io_uring: fail NOP if non-zero op flags is passed in

The NOP op flags should have been checked from beginning like any other
opcode, otherwise NOP may not be extended with the op flags.

Given both liburing and Rust io-uring crate always zeros SQE op flags, just
ignore users which play raw NOP uring interface without zeroing SQE, because
NOP is just for test purpose. Then we can save one NOP2 opcode.

Suggested-by: Jens Axboe <axboe@kernel.dk>
Fixes: 2b188cc ("Add io_uring IO interface")
Cc: stable@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20240510035031.78874-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

io_uring/nop.c

@@ -12,6 +12,8 @@
 
 int io_nop_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
+	if (READ_ONCE(sqe->rw_flags))
+		return -EINVAL;
 	return 0;
 }