[PATCH 5.8 244/255] io_uring: dont use poll handler if file cant be nonblocking read/written

[Greg Kroah-Hartman]

From: Jens Axboe <axboe@xxxxxxxxx>

[ Upstream commit 9dab14b81807a40dab8e464ec87043935c562c2c ]

There's no point in using the poll handler if we can't do a nonblocking
IO attempt of the operation, since we'll need to go async anyway. In
fact this is actively harmful, as reading from eg pipes won't return 0
to indicate EOF.

Cc: stable@xxxxxxxxxxxxxxx # v5.7+
Reported-by: Benedikt Ames <wisp3rwind@xxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/io_uring.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index c384caad64665..2b7018456091c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4503,12 +4503,20 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
 	struct async_poll *apoll;
 	struct io_poll_table ipt;
 	__poll_t mask, ret;
+	int rw;
 
 	if (!req->file || !file_can_poll(req->file))
 		return false;
 	if (req->flags & (REQ_F_MUST_PUNT | REQ_F_POLLED))
 		return false;
-	if (!def->pollin && !def->pollout)
+	if (def->pollin)
+		rw = READ;
+	else if (def->pollout)
+		rw = WRITE;
+	else
+		return false;
+	/* if we can't nonblock try, then no point in arming a poll handler */
+	if (!io_file_supports_async(req->file, rw))
 		return false;
 
 	apoll = kmalloc(sizeof(*apoll), GFP_ATOMIC);
-- 
2.25.1