[PATCHv1 7/8] cgroup: cgroup namespace setns support

[Serge E. Hallyn]

Quoting Aditya Kali (adityakali at google.com):
> On Thu, Oct 16, 2014 at 2:12 PM, Serge E. Hallyn <serge at hallyn.com> wrote:
> > Quoting Aditya Kali (adityakali at google.com):
> >> setns on a cgroup namespace is allowed only if
> >> * task has CAP_SYS_ADMIN in its current user-namespace and
> >>   over the user-namespace associated with target cgroupns.
> >> * task's current cgroup is descendent of the target cgroupns-root
> >>   cgroup.
> >
> > What is the point of this?
> >
> > If I'm a user logged into
> > /lxc/c1/user.slice/user-1000.slice/session-c12.scope and I start
> > a container which is in
> > /lxc/c1/user.slice/user-1000.slice/session-c12.scope/x1
> > then I will want to be able to enter the container's cgroup.
> > The container's cgroup root is under my own (satisfying the
> > below condition0 but my cgroup is not a descendent of the
> > container's cgroup.
> >
> This condition is there because we don't want to do implicit cgroup
> changes when a process attaches to another cgroupns. cgroupns tries to
> preserve the invariant that at any point, your current cgroup is
> always under the cgroupns-root of your cgroup namespace. But in your
> example, if we allow a process in "session-c12.scope" container to
> attach to cgroupns root'ed at "session-c12.scope/x1" container
> (without implicitly moving its cgroup), then this invariant won't
> hold.

Oh, I see.  Guess that should be workable.  Thanks.

-serge