io_uring/msg_ring: let target know allocated index

msg_ring requests transferring files support auto index selection via
IORING_FILE_INDEX_ALLOC, however they don't return the selected index
to the target ring and there is no other good way for the userspace to
know where is the receieved file.

Return the index for allocated slots and 0 otherwise, which is
consistent with other fixed file installing requests.

Cc: stable@vger.kernel.org # v6.0+
Fixes: e6130eb ("io_uring: add support for passing fixed file descriptors")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: axboe/liburing#809
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/msg_ring.c

@@ -202,7 +202,7 @@ static int io_msg_install_complete(struct io_kiocb *req, unsigned int issue_flag
 	 * completes with -EOVERFLOW, then the sender must ensure that a
 	 * later IORING_OP_MSG_RING delivers the message.
 	 */
-	if (!io_post_aux_cqe(target_ctx, msg->user_data, msg->len, 0))
+	if (!io_post_aux_cqe(target_ctx, msg->user_data, ret, 0))
 		ret = -EOVERFLOW;
 out_unlock:
 	io_double_unlock_ctx(target_ctx);
@@ -229,6 +229,8 @@ static int io_msg_send_fd(struct io_kiocb *req, unsigned int issue_flags)
 	struct io_ring_ctx *ctx = req->ctx;
 	struct file *src_file = msg->src_file;
 
+	if (msg->len)
+		return -EINVAL;
 	if (target_ctx == ctx)
 		return -EINVAL;
 	if (target_ctx->flags & IORING_SETUP_R_DISABLED)