[systemd-devel] logind device access weird behavior

[serenissi]

I noticed a phenomenon about logind managed devices (drm node). I have 
two users, localuser and testuser, the former has a session in seat0 
(this is important). I attached drm card1 to new seat `seat1` and set 
777 permission to the dev node /dev/dri/card1. Now the acl looks like

# file: dev/dri/card1
# owner: root
# group: video
user::rwx
group::---
mask::rwx
other::rwx

as expected. Now if I do from a localuser shell: sudo -u testuser cat 
/dev/dri/card1, the device opens as expected. However doing so as 
localuser results in permission denied.

But if I add another acl entry with setfacl -m u:localuser:rw 
/dev/dri/card1, cat /dev/dri/card1 suddenly works as expected. In this 
case the acl is

# file: dev/dri/card1
# owner: root
# group: video
user::rwx
user:localuser:rw-
group::---
mask::rw-
other::rwx

here the `other` entry makes the `user:localuser` entry pointless in 
common sense, which is not the case.

My hunch is ebpf but I couldn't find where this logic is defined in 
systemd tree. Could anyone here help me with that?


~ serene

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