** Description changed:
- ## Summary
+ [ Impact ]
When GDM 50 is configured to launch an X11 user session (e.g., Cinnamon),
- the spawned session inherits XDG_SESSION_TYPE=wayland from GDM's own
- greeter environment via the PAM session chain. gdm-x-session then broadcasts
- this incorrect value through dbus-update-activation-environment to all
- D-Bus-activated session services. Components like cinnamon-screensaver
+ the spawned session incorrectly sets XDG_SESSION_TYPE=wayland. gdm-x-session
then broadcasts this incorrect value through dbus-update-activation-environment
to all D-Bus-activated session services. Components like cinnamon-screensaver
honor XDG_SESSION_TYPE and exit immediately ("disabled in wayland sessions"),
causing required-component failure for cinnamon-session, and muffin
attempts to initialize a native Wayland backend instead of X11, hitting
EBUSY because GDM's Wayland compositor still holds DRM control.
Net effect: Cinnamon (and presumably other X11-only desktops) cannot be
launched from GDM on Ubuntu 26.04 — every login attempt times out at 30
seconds and returns to the greeter.
- ## Steps to reproduce
+ [ Test Plan ]
1. Fresh Ubuntu 26.04 install with GDM (default).
2. `sudo apt install cinnamon-desktop-environment`
3. Reboot.
4. At GDM greeter, select user → cog icon → choose "Cinnamon" (Xorg) → log in.
Expected: Cinnamon X11 session loads.
Actual: Black screen for ~30 seconds, then returns to GDM greeter.
+ [ Other info ]
+
## Evidence (from journalctl of the failed boot)
Environment GDM sets for the X session, captured from gdm-x-session's
dbus-update-activation-environment output:
- XDG_SESSION_DESKTOP=cinnamon
- XDG_SESSION_TYPE=wayland ← incorrect for an X11 session
- XDG_SESSION_EXTRA_DEVICE_ACCESS=render:accel
- GDMSESSION=cinnamon
- DESKTOP_SESSION=cinnamon
+ XDG_SESSION_DESKTOP=cinnamon
+ XDG_SESSION_TYPE=wayland ← incorrect for an X11 session
+ XDG_SESSION_EXTRA_DEVICE_ACCESS=render:accel
+ GDMSESSION=cinnamon
+ DESKTOP_SESSION=cinnamon
Then in the failing session:
- cinnamon[N]: Failed to create backend: Could not take control:
- GDBus.Error:System.Error.EBUSY: Device or resource busy
- org.cinnamon.ScreenSaver: cinnamon-screensaver is disabled in
- wayland sessions. Exiting.
- cinnamon-session-binary: WARNING: t+30s: Application
- 'cinnamon.desktop' failed to register before timeout
- cinnamon-session-binary: CRITICAL: Unable to start session: A program
- required by the session failed to start.
+ cinnamon[N]: Failed to create backend: Could not take control:
+ GDBus.Error:System.Error.EBUSY: Device or resource busy
+ org.cinnamon.ScreenSaver: cinnamon-screensaver is disabled in
+ wayland sessions. Exiting.
+ cinnamon-session-binary: WARNING: t+30s: Application
+ 'cinnamon.desktop' failed to register before timeout
+ cinnamon-session-binary: CRITICAL: Unable to start session: A program
+ required by the session failed to start.
