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

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Date:	Thu, 21 Jul 2016 09:33:30 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Aleksa Sarai <asarai@...e.de>
Cc:	Tejun Heo <tj@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Aditya Kali <adityakali@...gle.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Christian Brauner <cbrauner@...e.de>, dev@...ncontainers.org,
	James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [PATCH v1 3/3] cgroup: relax common ancestor restriction for
 direct descendants

Quoting Aleksa Sarai (asarai@...e.de):
> >>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

Ths is of course solvable using something like libpam-cgfs or
libpam-cgm (and others).  Since this sounds like a question of
policy, not mechanism, userspace seems like the right place.  Is
there a downside to that (or, as Tejun put it, "delegating explicitly")?