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![Wen Yang](/assets/img/avatar_default.png)
For eventfd with flag EFD_SEMAPHORE, when its ctx->count is 0, calling
eventfd_ctx_do_read will cause ctx->count to overflow to ULLONG_MAX.
An underflow can happen with EFD_SEMAPHORE eventfds in at least the
following three subsystems:
(1) virt/kvm/eventfd.c
(2) drivers/vfio/virqfd.c
(3) drivers/virt/acrn/irqfd.c
where (2) and (3) are just modeled after (1). An eventfd must be
specified for use with the KVM_IRQFD ioctl(). This can also be an
EFD_SEMAPHORE eventfd. When the eventfd count is zero or has been
decremented to zero an underflow can be triggered when the irqfd is shut
down by raising the KVM_IRQFD_FLAG_DEASSIGN flag in the KVM_IRQFD
ioctl():
// ctx->count == 0
kvm_vm_ioctl()
-> kvm_irqfd()
-> kvm_irqfd_deassign()
-> irqfd_deactivate()
-> irqfd_shutdown()
-> eventfd_ctx_remove_wait_queue(&cnt)
-> eventfd_ctx_do_read(&cnt)
Userspace polling on the eventfd wouldn't notice the underflow because 1
is always returned as the value from eventfd_read() while ctx->count
would've underflowed. It's not a huge deal because this should only be
happening when the irqfd is shutdown but we should still fix it and
avoid the spurious wakeup.
Fixes: cb289d6244
("eventfd - allow atomic read and waitqueue remove")
Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dylan Yudaken <dylany@fb.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Message-Id: <tencent_7588DFD1F365950A757310D764517A14B306@qq.com>
[brauner: rewrite commit message and add explanation how this underflow can happen]
Signed-off-by: Christian Brauner <brauner@kernel.org>
442 lines
12 KiB
C
442 lines
12 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* fs/eventfd.c
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*
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* Copyright (C) 2007 Davide Libenzi <davidel@xmailserver.org>
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*
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*/
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#include <linux/file.h>
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#include <linux/poll.h>
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#include <linux/init.h>
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#include <linux/fs.h>
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#include <linux/sched/signal.h>
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#include <linux/kernel.h>
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#include <linux/slab.h>
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#include <linux/list.h>
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#include <linux/spinlock.h>
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#include <linux/anon_inodes.h>
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#include <linux/syscalls.h>
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#include <linux/export.h>
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#include <linux/kref.h>
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#include <linux/eventfd.h>
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#include <linux/proc_fs.h>
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#include <linux/seq_file.h>
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#include <linux/idr.h>
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#include <linux/uio.h>
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static DEFINE_IDA(eventfd_ida);
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struct eventfd_ctx {
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struct kref kref;
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wait_queue_head_t wqh;
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/*
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* Every time that a write(2) is performed on an eventfd, the
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* value of the __u64 being written is added to "count" and a
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* wakeup is performed on "wqh". If EFD_SEMAPHORE flag was not
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* specified, a read(2) will return the "count" value to userspace,
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* and will reset "count" to zero. The kernel side eventfd_signal()
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* also, adds to the "count" counter and issue a wakeup.
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*/
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__u64 count;
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unsigned int flags;
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int id;
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};
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__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
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{
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unsigned long flags;
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/*
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* Deadlock or stack overflow issues can happen if we recurse here
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* through waitqueue wakeup handlers. If the caller users potentially
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* nested waitqueues with custom wakeup handlers, then it should
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* check eventfd_signal_allowed() before calling this function. If
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* it returns false, the eventfd_signal() call should be deferred to a
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* safe context.
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*/
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if (WARN_ON_ONCE(current->in_eventfd))
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return 0;
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spin_lock_irqsave(&ctx->wqh.lock, flags);
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current->in_eventfd = 1;
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if (ULLONG_MAX - ctx->count < n)
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n = ULLONG_MAX - ctx->count;
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ctx->count += n;
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if (waitqueue_active(&ctx->wqh))
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wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask);
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current->in_eventfd = 0;
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spin_unlock_irqrestore(&ctx->wqh.lock, flags);
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return n;
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}
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/**
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* eventfd_signal - Adds @n to the eventfd counter.
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* @ctx: [in] Pointer to the eventfd context.
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* @n: [in] Value of the counter to be added to the eventfd internal counter.
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* The value cannot be negative.
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*
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* This function is supposed to be called by the kernel in paths that do not
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* allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
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* value, and we signal this as overflow condition by returning a EPOLLERR
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* to poll(2).
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*
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* Returns the amount by which the counter was incremented. This will be less
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* than @n if the counter has overflowed.
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*/
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__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
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{
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return eventfd_signal_mask(ctx, n, 0);
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}
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EXPORT_SYMBOL_GPL(eventfd_signal);
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static void eventfd_free_ctx(struct eventfd_ctx *ctx)
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{
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if (ctx->id >= 0)
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ida_simple_remove(&eventfd_ida, ctx->id);
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kfree(ctx);
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}
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static void eventfd_free(struct kref *kref)
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{
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struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
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eventfd_free_ctx(ctx);
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}
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/**
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* eventfd_ctx_put - Releases a reference to the internal eventfd context.
