wayland-server: return new ID in wl_client_add_resource()

wl_client_add_resource() used to return no error even though the new
resource wasn't added to the client. This currently makes it very easy to
DOS weston by simply posting thousands of "create_surface" requests with
an invalid ID. Weston simply assumes the wl_client_add_resource() request
succeeds but will never destroy the surface again as the "destroy" signal
is never called (because the surface isn't linked into the wl_map).

This change makes wl_client_add_resource() return the new ID of the added
object and 0 on failure. Servers (like weston) can now correctly
immediately destroy the surface when this call fails instead of leaving
the surface around and producing memory-leaks.

Instead of returning -1 on failure and 0 on success, I made it return the
new ID as this seems more appropriate. We can directly use it when calling
it with new_id==0.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

src/wayland-server.c

@@ -387,23 +387,27 @@ wl_client_get_credentials(struct wl_client *client,
 		*gid = client->ucred.gid;
 }
 
-WL_EXPORT void
+WL_EXPORT uint32_t
 wl_client_add_resource(struct wl_client *client,
 		       struct wl_resource *resource)
 {
-	if (resource->object.id == 0)
+	if (resource->object.id == 0) {
 		resource->object.id =
 			wl_map_insert_new(&client->objects,
 					  WL_MAP_SERVER_SIDE, resource);
-	else if (wl_map_insert_at(&client->objects,
+	} else if (wl_map_insert_at(&client->objects,
-				  resource->object.id, resource) < 0)
+				  resource->object.id, resource) < 0) {
 		wl_resource_post_error(client->display_resource,
 				       WL_DISPLAY_ERROR_INVALID_OBJECT,
 				       "invalid new id %d",
 				       resource->object.id);
+		return 0;
+	}
 
 	resource->client = client;
 	wl_signal_init(&resource->destroy_signal);
+
+	return resource->object.id;
 }
 
 WL_EXPORT struct wl_resource *

src/wayland-server.h

@@ -354,7 +354,7 @@ void wl_resource_post_no_memory(struct wl_resource *resource);
 
 #include "wayland-server-protocol.h"
 
-void
+uint32_t
 wl_client_add_resource(struct wl_client *client,
 		       struct wl_resource *resource);