Re: [PATCH v2 1/2] cgroups: allow a cgroup subsystem to reject a fork

From: Aleksa Sarai
Date: Wed Mar 11 2015 - 01:16:54 EST

Hello Tejun,

On Wed, Mar 11, 2015 at 2:17 AM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Wed, Mar 11, 2015 at 01:51:06AM +1100, Aleksa Sarai wrote:
>> Actually, I'm fairly sure we can do it all inside cgroup_post_fork() because
>> inside cgroup_post_fork() we have access to both the old css_set and the new
>> one. Then it's just a matter of reverting and re-applying the charge to the
>> hierarchies.
> But the problem isn't whether we know both the old and new ones. The
> problem is that we can only abort before the fork commit point and the
> "old" one may change between the abort point and post-commit point so
> we need to trycharge the old one at the possible abort point, remember
> to which css it got charged and then check whether the association has
> changed inbetween at the post commit point and readjust if so.

Actually, it appears I was wrong. Until we hit cgroup_post_fork()'s setting up
of the task's css_set, cgroup_can_fork() ends up charging init_css_set *every
time*. Which means a check to see if it changed will always show that it had
changed. The issue is that we need to access the css_set which is going to be
saved as the task's css_set in order to decide if the task should fork.

We know that the task will have its css_set set to task_css_set(current), and
we could just use that in cgroup_can_fork(). The only question is, can
task_css_set(current) change between cgroup_can_fork() and cgroup_post_fork()?

If it can change between the two calls, then we're in trouble -- there'd be no
reliable way of checking that the future css_set allows for the fork without
going through the registration of the css_set *proper* in cgroup_post_fork()
unless we hold css_set_rwsem for the entirety of the can_fork() to post_fork()
segment (which I can't imagine is a good idea).

Aleksa Sarai (cyphar)
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