GDM status (again)

[Timothy Sample]

Hi Guix,

Two months ago, Andy Wingo did a bunch of work on getting GDM working as
a display manager for Guix [1]. Unfortunately, Andy had to step away
from the task before getting everything working.

I’ve been looking at this a bit, and have made some progress. Currently,
GDM starts, lists users, logs in, and launches Gnome. What’s more is
that locking the screen works! Hooray!, right? Well, not quite. There
are two serious problems. The first is that it allows me to log in
without asking for a password. I haven’t looked at why this is. The
second is that Gnome cannot launch applications like “gnome-terminal”
due to a DBUS error. I’ll explain a little bit about the error, but
first I want to write down what I did to get GDM to work at all.

There’s a bug in the GDM package specification, which prevents GDM from
finding its config file. (I think this is why Andy had trouble with the
debug output.) Basically, the “substitute*” call that looks like

    ;; Look for custom GDM conf in /run/current-system.
    (substitute* '("common/gdm-settings-backend.c")
      (("GDM_CUSTOM_CONF")
       "/run/current-system/etc/gdm/custom.conf"))

should be

    ;; Look for custom GDM conf in /run/current-system.
    (substitute* '("common/gdm-settings-desktop-backend.c")
      (("GDM_CUSTOM_CONF")
       "\"/run/current-system/etc/gdm/custom.conf\""))

After clearing that up, I found out that GDM was unable to find the
Gnome Shell. This is pretty tricky because Gnome Shell already depends
on GDM, so you have to avoid a circular dependency. Other distros (I
looked at Debian and Fedora) solve this by slicing out a “libgdm” output
from the GDM sources, so that you can build “libgdm” without Gnome
Shell, have Gnome Shell depend on “libgdm”, and then have GDM depend on
Gnome Shell. AFAIK, GDM does not provide something like an
“install-libgdm” make target, so the other distros just select the files
manually. GDM also requires access to DBUS, since it launches a session
bus for itself and every user who logs in. I skirted doing all the work
of setting this up in the package, and just added

    XDG_DATA_DIRS=/run/current-system/profile/share
    PATH=/run/current-system/profile/bin

to the service environment in the “shepherd-service” definition. This
only works because I am assuming that the system is using the Gnome
service. I thought that I would see how it goes, and then come back to
making it nice ;).

The next two issues were solved with configure flags. By default, GDM
tries to take over the first virtual terminal, which doesn’t work
because mingetty is already using it. This is fixed by passing
“--with-default-vt=7” to the configure script.

With this, GDM runs, but does not let anyone log in. It turns out it
needs an Xsession script, which it only provides if you give the
configure script “--enable-gdm-xsession”. (They said that they put it
behind a configure flag because most distributions ignore it and write a
custom one anyway [2].)

After making these changes, GDM should work and allow you to log
in. This is where the problems I described earlier come it. The DBUS
error happens because GDM never loads any “profile” scripts (e.g.,
“/etc/profile”). This means that DBUS can’t find any “.session” files,
which causes a lot of stuff to break. I tried to fix this by modifying
the GDM source code to launch certain processes through a login shell
(i.e., “bash -l -c "..."”), and indeed this let the DBUS session daemon
find the “.session” files. Sadly, it just revealed fatal locale
errors. (To be sure, I don’t really think it’s a good solution anyway. I
just wanted to see what would happen.) This is where I gave up.

If you’ve read this far, thank you! Sorry for the long story.

I plan to keep looking at this, but these environment problems look to
be pretty far above my pay grade (with respect to Guix expertise). If
any of this reminds anyone of a similar problem that’s already been
solved I’d love to know so I can cheat off the solution! Otherwise, I
hope this might help anyone else who wants to work on GDM.

Finally, some quick info about QEMU and testing GDM/Gnome. Gnome takes
up a lot of memory, so “-m 1024” is a good idea. GDM runs X as non-root
users, which means that X has to use the KMS/DRM system. Currently Guix
sets up the Linux kernel without a direct rendering driver for the
default QEMU graphics setup (“-vga std”). Fortunately, “-vga qxl” seems
to solve this. (It looks like a driver for “-vga std” can be configured
into the kernel with the “CONFIG_DRM_BOCHS” flag, but I have not tested
this.) In QEMU, you can switch VTs by pressing Crtl-Alt-2 to get to the
QEMU shell, and then typing “sendkey ctrl-alt-fN” where N is the VT you
want to switch to.


-- Tim


[1] https://lists.gnu.org/archive/html/guix-devel/2017-08/msg00268.html
[2] https://mail.gnome.org/archives/commits-list/2015-June/msg00115.html