lightdm or gdm?

[Bryan Quigley]

I did some testing in a VM with switching different flavors to/from
the default dm.  Mostly I found that my current VM setup isn't stable
(it's my own virt package I'm working on).

But I did also see:
Lubuntu -> gdm, Lock screen now falls to black screen
Gnome -> Lightdm, Wayland failed on the first run, worked on the second.

Now again, this could be my setup, my Unity tests simply didn't work.


One of my concerns for a gdm change is are we ready to reorg our VTs?
GDM appears to default to VT-1, and then the actual desktop session
usually goes to VT-2, but I also managed to get it to VT-4 (without
changing settings).  This appears to make it somewhat harder from a
debugging perspective and we would be doing that at the same time as
switching to wayland.
Lightdm - Both dm and desktop are on VT-7.


Looking at support requests the only one of note that I find is an
issue that came up with having both lightdm and gdm3 installed.  We
didn't debug to a root cause, just determined that we needed to purge
gdm3 (it was some NFS related, avahi, lightdm, dbus problem).  Will
look into it again as this would likely affect a lot more people
either way.


Lastly, I wanted to know if there are any security differences in how
the login/lock screens work?
Specifically:
Is their a process I can kill from a user session to break the lock?
If I'm able to crash the lock screen, is the user session destroyed or unlocked?
Which would be easier to add functionality to block new USB
drives/other devices from being connected when the screen is locked?

Thanks!
Bryan


On Wed, Apr 19, 2017 at 10:32 AM, Sebastien Bacher <seb128 at ubuntu.com> wrote:
> Hey there,
>
> That's a topic that was mentioned during the meeting yesterday and
> something we need to decide on early in the cycle since there is going
> to be work needed on that front to have a fully working session.
>
> I'm doing a small summary of what I think are the pro (+) and con (-) of
> each
>
> * lightdm
>
> + well tested in Ubuntu
> + we have people in the team knowing the codebase
> + shared with other flavors/greeters selection
> + guest session
> - divergence from upstream
> - we are the maintainers so it's more work for us
> - gnome-shell uses gdm for its lockscreen so work is needed to make it
> work with lightdm
>
> * gdm
>
> + that's the GNOME solution, works today with wayland & gnome-shell
> - we started lightdm because we found the gdm codebase not easy to work
> on, that might still be true
> - ?keeps an active session from the greeter even after logging which
> uses resources? (it was mentioned on IRC, to be confirmed, is that
> needed due to the lockscreen?)
> - no guest session, we need to work on that or decide to drop the
> feature from Ubuntu
>
>
> I talked a bit with Robert yesterday who said he could make lightdm use
> the gdm greeter (he has some work started on that a few cycles ago)
> which means it could be used as GNOME lockscreen instead of gdm. He's
> probably the best placed to comment on the work and pro/con of the
> solutions though so I'm going to let him go into the details when he
> replies.
>
> Cheers,
> Sebastien Bacher
>
>
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