Re: [PATCH RFC] io_uring: make signalfd work with io_uring (and aio) POLL

[Jens Axboe]

On 11/13/19 9:31 PM, Jens Axboe wrote:
> This is a case of "I don't really know what I'm doing, but this works
> for me". Caveat emptor, but I'd love some input on this.
> 
> I got a bug report that using the poll command with signalfd doesn't
> work for io_uring. The reporter also noted that it doesn't work with the
> aio poll implementation either. So I took a look at it.
> 
> What happens is that the original task issues the poll request, we call
> ->poll() (which ends up with signalfd for this fd), and find that
> nothing is pending. Then we wait, and the poll is passed to async
> context. When the requested signal comes in, that worker is woken up,
> and proceeds to call ->poll() again, and signalfd unsurprisingly finds
> no signals pending, since it's the async worker calling it.
> 
> That's obviously no good. The below allows you to pass in the task in
> the poll_table, and it does the right thing for me, signal is delivered
> and the correct mask is checked in signalfd_poll().
> 
> Similar patch for aio would be trivial, of course.

>From the probably-less-nasty category, Jann Horn helpfully pointed out
that it'd be easier if signalfd just looked at the task that originally
created the fd instead. That looks like the below, and works equally
well for the test case at hand.

diff --git a/fs/signalfd.c b/fs/signalfd.c
index 44b6845b071c..cc72b5b08946 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -50,6 +50,7 @@ void signalfd_cleanup(struct sighand_struct *sighand)
 
 struct signalfd_ctx {
 	sigset_t sigmask;
+	struct task_struct *task;
 };
 
 static int signalfd_release(struct inode *inode, struct file *file)
@@ -63,14 +64,14 @@ static __poll_t signalfd_poll(struct file *file, poll_table *wait)
 	struct signalfd_ctx *ctx = file->private_data;
 	__poll_t events = 0;
 
-	poll_wait(file, &current->sighand->signalfd_wqh, wait);
+	poll_wait(file, &ctx->task->sighand->signalfd_wqh, wait);
 
-	spin_lock_irq(&current->sighand->siglock);
-	if (next_signal(&current->pending, &ctx->sigmask) ||
-	    next_signal(&current->signal->shared_pending,
+	spin_lock_irq(&ctx->task->sighand->siglock);
+	if (next_signal(&ctx->task->pending, &ctx->sigmask) ||
+	    next_signal(&ctx->task->signal->shared_pending,
 			&ctx->sigmask))
 		events |= EPOLLIN;
-	spin_unlock_irq(&current->sighand->siglock);
+	spin_unlock_irq(&ctx->task->sighand->siglock);
 
 	return events;
 }
@@ -280,6 +281,7 @@ static int do_signalfd4(int ufd, sigset_t *mask, int flags)
 			return -ENOMEM;
 
 		ctx->sigmask = *mask;
+		ctx->task = current;
 
 		/*
 		 * When we call this, the initialization must be complete, since

-- 
Jens Axboe