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* @ctx: [in] Pointer to eventfd context.
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*
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* The eventfd context reference must have been previously acquired either
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* with eventfd_ctx_fdget() or eventfd_ctx_fileget().
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*/
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void eventfd_ctx_put(struct eventfd_ctx *ctx)
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{
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kref_put(&ctx->kref, eventfd_free);
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}
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EXPORT_SYMBOL_GPL(eventfd_ctx_put);
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static int eventfd_release(struct inode *inode, struct file *file)
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{
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struct eventfd_ctx *ctx = file->private_data;
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wake_up_poll(&ctx->wqh, EPOLLHUP);
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eventfd_ctx_put(ctx);
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return 0;
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}
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static __poll_t eventfd_poll(struct file *file, poll_table *wait)
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{
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struct eventfd_ctx *ctx = file->private_data;
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__poll_t events = 0;
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u64 count;
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poll_wait(file, &ctx->wqh, wait);
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/*
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* All writes to ctx->count occur within ctx->wqh.lock. This read
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* can be done outside ctx->wqh.lock because we know that poll_wait
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* takes that lock (through add_wait_queue) if our caller will sleep.
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*
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* The read _can_ therefore seep into add_wait_queue's critical
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* section, but cannot move above it! add_wait_queue's spin_lock acts
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* as an acquire barrier and ensures that the read be ordered properly
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* against the writes. The following CAN happen and is safe:
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*
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* poll write
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* ----------------- ------------
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* lock ctx->wqh.lock (in poll_wait)
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* count = ctx->count
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* __add_wait_queue
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* unlock ctx->wqh.lock
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* lock ctx->qwh.lock
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* ctx->count += n
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* if (waitqueue_active)
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* wake_up_locked_poll
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* unlock ctx->qwh.lock
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* eventfd_poll returns 0
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*
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* but the following, which would miss a wakeup, cannot happen:
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*
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* poll write
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* ----------------- ------------
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* count = ctx->count (INVALID!)
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* lock ctx->qwh.lock
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* ctx->count += n
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* **waitqueue_active is false**
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* **no wake_up_locked_poll!**
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* unlock ctx->qwh.lock
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* lock ctx->wqh.lock (in poll_wait)
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* __add_wait_queue
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* unlock ctx->wqh.lock
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* eventfd_poll returns 0
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*/
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count = READ_ONCE(ctx->count);
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if (count > 0)
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events |= EPOLLIN;
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if (count == ULLONG_MAX)
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events |= EPOLLERR;
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if (ULLONG_MAX - 1 > count)
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events |= EPOLLOUT;
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return events;
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}
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void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
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{
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lockdep_assert_held(&ctx->wqh.lock);
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*cnt = ((ctx->flags & EFD_SEMAPHORE) && ctx->count) ? 1 : ctx->count;
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ctx->count -= *cnt;
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}
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EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
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/**
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* eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
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* @ctx: [in] Pointer to eventfd context.
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* @wait: [in] Wait queue to be removed.
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* @cnt: [out] Pointer to the 64-bit counter value.
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*
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* Returns %0 if successful, or the following error codes:
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*
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* -EAGAIN : The operation would have blocked.
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*
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* This is used to atomically remove a wait queue entry from the eventfd wait
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* queue head, and read/reset the counter value.
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*/
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int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
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__u64 *cnt)
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{
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unsigned long flags;
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spin_lock_irqsave(&ctx->wqh.lock, flags);
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eventfd_ctx_do_read(ctx, cnt);
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__remove_wait_queue(&ctx->wqh, wait);
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if (*cnt != 0 && waitqueue_active(&ctx->wqh))
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wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
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spin_unlock_irqrestore(&ctx->wqh.lock, flags);
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return *cnt != 0 ? 0 : -EAGAIN;
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}
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EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
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static ssize_t eventfd_read(struct kiocb *iocb, struct iov_iter *to)
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{
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struct file *file = iocb->ki_filp;
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struct eventfd_ctx *ctx = file->private_data;
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__u64 ucnt = 0;
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if (iov_iter_count(to) < sizeof(ucnt))
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return -EINVAL;
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spin_lock_irq(&ctx->wqh.lock);
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if (!ctx->count) {
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if ((file->f_flags & O_NONBLOCK) ||
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(iocb->ki_flags & IOCB_NOWAIT)) {
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spin_unlock_irq(&ctx->wqh.lock);
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return -EAGAIN;
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}
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if (wait_event_interruptible_locked_irq(ctx->wqh, ctx->count)) {
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spin_unlock_irq(&ctx->wqh.lock);
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return -ERESTARTSYS;
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}
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}
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eventfd_ctx_do_read(ctx, &ucnt);
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current->in_eventfd = 1;
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if (waitqueue_active(&ctx->wqh))
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wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
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current->in_eventfd = 0;
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spin_unlock_irq(&ctx->wqh.lock);
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if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
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return -EFAULT;
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return sizeof(ucnt);
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}
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static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
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loff_t *ppos)
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{
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struct eventfd_ctx *ctx = file->private_data;
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ssize_t res;
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__u64 ucnt;
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if (count < sizeof(ucnt))
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return -EINVAL;
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if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
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return -EFAULT;
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if (ucnt == ULLONG_MAX)
