io_uring/poll: don't reissue in case of poll race on multishot request

A previous commit fixed a poll race that can occur, but it's only
applicable for multishot requests. For a multishot request, we can safely
ignore a spurious wakeup, as we never leave the waitqueue to begin with.

A blunt reissue of a multishot armed request can cause us to leak a
buffer, if they are ring provided. While this seems like a bug in itself,
it's not really defined behavior to reissue a multishot request directly.
It's less efficient to do so as well, and not required to rearm anything
like it is for singleshot poll requests.

Cc: [e-mail]
Fixes: 6e5aedb9324a ("io_uring/poll: attempt request issue after racy poll wakeup")
Reported-and-tested-by: Olivier Langlois <[e-mail]>
Link: https://github.com/axboe/liburing/issues/778
Signed-off-by: Jens Axboe <[e-mail]>

io_uring/poll.c

···
 			 * to the waitqueue, so if we get nothing back, we
 			 * should be safe and attempt a reissue.
 			 */
-			if (unlikely(!req->cqe.res))
+			if (unlikely(!req->cqe.res)) {
+				/* Multishot armed need not reissue */
+				if (!(req->apoll_events & EPOLLONESHOT))
+					continue;
 				return IOU_POLL_REISSUE;
+			}
 		}
 		if (req->apoll_events & EPOLLONESHOT)
 			return IOU_POLL_DONE;