io_uring/net: fix multishot accept overflow handling

If we hit CQ ring overflow when attempting to post a multishot accept
completion, we don't properly save the result or return code. This
results in losing the accepted fd value.

Instead, we return the result from the poll operation that triggered
the accept retry. This is generally POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND
which is 0xc3, or 195, which looks like a valid file descriptor, but it
really has no connection to that.

Handle this like we do for other multishot completions - assign the
result, and return IOU_STOP_MULTISHOT to cancel any further completions
from this request when overflow is hit. This preserves the result, as we
should, and tells the application that the request needs to be re-armed.

Cc: stable@vger.kernel.org
Fixes: 515e269 ("io_uring: revert "io_uring fix multishot accept ordering"")
Link: axboe/liburing#1062
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/net.c

@@ -1372,7 +1372,7 @@ int io_accept(struct io_kiocb *req, unsigned int issue_flags)
 			 * has already been done
 			 */
 			if (issue_flags & IO_URING_F_MULTISHOT)
-				ret = IOU_ISSUE_SKIP_COMPLETE;
+				return IOU_ISSUE_SKIP_COMPLETE;
 			return ret;
 		}
 		if (ret == -ERESTARTSYS)
@@ -1397,7 +1397,8 @@ int io_accept(struct io_kiocb *req, unsigned int issue_flags)
 				ret, IORING_CQE_F_MORE))
 		goto retry;
 
-	return -ECANCELED;
+	io_req_set_res(req, ret, 0);
+	return IOU_STOP_MULTISHOT;
 }
 
 int io_socket_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)