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return -EINVAL;
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spin_lock_irq(&ctx->wqh.lock);
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res = -EAGAIN;
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if (ULLONG_MAX - ctx->count > ucnt)
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res = sizeof(ucnt);
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else if (!(file->f_flags & O_NONBLOCK)) {
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res = wait_event_interruptible_locked_irq(ctx->wqh,
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ULLONG_MAX - ctx->count > ucnt);
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if (!res)
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res = sizeof(ucnt);
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}
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if (likely(res > 0)) {
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ctx->count += ucnt;
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current->in_eventfd = 1;
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if (waitqueue_active(&ctx->wqh))
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wake_up_locked_poll(&ctx->wqh, EPOLLIN);
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current->in_eventfd = 0;
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}
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spin_unlock_irq(&ctx->wqh.lock);
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return res;
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}
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#ifdef CONFIG_PROC_FS
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static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
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{
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struct eventfd_ctx *ctx = f->private_data;
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spin_lock_irq(&ctx->wqh.lock);
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seq_printf(m, "eventfd-count: %16llx\n",
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(unsigned long long)ctx->count);
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spin_unlock_irq(&ctx->wqh.lock);
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seq_printf(m, "eventfd-id: %d\n", ctx->id);
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seq_printf(m, "eventfd-semaphore: %d\n",
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!!(ctx->flags & EFD_SEMAPHORE));
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}
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#endif
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static const struct file_operations eventfd_fops = {
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#ifdef CONFIG_PROC_FS
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.show_fdinfo = eventfd_show_fdinfo,
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#endif
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.release = eventfd_release,
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.poll = eventfd_poll,
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.read_iter = eventfd_read,
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.write = eventfd_write,
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.llseek = noop_llseek,
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};
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/**
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* eventfd_fget - Acquire a reference of an eventfd file descriptor.
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* @fd: [in] Eventfd file descriptor.
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*
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* Returns a pointer to the eventfd file structure in case of success, or the
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* following error pointer:
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*
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* -EBADF : Invalid @fd file descriptor.
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* -EINVAL : The @fd file descriptor is not an eventfd file.
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*/
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struct file *eventfd_fget(int fd)
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{
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struct file *file;
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file = fget(fd);
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if (!file)
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return ERR_PTR(-EBADF);
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if (file->f_op != &eventfd_fops) {
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fput(file);
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return ERR_PTR(-EINVAL);
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}
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return file;
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}
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EXPORT_SYMBOL_GPL(eventfd_fget);
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/**
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* eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
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* @fd: [in] Eventfd file descriptor.
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*
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* Returns a pointer to the internal eventfd context, otherwise the error
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* pointers returned by the following functions:
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*
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* eventfd_fget
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*/
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struct eventfd_ctx *eventfd_ctx_fdget(int fd)
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{
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struct eventfd_ctx *ctx;
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struct fd f = fdget(fd);
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if (!f.file)
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return ERR_PTR(-EBADF);
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ctx = eventfd_ctx_fileget(f.file);
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fdput(f);
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return ctx;
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}
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EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
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/**
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* eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
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* @file: [in] Eventfd file pointer.
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*
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* Returns a pointer to the internal eventfd context, otherwise the error
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* pointer:
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*
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* -EINVAL : The @fd file descriptor is not an eventfd file.
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*/
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struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
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{
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struct eventfd_ctx *ctx;
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if (file->f_op != &eventfd_fops)
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return ERR_PTR(-EINVAL);
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ctx = file->private_data;
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kref_get(&ctx->kref);
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return ctx;
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}
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EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
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static int do_eventfd(unsigned int count, int flags)
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{
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struct eventfd_ctx *ctx;
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struct file *file;
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int fd;
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/* Check the EFD_* constants for consistency. */
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BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
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BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
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if (flags & ~EFD_FLAGS_SET)
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return -EINVAL;
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ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
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if (!ctx)
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return -ENOMEM;
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kref_init(&ctx->kref);
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init_waitqueue_head(&ctx->wqh);
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ctx->count = count;
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ctx->flags = flags;
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ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
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flags &= EFD_SHARED_FCNTL_FLAGS;
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flags |= O_RDWR;
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fd = get_unused_fd_flags(flags);
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if (fd < 0)
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goto err;
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file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, flags);
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if (IS_ERR(file)) {
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put_unused_fd(fd);
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fd = PTR_ERR(file);
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goto err;
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}
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file->f_mode |= FMODE_NOWAIT;
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fd_install(fd, file);
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return fd;
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err:
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eventfd_free_ctx(ctx);
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return fd;
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}
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SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
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{
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return do_eventfd(count, flags);
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}
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SYSCALL_DEFINE1(eventfd, unsigned int, count)
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{
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return do_eventfd(count, 0);
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}
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