xwayland: Check X11 clients prior to terminate Xwayland

Currently, mutter checks for the presence of X11 windows to decide
whether or not Xwayland can be terminated, when Xwayland is started on
demand.

Unfortunately, not all X11 clients will map a window all the time, an
X11 client may keep the X11 connection opened after closing all its
windows. In that case, we may terminate Xwayland while there are some
X11 client connected still, and terminating Xwayland will also kill
those X11 clients.

To avoid that issue, check the X11 clients actually connected using the
XRes extension. The XRes extension provides the PID of the (local) X11
clients connected to the Xserver, so we need to match that against the
actual executable names, and compare with a list of known executables
that we can safely ignore, such as ibus-x11 or gsd-xsettings.

We also check against our own executable name, considering that the X11
window manager is also an X11 client connected to the Xserver.

Also, XRes returning the PID of local clients only is not a problem
considering that Xwayland does not listen to remote connections.
However, if the user spawns a client remotely on another system using
ssh tunneling (ssh -X), only clients which actually map a window will
be accounted for.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1537
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1671>

config.h.meson

@@ -82,3 +82,6 @@
 
 /* Whether the memfd_create function exists */
 #mesondefine HAVE_MEMFD_CREATE
+
+/* List of executable names to ignore when terminating Xwayland */
+#mesondefine XWAYLAND_IGNORE_EXECUTABLES

meson.build

@@ -461,6 +461,10 @@ xwayland_grab_default_access_rules = get_option('xwayland_grab_default_access_ru
 cdata.set_quoted('XWAYLAND_GRAB_DEFAULT_ACCESS_RULES',
                  xwayland_grab_default_access_rules)
 
+xwayland_ignore_executables = get_option('xwayland_ignore_executables')
+cdata.set_quoted('XWAYLAND_IGNORE_EXECUTABLES',
+                 xwayland_ignore_executables)
+
 cdata.set_quoted('MUTTER_PLUGIN_DIR', join_paths(pkglibdir, 'plugins'))
 cdata.set_quoted('MUTTER_LOCALEDIR', localedir)
 cdata.set_quoted('MUTTER_LIBEXECDIR', libexecdir)

meson_options.txt

@@ -158,3 +158,9 @@ option('xwayland_initfd',
   value: 'auto',
   description: 'Whether -initfd argument is passed to Xwayland to guarantee services (e.g. gsd-xsettings) startup before applications'
 )
+
+option('xwayland_ignore_executables',
+  type: 'string',
+  value: 'gsd-xsettings,ibus-x11,pulseaudio,Xwayland',
+  description: 'Comma delimited list of executable names to ignore when terminating Xwayland'
+)

src/wayland/meta-xwayland.c

@@ -41,6 +41,9 @@
 #include <unistd.h>
 #include <X11/extensions/Xrandr.h>
 #include <X11/Xauth.h>
+#include <X11/Xlib-xcb.h>
+
+#include <xcb/res.h>
 
 #include "backends/meta-monitor-manager-private.h"
 #include "backends/meta-settings-private.h"
@@ -126,6 +129,146 @@ meta_xwayland_is_xwayland_surface (MetaWaylandSurface *surface)
   return wl_resource_get_client (surface->resource) == manager->client;
 }
 
