wayland-server: Add API to control globals visibility

Add a new API to let compositor decide whether or not a wl_global
should be advertised to the clients via wl_registry_bind() or
display_get_registry()

By using its own filter, the compositor can decide which wl_global would
be listed to clients.

Compositors can use this mechanism to hide their own private interfaces
that regular clients should not use.

- Hiding interfaces that expose compositor implementation details
  makes it harder for clients to identify the compositor. Therefore
  clients are a little less likely to develop compositor-specific
  workarounds instead of reporting problems upstream.

- Hiding can be used to diminish the problems from missing namespacing:
  if two compositors happen to use the same named global with
  different interfaces for their special-purpose clients, the client
  expecting the different interface would probably never see it
  advertised.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/wayland-server-core.h

@@ -28,6 +28,7 @@
 
 #include <sys/types.h>
 #include <stdint.h>
+#include <stdbool.h>
 #include "wayland-util.h"
 #include "wayland-version.h"
 
@@ -168,6 +169,30 @@ wl_global_create(struct wl_display *display,
 void
 wl_global_destroy(struct wl_global *global);
 
+/** A filter function for wl_global objects
+ *
+ * \param client The client object
+ * \param global The global object to show or hide
+ * \param data   The user data pointer
+ *
+ * A filter function enables the server to decide which globals to
+ * advertise to each client.
+ *
+ * When a wl_global filter is set, the given callback funtion will be
+ * called during wl_global advertisment and binding.
+ *
+ * This function should return true if the global object should be made
+ * visible to the client or false otherwise.
+ */
+typedef bool (*wl_display_global_filter_func_t)(const struct wl_client *client,
+						const struct wl_global *global,
+						void *data);
+
+void
+wl_display_set_global_filter(struct wl_display *display,
+			     wl_display_global_filter_func_t filter,
+			     void *data);
+
 struct wl_client *
 wl_client_create(struct wl_display *display, int fd);
 

src/wayland-server.c

@@ -101,6 +101,9 @@ struct wl_display {
 	struct wl_signal create_client_signal;
 
 	struct wl_array additional_shm_formats;
+
+	wl_display_global_filter_func_t global_filter;
+	void *global_filter_data;
 };
 
 struct wl_global {
@@ -769,6 +772,21 @@ wl_client_destroy(struct wl_client *client)
 	free(client);
 }
 
+/* Check if a global filter is registered and use it if any.
+ *
+ * If no wl_global filter has been registered, this funtion will
+ * return true, allowing the wl_global to be visible to the wl_client
+ */
+static bool
+wl_global_is_visible(const struct wl_client *client,
+	      const struct wl_global *global)
+{
+	struct wl_display *display = client->display;
+
+	return (display->global_filter == NULL ||
+		display->global_filter(client, global, display->global_filter_data));
+}
+
 static void
 registry_bind(struct wl_client *client,
 	      struct wl_resource *resource, uint32_t name,
@@ -795,6 +813,10 @@ registry_bind(struct wl_client *client,
 				       WL_DISPLAY_ERROR_INVALID_OBJECT,
 				       "invalid version for global %s (%d): have %d, wanted %d",
 				       interface, name, global->version, version);
+	else if (!wl_global_is_visible(client, global))
+		wl_resource_post_error(resource,
+				       WL_DISPLAY_ERROR_INVALID_OBJECT,
+				       "invalid global %s (%d)", interface, name);
 	else
 		global->bind(client, global->data, version, id);
 }
@@ -850,11 +872,12 @@ display_get_registry(struct wl_client *client,
 		       &registry_resource->link);
 
 	wl_list_for_each(global, &display->global_list, link)
-		wl_resource_post_event(registry_resource,
+		if (wl_global_is_visible(client, global))
-				       WL_REGISTRY_GLOBAL,
+			wl_resource_post_event(registry_resource,
-				       global->name,
+					       WL_REGISTRY_GLOBAL,
-				       global->interface->name,
+					       global->name,
-				       global->version);
+					       global->interface->name,
+					       global->version);
 }
 
 static const struct wl_display_interface display_interface = {
@@ -925,6 +948,9 @@ wl_display_create(void)
 	display->id = 1;
 	display->serial = 0;
 
+	display->global_filter = NULL;
+	display->global_filter_data = NULL;
+
 	wl_array_init(&display->additional_shm_formats);
 
 	return display;
@@ -999,6 +1025,37 @@ wl_display_destroy(struct wl_display *display)
 	free(display);
 }
 
+/** Set a filter function for global objects
+ *
+ * \param display The Wayland display object.
+ * \param filter  The global filter funtion.
+ * \param data User data to be associated with the global filter.
+ * \return None.
+ *
+ * Set a filter for the wl_display to advertise or hide global objects
+ * to clients.
+ * The set filter will be used during wl_global advertisment to
+ * determine whether a global object should be advertised to a
+ * given client, and during wl_global binding to determine whether
+ * a given client should be allowed to bind to a global.
+ *
+ * Clients that try to bind to a global that was filtered out will
+ * have an error raised.
+ *
+ * Setting the filter NULL will result in all globals being
+ * advertised to all clients. The default is no filter.
+ *
+ * \memberof wl_display
+ */
+WL_EXPORT void
+wl_display_set_global_filter(struct wl_display *display,
+			     wl_display_global_filter_func_t filter,
+			     void *data)
+{
+	display->global_filter = filter;
+	display->global_filter_data = data;
+}
+
 WL_EXPORT struct wl_global *
 wl_global_create(struct wl_display *display,
 		 const struct wl_interface *interface, int version,