Commit 9f0deaa1 authored by Dylan Yudaken's avatar Dylan Yudaken Committed by Jens Axboe
Browse files

eventfd: guard wake_up in eventfd fs calls as well 



Guard wakeups that the user can trigger, and that may end up triggering a
call back into eventfd_signal. This is in addition to the current approach
that only guards in eventfd_signal.

Rename in_eventfd_signal -> in_eventfd at the same time to reflect this.

Without this there would be a deadlock in the following code using libaio:

int main()
{
	struct io_context *ctx = NULL;
	struct iocb iocb;
	struct iocb *iocbs[] = { &iocb };
	int evfd;
        uint64_t val = 1;

	evfd = eventfd(0, EFD_CLOEXEC);
	assert(!io_setup(2, &ctx));
	io_prep_poll(&iocb, evfd, POLLIN);
	io_set_eventfd(&iocb, evfd);
	assert(1 == io_submit(ctx, 1, iocbs));
        write(evfd, &val, 8);
}

Signed-off-by: default avatarDylan Yudaken <dylany@fb.com>
Reviewed-by: default avatarJens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20220816135959.1490641-1-dylany@fb.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 521a547c

fs/eventfd.c

+7 −3
Original line number Diff line number Diff line
@@ -69,17 +69,17 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) 
	 * it returns false, the eventfd_signal() call should be deferred to a

	 * safe context.

	 */

	if (WARN_ON_ONCE(current->in_eventfd_signal))

	if (WARN_ON_ONCE(current->in_eventfd))

		return 0;



	spin_lock_irqsave(&ctx->wqh.lock, flags);

	current->in_eventfd_signal = 1;

	current->in_eventfd = 1;

	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);

	current->in_eventfd_signal = 0;

	current->in_eventfd = 0;

	spin_unlock_irqrestore(&ctx->wqh.lock, flags);



	return n;
@@ -253,8 +253,10 @@ static ssize_t eventfd_read(struct kiocb *iocb, struct iov_iter *to) 
		__set_current_state(TASK_RUNNING);

	}

	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;
@@ -301,8 +303,10 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c 
	}

	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);

include/linux/eventfd.h

+1 −1
Original line number Diff line number Diff line
@@ -46,7 +46,7 @@ void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt); 


static inline bool eventfd_signal_allowed(void)

{

	return !current->in_eventfd_signal;

	return !current->in_eventfd;

}



#else /* CONFIG_EVENTFD */

include/linux/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -936,7 +936,7 @@ struct task_struct { 
#endif

#ifdef CONFIG_EVENTFD

	/* Recursion prevention for eventfd_signal() */

	unsigned			in_eventfd_signal:1;

	unsigned			in_eventfd:1;

#endif

#ifdef CONFIG_IOMMU_SVA

	unsigned			pasid_activated:1;