From 555813917ce22d51d72a324d6d5fc17cb47fa8c5 Mon Sep 17 00:00:00 2001
From: Joan Torres Lopez <joantolo@redhat.com>
Date: Thu, 19 Feb 2026 16:36:58 +0100
Subject: [PATCH] local-display-factory: Ignore WaylandEnable key when GNOME
 doesn't support X11

When GNOME is built without X11 support for GNOME sessions but GDM is
built with both Wayland and X11 support (to allow other desktop
environments to use X11), the greeter must always use Wayland.

This adds a runtime check for GNOME X11 support by verifying the
existence of the gnome.desktop xsession file. When this file doesn't
exist, the WaylandEnable setting is ignored and the greeter falls back
to Wayland.

This allows distributions to:
- Build GDM with X11 support for non-GNOME desktop environments
- Build GNOME without X11 session support
- Ensure the greeter always starts correctly

Closes: https://gitlab.gnome.org/GNOME/gdm/-/issues/1027
Part-of: <https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/354>
---
 daemon/gdm-local-display-factory.c | 40 ++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/daemon/gdm-local-display-factory.c b/daemon/gdm-local-display-factory.c
index ad2e65cf7..3f33402e3 100644
--- a/daemon/gdm-local-display-factory.c
+++ b/daemon/gdm-local-display-factory.c
@@ -55,6 +55,8 @@
 #define WAIT_TO_FINISH_TIMEOUT 10 /* seconds */
 #define SEAT0_GRAPHICS_CHECK_TIMEOUT 10 /* seconds */
 
+#define GNOME_X11_SESSION_DESKTOP DATADIR "/xsessions/gnome.desktop"
+
 struct _GdmLocalDisplayFactory
 {
         GdmDisplayFactory  parent;
@@ -207,6 +209,14 @@ take_next_display_number (GdmLocalDisplayFactory *factory)
         return ret;
 }
 
+#if defined (ENABLE_WAYLAND_SUPPORT) && defined (ENABLE_X11_SUPPORT)
+static gboolean
+gnome_session_supports_x11 (void)
+{
+        return g_file_test (GNOME_X11_SESSION_DESKTOP, G_FILE_TEST_EXISTS);
+}
+#endif
+
 static char *
 get_preferred_display_server (GdmLocalDisplayFactory *factory)
 {
@@ -216,6 +226,11 @@ get_preferred_display_server (GdmLocalDisplayFactory *factory)
 #if defined (ENABLE_WAYLAND_SUPPORT) && defined (ENABLE_X11_SUPPORT)
         gdm_settings_direct_get_boolean (GDM_KEY_WAYLAND_ENABLE, &wayland_enabled);
         gdm_settings_direct_get_boolean (GDM_KEY_XORG_ENABLE, &xorg_enabled);
+
+        if (!wayland_enabled && !gnome_session_supports_x11 ()) {
+                g_debug ("GdmLocalDisplayFactory: GNOME Session doesn't support X11, fallback to Wayland");
+                wayland_enabled = TRUE;
+        }
 #elif defined (ENABLE_WAYLAND_SUPPORT)
         wayland_enabled = TRUE;
 #elif defined (ENABLE_X11_SUPPORT)
@@ -262,19 +277,32 @@ get_preferred_display_server (GdmLocalDisplayFactory *factory)
         return g_strdup ("none");
 }
 
+static gboolean
+check_key_always (void)
+{
+        return TRUE;
+}
+
 struct GdmDisplayServerConfiguration {
         const char *display_server;
         const char *key;
         const char *binary;
         const char *session_type;
+        gboolean (*check_key) (void);
 } display_server_configuration[] = {
 #ifdef ENABLE_WAYLAND_SUPPORT
-        { "wayland", GDM_KEY_WAYLAND_ENABLE, NULL, "wayland" },
+        { "wayland", GDM_KEY_WAYLAND_ENABLE, NULL, "wayland",
+#ifdef ENABLE_X11_SUPPORT
+          gnome_session_supports_x11,
+#else
+          check_key_always,
+#endif
+        },
 #endif
 #ifdef ENABLE_X11_SUPPORT
-        { "xorg", GDM_KEY_XORG_ENABLE, "/usr/bin/Xorg", "x11" },
+        { "xorg", GDM_KEY_XORG_ENABLE, "/usr/bin/Xorg", "x11", check_key_always },
 #endif
-        { NULL, NULL, NULL },
+        { NULL, NULL, NULL, NULL, check_key_always },
 };
 
 static gboolean
@@ -293,8 +321,10 @@ display_server_enabled (GdmLocalDisplayFactory *factory,
                         continue;
 
 #if defined (ENABLE_WAYLAND_SUPPORT) && defined (ENABLE_X11_SUPPORT)
-                if (!gdm_settings_direct_get_boolean (key, &enabled) || !enabled)
-                        return FALSE;
+                if (display_server_configuration[i].check_key ()) {
+                        if (!gdm_settings_direct_get_boolean (key, &enabled) || !enabled)
+                                return FALSE;
+                }
 #endif
 
                 if (binary && !g_file_test (binary, G_FILE_TEST_IS_EXECUTABLE))
-- 
GitLab