[PATCH v2 for-next 08/12] io_uring: fix multishot poll on overflow

[Dylan Yudaken <dylany@fb.com>]

On overflow, multishot poll can still complete with the IORING_CQE_F_MORE
flag set.
If in the meantime the user clears a CQE and a the poll was cancelled then
the poll will post a CQE without the IORING_CQE_F_MORE (and likely result
-ECANCELED).

However when processing the application will encounter the non-overflow
CQE which indicates that there will be no more events posted. Typical
userspace applications would free memory associated with the poll in this
case.
It will then subsequently receive the earlier CQE which has overflowed,
which breaks the contract given by the IORING_CQE_F_MORE flag.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
---
 io_uring/poll.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/io_uring/poll.c b/io_uring/poll.c
index e8f922a4f6d7..57747d92bba4 100644
--- a/io_uring/poll.c
+++ b/io_uring/poll.c
@@ -243,8 +243,10 @@ static int io_poll_check_events(struct io_kiocb *req, bool *locked)
 						    req->apoll_events);
 
 			if (!io_post_aux_cqe(ctx, req->cqe.user_data,
-					     mask, IORING_CQE_F_MORE, true))
-				return -ECANCELED;
+					     mask, IORING_CQE_F_MORE, false)) {
+				io_req_set_res(req, mask, 0);
+				return IOU_POLL_REMOVE_POLL_USE_RES;
+			}
 		} else {
 			ret = io_poll_issue(req, locked);
 			if (ret == IOU_STOP_MULTISHOT)
-- 
2.30.2