## Causal chain (verified)
- GDM greeter (Wayland) ──PAM session──▶ gdm-session-worker
- │ inherits
XDG_SESSION_TYPE=wayland
- ▼
- gdm-x-session
- (starts Xorg correctly,
- but inherited env unchanged)
- │
- ▼
- dbus-update-activation-environment
- broadcasts XDG_SESSION_TYPE=wayland
- to all D-Bus services in the session
- │
- ┌────────────────────┴────────────────────┐
- ▼ ▼
- cinnamon (muffin) cinnamon-screensaver
- tries native Wayland backend exits: "disabled in
- → DRM EBUSY wayland sessions"
- → no compositor → required-component
- → no shell, blank X screen failure cascade
+ GDM greeter (Wayland) ──PAM session──▶ gdm-session-worker
+ │ sets XDG_SESSION_TYPE=wayland
+ ▼
+ gdm-x-session
+ (starts Xorg correctly,
+ but inherited env unchanged)
+ │
+ ▼
+ dbus-update-activation-environment
+ broadcasts XDG_SESSION_TYPE=wayland
+ to all D-Bus services in the session
+ │
+ ┌────────────────────┴────────────────────┐
+ ▼ ▼
+ cinnamon (muffin) cinnamon-screensaver
+ tries native Wayland backend exits: "disabled in
+ → DRM EBUSY wayland sessions"
+ → no compositor → required-component
+ → no shell, blank X screen failure cascade
Note: `loginctl show-session $N` reports Type=x11 correctly for the user
session. The bug is specifically in the D-Bus activation environment
- gdm-x-session broadcasts, which is inherited up the PAM chain rather
- than set fresh based on session type.
+ gdm-x-session broadcasts.
## Workaround
Replace GDM with LightDM:
- sudo apt install lightdm lightdm-gtk-greeter slick-greeter
- sudo dpkg-reconfigure lightdm # choose lightdm
- sudo systemctl reboot
+ sudo apt install lightdm lightdm-gtk-greeter slick-greeter
+ sudo dpkg-reconfigure lightdm # choose lightdm
+ sudo systemctl reboot
LightDM does not run a Wayland greeter, so XDG_SESSION_TYPE=x11 propagates
correctly and Cinnamon launches normally. Confirmed working on the same
hardware.
WaylandEnable=false in /etc/gdm3/custom.conf does NOT fix the bug. Its
semantics changed in GDM ~44 to only hide Wayland from the user session
chooser; GDM's own greeter still runs Wayland regardless.
## System info
- Ubuntu 26.04 LTS (Resolute Raccoon)
- Kernel 7.0.0-17-generic
- GDM3 50.0-0ubuntu1
- Cinnamon 6.4.13 (universe)
- Hardware: Razer Blade 15 Early 2020 (Intel UHD + GTX 1660 Ti hybrid, PRIME
on-demand)
- Nvidia driver: 595.58.03
## Notes for triage
- Reproducible: yes, every boot
- The bug likely affects any X11-only desktop (MATE, XFCE may be similar),
- not just Cinnamon — anything whose components check XDG_SESSION_TYPE
- or whose compositor uses the env-var to pick backend.
+ not just Cinnamon — anything whose components check XDG_SESSION_TYPE
+ or whose compositor uses the env-var to pick backend.
- Possible upstream fix path: gdm-x-session should override
- XDG_SESSION_TYPE=x11 in the environment it broadcasts via D-Bus,
- rather than inheriting it from the parent greeter session.
+ XDG_SESSION_TYPE=x11 in the environment
- I have additional logs available if useful.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: gdm3 50.0-0ubuntu1
ProcVersionSignature: Ubuntu 7.0.0-17.17-generic 7.0.2
Uname: Linux 7.0.0-17-generic x86_64
ApportVersion: 2.34.0-0ubuntu2
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: X-Cinnamon
Date: Sun May 17 13:53:27 2026
InstallationDate: Installed on 2025-05-31 (351 days ago)
InstallationMedia: Ubuntu 25.04 "Plucky Puffin" - Release amd64 (20250415.3)
ProcEnviron:
- LANG=en_US.UTF-8
- PATH=(custom, no user)
- SHELL=/bin/bash
- TERM=xterm-256color
- XDG_RUNTIME_DIR=<set>
+ LANG=en_US.UTF-8
+ PATH=(custom, no user)
+ SHELL=/bin/bash
+ TERM=xterm-256color
+ XDG_RUNTIME_DIR=<set>
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.gdm3.custom.conf: 2026-05-17T03:41:22.118500
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You received this bug notification because you are a member of Ubuntu
Desktop Bugs, which is subscribed to gdm3 in Ubuntu.
https://bugs.launchpad.net/bugs/2152848
Title:
GDM 50 incorrectly sets XDG_SESSION_TYPE=wayland for X11 user
sessions, breaking Cinnamon and other X11-only desktops
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