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

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Message-ID: <7e004486-b402-6e01-9893-c8751bcdb316@suse.de>
Date:	Fri, 22 Jul 2016 18:24:25 +1000
From:	Aleksa Sarai <asarai@...e.de>
To:	Tejun Heo <tj@...nel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	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
Subject: Re: [PATCH v1 3/3] cgroup: relax common ancestor restriction for
 direct descendants

>>  It's about the debris left behind if the admin (or someone with
>> delegated authority) moves the task to a wholly different cgroup.
>>
>> Now we have a cgroup directory in the old cgroup, which the current
>> task has been removed from, for which the current user has permissions
>> and could then move the task back to.  Is that the essence of the
>> problem?
>
> That'd be one side.  The other side is the one moving.  Let's say the
> system admin thing wants to move all processe from A proper to B.  It
> would do that by draining processes from A's procs file into B's and
> even that is multistep and can race.

Once freezer is ported, wouldn't that allow you to stop the processes so 
you can drain them? I understand your concern with draining, but surely 
the same races occur if you fork? How many times would you need to scan 
cgroup.procs to make sure that you didn't miss anything (and if there's 
enough processes then cgroup.procs reads aren't atomic either).

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/