Public bug reported:
========================================================================
DESCRIPTION:
========================================================================
After upgrading gdm3 from 49.2 to 50 on Ubuntu 26.04, the login screen
does not appear. Instead the system launches a full GNOME desktop session
as the greeter user. This affects all Ubuntu 26.04 systems running
gdm3 50~beta-0ubuntu3, regardless of domain membership.
Package versions:
-----------------
$ dpkg -l gdm3 gnome-session-bin
ii gdm3 50~beta-0ubuntu3 amd64
ii gnome-session-bin 50~beta-0ubuntu1 amd64
Environment:
------------
- Ubuntu 26.04 (Resolute)
- gdm3 50~beta-0ubuntu3
- Previous working version: gdm3 49.2-1ubuntu4
What is happening:
------------------
The greeter session is defined in:
/usr/share/gdm/greeter/wayland-sessions/gnome-greeter.desktop
This file calls gnome-session with no --session argument:
Exec=gnome-session
gnome-session requires an explicit --session argument to know which
session definition to load:
$ gnome-session --help
Application Options:
--session=SESSION_NAME Session to use
Without it, gnome-session has no session name and falls back to the
default "ubuntu" session, causing gnome-shell to launch with
--mode=ubuntu instead of --mode=gdm.
Confirmed by logs:
------------------
$ journalctl -b | grep "gnome-shell.*mode\|Shell@"
systemd: Starting [email protected] - GNOME Shell...
gnome-shell: /usr/bin/gnome-shell --mode=ubuntu
gnome-shell: Running GNOME Shell (using mutter 50.beta) as a Wayland
display server
Instead of the expected:
[email protected]
/usr/bin/gnome-shell --mode=gdm
Verified as a packaging bug:
-----------------------------
The file was downloaded fresh from the Ubuntu archive and confirmed to
ship with the missing argument:
$ apt-get download gdm3=50~beta-0ubuntu3
$ dpkg-deb -x gdm3_50~beta-0ubuntu3_amd64.deb /tmp/gdm3-50-check
$ cat
/tmp/gdm3-50-check/usr/share/gdm/greeter/wayland-sessions/gnome-greeter.desktop
[Desktop Entry]
Name=GNOME Greeter
Comment=The default GNOME Greeter session
Exec=gnome-session
TryExec=gnome-session
Type=Application
DesktopNames=GNOME-Greeter:GNOME
NoDisplay=true
X-GDM-SessionRegisters=true
X-GDM-CanRunHeadless=true
X-Ubuntu-Gettext-Domain=gdm
This is not a local modification — the file shipped broken in the
package.
Why this is a regression from 49.2:
------------------------------------
gdm3 49.2 had no /usr/share/gdm/greeter/wayland-sessions/ directory at
all. The greeter was launched differently and did not depend on
gnome-session selecting the correct session by name. This is a new
code path introduced in gdm3 50 that was not present in 49.2.
Fix:
----
The Exec line in gnome-greeter.desktop should be:
Exec=gnome-session --session=gnome-login
The corrected file should read:
[Desktop Entry]
Name=GNOME Greeter
Comment=The default GNOME Greeter session
Exec=gnome-session --session=gnome-login
TryExec=gnome-session
Type=Application
DesktopNames=GNOME-Greeter:GNOME
NoDisplay=true
X-GDM-SessionRegisters=true
X-GDM-CanRunHeadless=true
X-Ubuntu-Gettext-Domain=gdm
This fix was verified to resolve the issue. After applying the change
and restarting gdm3, gnome-shell correctly launched with --mode=gdm and
the login screen appeared as expected.
Workaround:
-----------
Edit /usr/share/gdm/greeter/wayland-sessions/gnome-greeter.desktop and
change:
Exec=gnome-session
to:
Exec=gnome-session --session=gnome-login
Then restart gdm3:
sudo systemctl restart gdm3
Note: this file is owned by the gdm3 package and will be overwritten
on the next gdm3 package upgrade.
** Affects: gdm3 (Ubuntu)
Importance: Undecided
Status: New
** Tags: gdm3 gnome-greeter session
--
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https://bugs.launchpad.net/bugs/2143268
Title:
gdm3 50: gnome-greeter.desktop missing --session=gnome-login argument
causes gnome-shell to launch in desktop mode instead of greeter mode
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