// SPDX-License-Identifier: GPL-2.0-only
/*
 *  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>
#include <linux/idr.h>
#include <linux/uio.h>

static DEFINE_IDA(eventfd_ida);

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". If EFD_SEMAPHORE flag was not
         * specified, 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;
        int id;
};

/**
 * eventfd_signal_mask - Increment the event counter
 * @ctx: [in] Pointer to the eventfd context.
 * @mask: [in] poll mask
 *
 * 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).
 */
void eventfd_signal_mask(struct eventfd_ctx *ctx, __poll_t mask)
{
        unsigned long flags;

        /*
         * Deadlock or stack overflow issues can happen if we recurse here
         * through waitqueue wakeup handlers. If the caller users potentially
         * nested waitqueues with custom wakeup handlers, then it should
         * check eventfd_signal_allowed() before calling this function. If
         * it returns false, the eventfd_signal() call should be deferred to a
         * safe context.
         */
        if (WARN_ON_ONCE(current->in_eventfd))
                return;

        spin_lock_irqsave(&ctx->wqh.lock, flags);
        current->in_eventfd = 1;
        if (ctx->count < ULLONG_MAX)
                ctx->count++;
        if (waitqueue_active(&ctx->wqh))
                wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask);
        current->in_eventfd = 0;
        spin_unlock_irqrestore(&ctx->wqh.lock, flags);
}
EXPORT_SYMBOL_GPL(eventfd_signal_mask);

static void eventfd_free_ctx(struct eventfd_ctx *ctx)
{
        if (ctx->id >= 0)
                ida_free(&eventfd_ida, ctx->id);
        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;
}

void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
{
        lockdep_assert_held(&ctx->wqh.lock);

        *cnt = ((ctx->flags & EFD_SEMAPHORE) && ctx->count) ? 1 : ctx->count;
        ctx->count -= *cnt;
}
EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);

/**
 * 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 kiocb *iocb, struct iov_iter *to)
{
        struct file *file = iocb->ki_filp;
        struct eventfd_ctx *ctx = file->private_data;
        __u64 ucnt = 0;

        if (iov_iter_count(to) < sizeof(ucnt))
                return -EINVAL;
        spin_lock_irq(&ctx->wqh.lock);
        if (!ctx->count) {
                if ((file->f_flags & O_NONBLOCK) ||
                    (iocb->ki_flags & IOCB_NOWAIT)) {
                        spin_unlock_irq(&ctx->wqh.lock);
                        return -EAGAIN;
                }

                if (wait_event_interruptible_locked_irq(ctx->wqh, ctx->count)) {
                        spin_unlock_irq(&ctx->wqh.lock);
                        return -ERESTARTSYS;
                }
        }
        eventfd_ctx_do_read(ctx, &ucnt);
        current->in_eventfd = 1;
        if (waitqueue_active(&ctx->wqh))
                wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
        current->in_eventfd = 0;
        spin_unlock_irq(&ctx->wqh.lock);
        if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
                return -EFAULT;

        return sizeof(ucnt);
}

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;

        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)) {
                res = wait_event_interruptible_locked_irq(ctx->wqh,
                                ULLONG_MAX - ctx->count > ucnt);
                if (!res)
                        res = sizeof(ucnt);
        }
        if (likely(res > 0)) {
                ctx->count += ucnt;
                current->in_eventfd = 1;
                if (waitqueue_active(&ctx->wqh))
                        wake_up_locked_poll(&ctx->wqh, EPOLLIN);
                current->in_eventfd = 0;
        }
        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;
        __u64 cnt;

        spin_lock_irq(&ctx->wqh.lock);
        cnt = ctx->count;
        spin_unlock_irq(&ctx->wqh.lock);

        seq_printf(m,
                   "eventfd-count: %16llx\n"
                   "eventfd-id: %d\n"
                   "eventfd-semaphore: %d\n",
                   cnt,
                   ctx->id,
                   !!(ctx->flags & EFD_SEMAPHORE));
}
#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_iter      = 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)
{
        CLASS(fd, f)(fd);
        if (fd_empty(f))
                return ERR_PTR(-EBADF);
        return eventfd_ctx_fileget(fd_file(f));
}
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 __free(kfree) = NULL;

        /* Check the EFD_* constants for consistency.  */
        BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
        BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
        BUILD_BUG_ON(EFD_SEMAPHORE != (1 << 0));

        if (flags & ~EFD_FLAGS_SET)
                return -EINVAL;

        ctx = kmalloc_obj(*ctx);
        if (!ctx)
                return -ENOMEM;

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

        flags &= EFD_SHARED_FCNTL_FLAGS;
        flags |= O_RDWR;

        FD_PREPARE(fdf, flags,
                   anon_inode_getfile_fmode("[eventfd]", &eventfd_fops, ctx,
                                            flags, FMODE_NOWAIT));
        if (fdf.err)
                return fdf.err;

        ctx->id = ida_alloc(&eventfd_ida, GFP_KERNEL);
        retain_and_null_ptr(ctx);
        return fd_publish(fdf);
}

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

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