is safe:
         *
         *     poll                               write
         *     -----------------                  ------------
         *     lock ctx->wqh.lock (in poll_wait)
         *     count = ctx->count
         *     __add_wait_queue
         *     unlock ctx->wqh.lock
         *                                        lock ctx->qwh.lock
         *                                        ctx->count += n
         *                                        if (waitqueue_active)
         *                                          wake_up_locked_poll
         *                                        unlock ctx->qwh.lock
         *     eventfd_poll returns 0
         *
         * but the following, which would miss a wakeup, cannot happen:
         *
         *     poll                               write
         *     -----------------                  ------------
         *     count = ctx->count (INVALID!)
         *                                        lock ctx->qwh.lock
         *                                        ctx->count += n
         *                                        **waitqueue_active is false**
         *                                        **no wake_up_locked_poll!**
         *                                        unlock ctx->qwh.lock
         *     lock ctx->wqh.lock (in poll_wait)
         *     __add_wait_queue
         *     unlock ctx->wqh.lock
         *     eventfd_poll returns 0
         */
        count = READ_ONCE(ctx->count);

        if (count > 0)
                events |= EPOLLIN;
        if (count == ULLONG_MAX)
                events |= EPOLLERR;
        if (ULLONG_MAX - 1 > count)
                events |= EPOLLOUT;

        return events;
}

static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
{
        *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
        ctx->count -= *cnt;
}

/**
 * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
 * @ctx: [in] Pointer to eventfd context.
 * @wait: [in] Wait queue to be removed.
 * @cnt: [out] Pointer to the 64-bit counter value.
 *
 * Returns %0 if successful, or the following error codes:
 *
 * -EAGAIN      : The operation would have blocked.
 *
 * This is used to atomically remove a wait queue entry from the eventfd wait
 * queue head, and read/reset the counter value.
 */
int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
                                  __u64 *cnt)
{
        unsigned long flags;

        spin_lock_irqsave(&ctx->wqh.lock, flags);
        eventfd_ctx_do_read(ctx, cnt);
        __remove_wait_queue(&ctx->wqh, wait);
        if (*cnt != 0 && waitqueue_active(&ctx->wqh))
                wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
        spin_unlock_irqrestore(&ctx->wqh.lock, flags);

        return *cnt != 0 ? 0 : -EAGAIN;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);

static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
                            loff_t *ppos)
{
        struct eventfd_ctx *ctx = file->private_data;
        ssize_t res;
        __u64 ucnt = 0;
        DECLARE_WAITQUEUE(wait, current);

        if (count < sizeof(ucnt))
                return -EINVAL;

        spin_lock_irq(&ctx->wqh.lock);
        res = -EAGAIN;
        if (ctx->count > 0)
                res = sizeof(ucnt);
        else if (!(file->f_flags & O_NONBLOCK)) {
                __add_wait_queue(&ctx->wqh, &wait);
                for (;;) {
                        set_current_state(TASK_INTERRUPTIBLE);
                        if (ctx->count > 0) {
                                res = sizeof(ucnt);
                                break;
                        }
                        if (signal_pending(current)) {
                                res = -ERESTARTSYS;
                                break;
                        }
                        spin_unlock_irq(&ctx->wqh.lock);
                        schedule();
                        spin_lock_irq(&ctx->wqh.lock);
                }
                __remove_wait_queue(&ctx->wqh, &wait);
                __set_current_state(TASK_RUNNING);
        }
        if (likely(res > 0)) {
                eventfd_ctx_do_read(ctx, &ucnt);
                if (waitqueue_active(&ctx->wqh))
                        wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
        }
        spin_unlock_irq(&ctx->wqh.lock);

        if (res > 0 && put_user(ucnt, (__u64 __user *)buf))
                return -EFAULT;

        return res;
}

static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
                             loff_t *ppos)
{
        struct eventfd_ctx *ctx = file->private_data;
        ssize_t res;
        __u64 ucnt;
        DECLARE_WAITQUEUE(wait, current);

