io_uring: defer msg-ring file validity check until command issue

In preparation for not using the file at prep time, defer checking if this
file refers to a valid io_uring instance until issue time.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4473,9 +4473,6 @@ static int io_msg_ring_prep(struct io_kiocb *req,
 		     sqe->splice_fd_in || sqe->buf_index || sqe->personality))
 		return -EINVAL;
 
-	if (req->file->f_op != &io_uring_fops)
-		return -EBADFD;
-
 	req->msg.user_data = READ_ONCE(sqe->off);
 	req->msg.len = READ_ONCE(sqe->len);
 	return 0;
@@ -4485,9 +4482,14 @@ static int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct io_ring_ctx *target_ctx;
 	struct io_msg *msg = &req->msg;
-	int ret = -EOVERFLOW;
 	bool filled;
+	int ret;
 
+	ret = -EBADFD;
+	if (req->file->f_op != &io_uring_fops)
+		goto done;
+
+	ret = -EOVERFLOW;
 	target_ctx = req->file->private_data;
 
 	spin_lock(&target_ctx->completion_lock);
@@ -4500,6 +4502,7 @@ static int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags)
 		ret = 0;
 	}
 
+done:
 	if (ret < 0)
 		req_set_fail(req);
 	__io_req_complete(req, issue_flags, ret, 0);