Re: [PATCH v1 3/3] cgroup: relax common ancestor restriction for direct descendants

From: Aleksa Sarai
Date: Thu Jul 21 2016 - 03:44:33 EST

I feel like the permission model makes sense in certain cases (the common
ancestor restriction, as well as the ability for a parent to apply limits to
children by setting its own limits). Neither of those are violated (if you
read the commit that introduced the common ancestor restriction).

Maybe if you give me a usecase of when it might be important that a process
must not be able to move to a sub-cgroup of its current one, I might be able
to understand your concerns? From my perspective, I think that's actually
quite useful.

cgroup is used to keep track of which processes belong where and
allowing processes to be moved out of its cgroup like this would be
surprising to say the least.

Would you find it acceptable if we added a bit that would make this not happen (you could specify that a cgroup should not allow a process to move itself to a sub-cgroup)? Or an aggregate cgroup.procs that gives you all of the processes in the entire branch of the tree? Surely this is something that can be fixed without unnecessarily restricting users from doing useful things.

The reason I'm doing this is so that we might be able to _practically_ use
cgroups as an unprivileged user (something that will almost certainly be
useful to not just the container crowd, but people also planning on using
cgroups as advanced forms of rlimits).

I don't get why we need this fragile dance with permissions at all
when the same functionality can be achieved by delegating explicitly.

The key words being "unprivileged user". Currently, if I am a regular user on a system and I want to use the freezer cgroup to pause a process I am running, I have to *go to the administrator and ask them to give me permission to do that*. Why is that necessary? I find it quite troubling that the usecase of an ordinary user on a system trying to use something as useful as cgroups is considered to be "solved" by asking your administrator (or systemd) to do it for you. "Delegating explicitly" is punting on the problem, by saying "just get the administrator to do the setup for you". What if you don't have the opportunity to do that, and it takes you 4 weeks of sending emails for you to get the administrator to do _anything_?

This is something I'm trying to fix with my recent work with rootless containers (and quite a few other people are trying to fix it too). Currently we just simply can't do certain operations as an unprivileged user that would be possible *if we could just use cgroups*. Things like the freezer cgroup would be invaluable for containers, and I guarantee that the Chromium and Firefox folks would find it useful to be able to limit browser processes in a similar way.

Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH