On Fri, 3 Jun 2016, Lennart Poettering wrote:
You are redefining the meaning of (a graphical) logout. It simply
means another user can use the mouse, keyboard and screen of this
device. It makes no statement on whether the machines resources are
shared or not.
Actually, with logind, current kernel, current X11 and/or wayland
there's a very clear statement on sharing devices: logind will ensure
that only the fg session can access the various evdev and DRM devices,
and will suspend access for all sessions not currently in the
fg. Similar, ACLs for a couple of other device nodes are patched
depending on the fg session (but only for DRM and evdev the ongoing
connection of bg users is suspended, as there's no concept of a
generic revoke() in the Linux kernel, but only DRM and evdev-specific
mechanisms). Locking this down properly, so that background sessions
or even non-console logins don't get access to your devices has been
something various folks from various communities have been working
on for a while.
So yeah, sessions (as defined by logind) are a security concept
already, and they will make sure that only the right users get access
to the devices at the right times.
That's great. It has however, absolutely nothing to do with backgrounded
processes, and their interpretation of good vs evil by systemd.
No one is saying when a graphical session ends, you cannot reclaim the
devices required for the next graphical session to start.
No one is saying you cannot protect physical devices from graphical or
network logins.
What it is offered now is garbage collection of the global process list,
and people are stating systemd does not have the required to knowledge to
successfully perform that task - and therefore should not try.
Paul
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