Bug#973812: Bug#973485: gdm3: options for both "System X11 Default" and "Default X11 Session"

[Simon McVittie]

Control: tags -1 + moreinfo

On Thu, 05 Nov 2020 at 11:39:59 +0000, Simon McVittie wrote:
> Control: retitle -2 gdm3: options for both "System X11 Default" and "Default X11 Session"
> 
> On Thu, 05 Nov 2020 at 11:32:37 +0100, Christoph Berg wrote:
> > I confirm that both the "System X11 Default" and "Default X11 Session"
> > options start my .Xsession file. (Iirc the second option was missing
> > in the old version.)
>
> I don't have this. I currently have options for:
> 
> * System X11 Default
> * GNOME
> [and some others]
> 
> which come from
> 
> * /usr/share/gdm/BuiltInSessions/default.desktop
> * /usr/share/wayland-sessions/gnome.desktop or /usr/share/xsessions/gnome.desktop
> [and some others]

I think I might have found the root cause for this. Do you perhaps have
lightdm installed, even though you are not actively using it?

lightdm installs /usr/share/xsessions/lightdm-xsession.desktop, labelled
"Default Xsession", which starts the traditional x11-common session
(~/.xsessionrc or similar).

Unfortunately, it installs this into a location where *all* display
managers that work like this (including gdm3, sddm, lxdm, slim) will
load that session definition, even though it is not intended for them.

I think this is a lightdm bug (#1004559) and it should either not install
that session definition at all, or install it into a location where only
lightdm will see it (such as /usr/share/lightdm/sessions).

> It isn't ideal that we have an option for "it does something but we
> can't tell you what", but we don't have enough information to do better,
> and even if we could look into individual users' home directories to
> see whether they have ~/.xsession and display that in the greeter,
> that would be an information leak.

Following this logic, I removed the "System X11 Default" option in
gdm3/41.0-1. Instead, there is now an example file
/usr/share/doc/gdm3/examples/custom-x11-session.desktop which can be
copied into /etc/X11/sessions and customized as needed. This is the
method recommended in upstream GNOME documentation for defining local
sessions.

    smcv