        if (count < sizeof(ucnt))
                return -EINVAL;
        if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
                return -EFAULT;
        if (ucnt == ULLONG_MAX)
                return -EINVAL;
        spin_lock_irq(&ctx->wqh.lock);
        res = -EAGAIN;
        if (ULLONG_MAX - ctx->count > ucnt)
                res = sizeof(ucnt);
        else if (!(file->f_flags & O_NONBLOCK)) {
                __add_wait_queue(&ctx->wqh, &wait);
                for (res = 0;;) {
                        set_current_state(TASK_INTERRUPTIBLE);
                        if (ULLONG_MAX - ctx->count > ucnt) {
                                res = sizeof(ucnt);
                                break;
                        }
                        if (signal_pending(current)) {
                                res = -ERESTARTSYS;
                                break;
                        }
                        spin_unlock_irq(&ctx->wqh.lock);
                        schedule();
                        spin_lock_irq(&ctx->wqh.lock);
                }
                __remove_wait_queue(&ctx->wqh, &wait);
                __set_current_state(TASK_RUNNING);
        }
        if (likely(res > 0)) {
                ctx->count += ucnt;
                if (waitqueue_active(&ctx->wqh))
                        wake_up_locked_poll(&ctx->wqh, EPOLLIN);
        }
        spin_unlock_irq(&ctx->wqh.lock);

        return res;
}

#ifdef CONFIG_PROC_FS
static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
{
        struct eventfd_ctx *ctx = f->private_data;

        spin_lock_irq(&ctx->wqh.lock);
        seq_printf(m, "eventfd-count: %16llx\n",
                   (unsigned long long)ctx->count);
        spin_unlock_irq(&ctx->wqh.lock);
}
#endif

static const struct file_operations eventfd_fops = {
#ifdef CONFIG_PROC_FS
        .show_fdinfo    = eventfd_show_fdinfo,
#endif
        .release        = eventfd_release,
        .poll           = eventfd_poll,
        .read           = eventfd_read,
        .write          = eventfd_write,
        .llseek         = noop_llseek,
};

/**
 * eventfd_fget - Acquire a reference of an eventfd file descriptor.
 * @fd: [in] Eventfd file descriptor.
 *
 * Returns a pointer to the eventfd file structure in case of success, or the
 * following error pointer:
 *
 * -EBADF    : Invalid @fd file descriptor.
 * -EINVAL   : The @fd file descriptor is not an eventfd file.
 */
struct file *eventfd_fget(int fd)
{
        struct file *file;

        file = fget(fd);
        if (!file)
                return ERR_PTR(-EBADF);
        if (file->f_op != &eventfd_fops) {
                fput(file);
                return ERR_PTR(-EINVAL);
        }

        return file;
}
EXPORT_SYMBOL_GPL(eventfd_fget);

/**
 * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
 * @fd: [in] Eventfd file descriptor.
 *
 * Returns a pointer to the internal eventfd context, otherwise the error
 * pointers returned by the following functions:
 *
 * eventfd_fget
 */
struct eventfd_ctx *eventfd_ctx_fdget(int fd)
{
        struct eventfd_ctx *ctx;
        struct fd f = fdget(fd);
        if (!f.file)
                return ERR_PTR(-EBADF);
        ctx = eventfd_ctx_fileget(f.file);
        fdput(f);
        return ctx;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);

/**
 * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
 * @file: [in] Eventfd file pointer.
 *
 * Returns a pointer to the internal eventfd context, otherwise the error
 * pointer:
 *
 * -EINVAL   : The @fd file descriptor is not an eventfd file.
 */
struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
{
        struct eventfd_ctx *ctx;

        if (file->f_op != &eventfd_fops)
                return ERR_PTR(-EINVAL);

        ctx = file->private_data;
        kref_get(&ctx->kref);
        return ctx;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);

static int do_eventfd(unsigned int count, int flags)
{
        struct eventfd_ctx *ctx;
        int fd;

        /* Check the EFD_* constants for consistency.  */
        BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
        BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);

        if (flags & ~EFD_FLAGS_SET)
                return -EINVAL;

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

        kref_init(&ctx->kref);
        init_waitqueue_head(&ctx->wqh);
        ctx->count = count;
        ctx->flags = flags;

        fd = anon_inode_getfd("[eventfd]", &eventfd_fops, ctx,
                              O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
        if (fd < 0)
                eventfd_free_ctx(ctx);

        return fd;
}

SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
{
        return do_eventfd(count, flags);
}

SYSCALL_DEFINE1(eventfd, unsigned int, count)
{
        return do_eventfd(count, 0);
}