backend: fix use-after-free when destroying backends

The backend destroy signal is emitted before the output_remove
signal is. When the destroy signal is emitted listeners remove
their output_remove listener, so the output_remove signal is never
received and listeners have an invalid output pointer.

The correct way to solve this would be to remove the output_remove
signal completely and use the wlr_output.events.destroy signal
instead. This isn't yet possible because wl_signal_emit is unsafe
and listeners cannot be removed in listeners.

backend/backend.c

@@ -37,7 +37,6 @@ void wlr_backend_destroy(struct wlr_backend *backend) {
 		return;
 	}
 
-	wl_signal_emit(&backend->events.destroy, backend);
 	if (backend->impl && backend->impl->destroy) {
 		backend->impl->destroy(backend);
 	} else {

backend/drm/backend.c

@@ -34,6 +34,8 @@ static void wlr_drm_backend_destroy(struct wlr_backend *backend) {
 		wlr_output_destroy(&conn->output);
 	}
 
+	wl_signal_emit(&backend->events.destroy, backend);
+
 	wl_list_remove(&drm->display_destroy.link);
 	wl_list_remove(&drm->session_signal.link);
 	wl_list_remove(&drm->drm_invalidated.link);

backend/headless/backend.c

@@ -51,6 +51,8 @@ static void backend_destroy(struct wlr_backend *wlr_backend) {
 		wlr_input_device_destroy(&input_device->wlr_input_device);
 	}
 
+	wl_signal_emit(&wlr_backend->events.destroy, backend);
+
 	wlr_egl_finish(&backend->egl);
 	free(backend);
 }

backend/headless/output.c

@@ -62,8 +62,6 @@ static bool output_swap_buffers(struct wlr_output *wlr_output) {
 static void output_destroy(struct wlr_output *wlr_output) {
 	struct wlr_headless_output *output =
 		(struct wlr_headless_output *)wlr_output;
-	wl_signal_emit(&output->backend->backend.events.output_remove,
-		&output->wlr_output);
 
 	wl_list_remove(&output->link);
 

backend/libinput/backend.c

@@ -95,12 +95,12 @@ static bool wlr_libinput_backend_start(struct wlr_backend *_backend) {
 	return true;
 }
 
-static void wlr_libinput_backend_destroy(struct wlr_backend *_backend) {
-	if (!_backend) {
+static void wlr_libinput_backend_destroy(struct wlr_backend *wlr_backend) {
+	if (!wlr_backend) {
 		return;
 	}
 	struct wlr_libinput_backend *backend =
-		(struct wlr_libinput_backend *)_backend;
+		(struct wlr_libinput_backend *)wlr_backend;
 
 	for (size_t i = 0; i < backend->wlr_device_lists.length; i++) {
 		struct wl_list *wlr_devices = backend->wlr_device_lists.items[i];
@@ -112,6 +112,8 @@ static void wlr_libinput_backend_destroy(struct wlr_backend *_backend) {
 		free(wlr_devices);
 	}
 
+	wl_signal_emit(&wlr_backend->events.destroy, wlr_backend);
+
 	wl_list_remove(&backend->display_destroy.link);
 	wl_list_remove(&backend->session_signal.link);
 

backend/multi/backend.c

@@ -42,11 +42,16 @@ static void subbackend_state_destroy(struct subbackend_state *sub) {
 
 static void multi_backend_destroy(struct wlr_backend *wlr_backend) {
 	struct wlr_multi_backend *backend = (struct wlr_multi_backend *)wlr_backend;
+
 	wl_list_remove(&backend->display_destroy.link);
+
 	struct subbackend_state *sub, *next;
 	wl_list_for_each_safe(sub, next, &backend->backends, link) {
 		wlr_backend_destroy(sub->backend);
 	}
+
+	// Destroy this backend only after removing all sub-backends
+	wl_signal_emit(&wlr_backend->events.destroy, backend);
 	free(backend);
 }
 

backend/wayland/backend.c

@@ -64,9 +64,9 @@ static bool wlr_wl_backend_start(struct wlr_backend *_backend) {
 	return true;
 }
 
-static void wlr_wl_backend_destroy(struct wlr_backend *_backend) {
-	struct wlr_wl_backend *backend = (struct wlr_wl_backend *)_backend;
-	if (!_backend) {
+static void wlr_wl_backend_destroy(struct wlr_backend *wlr_backend) {
+	struct wlr_wl_backend *backend = (struct wlr_wl_backend *)wlr_backend;
+	if (backend == NULL) {
 		return;
 	}
 
@@ -80,6 +80,8 @@ static void wlr_wl_backend_destroy(struct wlr_backend *_backend) {
 		wlr_input_device_destroy(input_device);
 	}
 
+	wl_signal_emit(&wlr_backend->events.destroy, wlr_backend);
+
 	wl_list_remove(&backend->local_display_destroy.link);
 
 	free(backend->seat_name);

backend/wayland/output.c

@@ -161,11 +161,12 @@ static bool wlr_wl_output_set_cursor(struct wlr_output *_output,
 	return true;
 }
 
-static void wlr_wl_output_destroy(struct wlr_output *_output) {
+static void wlr_wl_output_destroy(struct wlr_output *wlr_output) {
 	struct wlr_wl_backend_output *output =
-		(struct wlr_wl_backend_output *)_output;
-	wl_signal_emit(&output->backend->backend.events.output_remove,
-		&output->wlr_output);
+		(struct wlr_wl_backend_output *)wlr_output;
+	if (output == NULL) {
+		return;
+	}
 
 	wl_list_remove(&output->link);
 

backend/x11/backend.c

@@ -259,6 +259,8 @@ static void wlr_x11_backend_destroy(struct wlr_backend *backend) {
 		xkb_state_unref(x11->keyboard_dev.keyboard->xkb_state);
 	}
 
+	wl_signal_emit(&backend->events.destroy, backend);
+
 	wl_list_remove(&x11->display_destroy.link);
 
 	wl_event_source_remove(x11->frame_timer);

types/wlr_output.c

@@ -286,6 +286,7 @@ void wlr_output_destroy(struct wlr_output *output) {
 	wlr_output_destroy_global(output);
 	wlr_output_set_fullscreen_surface(output, NULL);
 
+	wl_signal_emit(&output->backend->events.output_remove, output);
 	wl_signal_emit(&output->events.destroy, output);
 
 	struct wlr_output_mode *mode, *tmp_mode;