Permissions adjustments for a greeter session

[Yves-Alexis Perez <corsac@xxxxxxxxxx>]

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Hey there,

I'm writing here as the Debian LightDM maintainer following a bug opened
against it (https://bugs.debian.org/1095999)

The reporter has issues with the greeter not showing on their install,
apparently because the lightdm-gtk-greeter process (runing as the lightdm
user) doesn't have permissions to access the /dev/dri/renderD128 node (and
it's apparently required for their mali gpu, and others).

Nowadays, afaict, the permissions on device nodes are usually handled by a
combination of udev and logind. For example on my graphical session the above
node has RW ACLs for my users, following (I guess) rules from
/lib/udev/rules.d/70-uaccess.rules

It seems that the greeter process doesn't have the same permissions (no ACLs)
and I'm not too sure why. In loginctl it's shown as a session of type
'greeter' so maybe it's related? Or maybe something else?

In any case, any help / pointer would be appreciated in order to properly fix
this issue for our users.

Thanks in advance, and if you need more information please ask!

Regards,
- -- 
Yves-Alexis
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