Public bug reported:
## Summary
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
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
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.
## 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
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.
## 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
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.
## Workaround
Replace GDM with LightDM:
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.
- 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.
- 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>
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.gdm3.custom.conf: 2026-05-17T03:41:22.118500
** Affects: gdm3 (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute
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https://bugs.launchpad.net/bugs/2152848
Title:
GDM 50 leaks XDG_SESSION_TYPE=wayland from Wayland greeter into X11
user sessions, breaking Cinnamon and other X11-only desktops
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