Quoting Tejun Heo (tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org):

Hello, Aleksa.

On Fri, Jul 22, 2016 at 06:30:07PM +1000, Aleksa Sarai wrote:

quoted

Just to be clear, the "ns subdir operation" is a cgroup namespaced process
moving A -> A_subdir which is racing against some administrative process
moving everything from A -> B (but not wanting to move A -> A_subdir)?

Yes.

quoted

So should there be policy within the kernel to not permit a process outside
a cgroup namespace to move processes inside the namespace? Or would you be
concerned about people escaping the administrator's attempts to reorganise
the hierarchy?

Pushed that far, I frankly can't assess what the implications and
side-effects would be.

quoted

What if we extended rename(2) so that it /does/ allow for reorganisation of
the hierarchy? So an administrator could use rename to change the point at
which a cgroupns root is rooted at, but not be able to move the actual
processes within the cgroup namespace around? The administrator could also
join the cgroupns (without needing to join the userns) and then just move
things around that way?

Do any of those suggestions seem reasonable?

Unfortunately not.  I get what you're trying to do and am sure we can
make some specific scenarios work with the right set of hacks and
holes, but this type of approach is very dangerous in the long term.

The downside we have now is that we need an explicit delegation from
userland and that stems from the architectural constraints of
cgroupfs.  It's not ideal but an acceptable situation.  Let's please
not riddle the whole thing with holes that we don't understand for an
inconvenience which can be worked around otherwise.

It's something which distros, post-machine-install scripts, and
 post-pkg-install scripts can all very easily setup.  They have to
setup subuid and subgid too if you want safe unprivileged containers,
so I really don't feel this is so arduous.  What is the use case where
none of these are an option?  (I don't doubt that it exists since you
are pushing pretty hard)

-serge