xwayland: add a size-safe wrapper for xcb_send_event

xcb_send_event expects the caller to always provide 32 byte data, even if the actual event struct is
smaller than that.

Reference: https://gitlab.freedesktop.org/xorg/lib/libxcb/-/issues/18

include/xwayland/xwm.h

@@ -162,4 +162,8 @@ char *xwm_get_atom_name(struct wlr_xwm *xwm, xcb_atom_t atom);
 bool xwm_atoms_contains(struct wlr_xwm *xwm, xcb_atom_t *atoms,
 	size_t num_atoms, enum atom_name needle);
 
+xcb_void_cookie_t xwm_send_event_with_size(xcb_connection_t *c,
+	uint8_t propagate, xcb_window_t destination,
+	uint32_t event_mask, const void *event, uint32_t length);
+
 #endif

xwayland/selection/dnd.c

@@ -49,11 +49,12 @@ static void xwm_dnd_send_event(struct wlr_xwm *xwm, xcb_atom_t type,
 		.data = *data,
 	};
 
-	xcb_send_event(xwm->xcb_conn,
+	xwm_send_event_with_size(xwm->xcb_conn,
 		0, // propagate
 		dest->window_id,
 		XCB_EVENT_MASK_NO_EVENT,
-		(const char *)&event);
+		&event,
+		sizeof(event));
 	xcb_flush(xwm->xcb_conn);
 }
 

xwayland/selection/outgoing.c

@@ -27,11 +27,12 @@ static void xwm_selection_send_notify(struct wlr_xwm *xwm,
 		" requestor=%" PRIu32 " selection=%" PRIu32 " target=%" PRIu32 " property=%" PRIu32,
 		req->requestor, req->time, req->requestor, req->selection, req->target,
 		selection_notify.property);
-	xcb_send_event(xwm->xcb_conn,
+	xwm_send_event_with_size(xwm->xcb_conn,
 		0, // propagate
 		req->requestor,
 		XCB_EVENT_MASK_NO_EVENT,
-		(const char *)&selection_notify);
+		&selection_notify,
+		sizeof(selection_notify));
 	xcb_flush(xwm->xcb_conn);
 }
 

xwayland/xwm.c

@@ -257,6 +257,25 @@ static void xwm_set_net_active_window(struct wlr_xwm *xwm,
 			xwm->atoms[WINDOW], 32, 1, &window);
 }
 
+/*
+ * Wrapper for xcb_send_event, which ensures that the event data is 32 byte big.
+ */
+xcb_void_cookie_t xwm_send_event_with_size(xcb_connection_t *c,
+		uint8_t propagate, xcb_window_t destination,
+		uint32_t event_mask, const void *event, uint32_t length)
+{
+	if (length == 32) {
+		return xcb_send_event(c, propagate, destination, event_mask, event);
+	} else if (length < 32) {
+		char buf[32];
+		memcpy(buf, event, length);
+		memset(buf + length, 0, 32 - length);
+		return xcb_send_event(c, propagate, destination, event_mask, buf);
+	} else {
+		assert(false && "Event too long");
+	}
+}
+
 static void xwm_send_wm_message(struct wlr_xwayland_surface *surface,
 		xcb_client_message_data_t *data, uint32_t event_mask) {
 	struct wlr_xwm *xwm = surface->xwm;
@@ -270,11 +289,12 @@ static void xwm_send_wm_message(struct wlr_xwayland_surface *surface,
 		.data = *data,
 	};
 
-	xcb_send_event(xwm->xcb_conn,
+	xwm_send_event_with_size(xwm->xcb_conn,
 		0, // propagate
 		surface->window_id,
 		event_mask,
-		(const char *)&event);
+		&event,
+		sizeof(event));
 	xcb_flush(xwm->xcb_conn);
 }
 
@@ -1824,9 +1844,10 @@ void wlr_xwayland_surface_configure(struct wlr_xwayland_surface *xsurface,
 			.height = height,
 		};
 
-		xcb_send_event(xwm->xcb_conn, 0, xsurface->window_id,
+		xwm_send_event_with_size(xwm->xcb_conn, 0, xsurface->window_id,
 			XCB_EVENT_MASK_STRUCTURE_NOTIFY,
-			(const char *)&configure_notify);
+			&configure_notify,
+			sizeof(configure_notify));
 	}
 
 	xcb_flush(xwm->xcb_conn);