lxdm and pam_group.so not working correctly

[antubis]

We have a bunch of clients all configured identically. To give our users the choice and comfort they need for working smooth we have installed a big set of windows managers on these clients

Currently I'm preparing the upgrade from Precise to Trusty, so I've upgraded a few of the clients for testing. Because of the bugs https://bugs.launchpad.net/ubuntu/+source/unity-greeter/+bug/1068605 and https://bugs.launchpad.net/ubuntu/+source/gnome-session/+bug/1293629 I was forced to switch to lxdm on the Trusty clients.

Lxdm works fine for login and displays all available desktops with a proper scroll bar. Unfortunally it doesn't work together with the pam_group.so correctly.

First, I encountered problems with the 3d acceleration of the client falling back to llvmpipe. Analyzing the problem, I recognized that my pam_group assignments (we have ldap groups mapped to local ones) were not active, especially the membership in the video group (required for the intel i915 drivers) was not set.

Comparing the pam config files of lightdm and lxdm I recognized that /etc/pam.d/lxdm is including 'common-session-noninteractive' instead of 'common-session'. Fixing this, the 3d acceleration started working again. Although the group mappings are still not set e.g. in a terminal.

In the /var/log/auth.log we have messages like this:
xscreensaver[12611]: pam_group(xscreensaver:setcred): unable to set the group membership for user: Operation not permitted
or
cinnamon-screensaver-dialog: pam_group(cinnamon-screensaver:setcred): unable to set the group membership for user: Operation not permitted

It looks like the bug is similar to an old bug in lightdm (https://bugs.launchpad.net/lightdm/+bug/880104).