+static char *
+meta_xwayland_get_exe_from_proc_entry (const char *proc_entry)
+{
+  g_autofree char *exepath;
+  char *executable;
+  char *p;
+
+  exepath = g_file_read_link (proc_entry, NULL);
+  if (!exepath)
+    return NULL;
+
+  p = strrchr (exepath, G_DIR_SEPARATOR);
+  if (p)
+    executable = g_strdup (++p);
+  else
+    executable = g_strdup (exepath);
+
+  return executable;
+}
+
+static char *
+meta_xwayland_get_exe_from_pid (uint32_t pid)
+{
+  g_autofree char *proc_entry;
+  char *executable;
+
+  proc_entry = g_strdup_printf ("/proc/%i/exe", pid);
+  executable = meta_xwayland_get_exe_from_proc_entry (proc_entry);
+
+  return executable;
+}
+
+static char *
+meta_xwayland_get_self_exe (void)
+{
+  g_autofree char *proc_entry;
+  char *executable;
+
+  proc_entry = g_strdup_printf ("/proc/self/exe");
+  executable = meta_xwayland_get_exe_from_proc_entry (proc_entry);
+
+  return executable;
+}
+
+static gboolean
+can_xwayland_ignore_exe (const char *executable,
+                         const char *self)
+{
+  char ** ignore_executables;
+  gboolean ret;
+
+  if (!g_strcmp0 (executable, self))
+    return TRUE;
+
+  ignore_executables = g_strsplit_set (XWAYLAND_IGNORE_EXECUTABLES, ",", -1);
+  ret = g_strv_contains ((const char * const *) ignore_executables, executable);
+  g_strfreev (ignore_executables);
+
+  return ret;
+}
+
+static uint32_t
+meta_xwayland_get_client_pid (xcb_connection_t *xcb,
+                              uint32_t          client)
+{
+  xcb_res_client_id_spec_t spec = { 0 };
+  xcb_res_query_client_ids_cookie_t cookie;
+  xcb_res_query_client_ids_reply_t *reply = NULL;
+  uint32_t pid = 0, *value;
+
+  spec.client = client;
+  spec.mask = XCB_RES_CLIENT_ID_MASK_LOCAL_CLIENT_PID;
+
+  cookie = xcb_res_query_client_ids (xcb, 1, &spec);
+  reply = xcb_res_query_client_ids_reply (xcb, cookie, NULL);
+
+  if (reply == NULL)
+    return 0;
+
+  xcb_res_client_id_value_iterator_t it;
+  for (it = xcb_res_query_client_ids_ids_iterator (reply);
+       it.rem;
+       xcb_res_client_id_value_next (&it))
+    {
+      spec = it.data->spec;
+      if (spec.mask & XCB_RES_CLIENT_ID_MASK_LOCAL_CLIENT_PID)
+        {
+          value = xcb_res_client_id_value_value (it.data);
+          pid = *value;
+          break;
+        }
+    }
+  free (reply);
+
+  return pid;
+}
+
+static gboolean
+can_terminate_xwayland (Display *xdisplay)
+{
+  xcb_connection_t *xcb = XGetXCBConnection (xdisplay);
+  xcb_res_query_clients_cookie_t cookie;
+  xcb_res_query_clients_reply_t *reply = NULL;
+  xcb_res_client_iterator_t it;
+  gboolean can_terminate;
+  char *self;
+
+  cookie = xcb_res_query_clients (xcb);
+  reply = xcb_res_query_clients_reply (xcb, cookie, NULL);
+
+  /* Could not get the list of X11 clients, better not terminate Xwayland */
+  if (reply == NULL)
+    return FALSE;
+
+  can_terminate = TRUE;
+  self = meta_xwayland_get_self_exe ();
+  for (it = xcb_res_query_clients_clients_iterator (reply);
+       it.rem && can_terminate;
+       xcb_res_client_next (&it))
+    {
+      uint32_t pid;
+      char *executable;
+
+      pid = meta_xwayland_get_client_pid (xcb, it.data->resource_base);
+      if (pid == 0)
+        {
+          /* Unknown PID, don't risk terminating it */
+          can_terminate = FALSE;
+          break;
+        }
+
+      executable = meta_xwayland_get_exe_from_pid (pid);
+      can_terminate = can_xwayland_ignore_exe (executable, self);
+      g_free (executable);
+    }
+  free (reply);
+
+  return can_terminate;
+}
+
 static gboolean
 try_display (int      display,
              char   **filename_out,
@@ -385,10 +528,14 @@ static gboolean
 shutdown_xwayland_cb (gpointer data)
 {
   MetaXWaylandManager *manager = data;
+  MetaDisplay *display = meta_get_display ();
+
+  if (!can_terminate_xwayland (display->x11_display->xdisplay))
+    return G_SOURCE_CONTINUE;
 
   meta_verbose ("Shutting down Xwayland");
   manager->xserver_grace_period_id = 0;
-  meta_display_shutdown_x11 (meta_get_display ());
+  meta_display_shutdown_x11 (display);
   meta_xwayland_stop_xserver (manager);
   return G_SOURCE_REMOVE;
 }