Re: CGroup Namespaces (v4)

[Serge E. Hallyn]

On Mon, Nov 16, 2015 at 04:24:27PM -0600, Eric W. Biederman wrote:
> "Serge E. Hallyn" <serge@xxxxxxxxxx> writes:
> 
> > On Mon, Nov 16, 2015 at 09:50:55PM +0100, Richard Weinberger wrote:
> >> Am 16.11.2015 um 21:46 schrieb Serge E. Hallyn:
> >> > On Mon, Nov 16, 2015 at 09:41:15PM +0100, Richard Weinberger wrote:
> >> >> Serge,
> >> >>
> >> >> On Mon, Nov 16, 2015 at 8:51 PM,  <serge@xxxxxxxxxx> wrote:
> >> >>> To summarize the semantics:
> >> >>>
> >> >>> 1. CLONE_NEWCGROUP re-uses 0x02000000, which was previously CLONE_STOPPED
> >> >>>
> >> >>> 2. unsharing a cgroup namespace makes all your current cgroups your new
> >> >>> cgroup root.
> >> >>>
> >> >>> 3. /proc/pid/cgroup always shows cgroup paths relative to the reader's
> >> >>> cgroup namespce root.  A task outside of  your cgroup looks like
> >> >>>
> >> >>>         8:memory:/../../..
> >> >>>
> >> >>> 4. when a task mounts a cgroupfs, the cgroup which shows up as root depends
> >> >>> on the mounting task's  cgroup namespace.
> >> >>>
> >> >>> 5. setns to a cgroup namespace switches your cgroup namespace but not
> >> >>> your cgroups.
> >> >>>
> >> >>> With this, using github.com/hallyn/lxc #2015-11-09/cgns (and
> >> >>> github.com/hallyn/lxcfs #2015-11-10/cgns) we can start a container in a full
> >> >>> proper cgroup namespace, avoiding either cgmanager or lxcfs cgroup bind mounts.
> >> >>>
> >> >>> This is completely backward compatible and will be completely invisible
> >> >>> to any existing cgroup users (except for those running inside a cgroup
> >> >>> namespace and looking at /proc/pid/cgroup of tasks outside their
> >> >>> namespace.)
> >> >>>    cgroupns-root.
> >> >>
> >> >> IIRC one downside of this series was that only the new "sane" cgroup
> >> >> layout was supported
> >> >> and hence it was useless for everything which expected the default layout.
> >> >> Hence, still no systemd for us. :)
> >> >>
> >> >> Is this now different?
> >> > 
> >> > Yes, all hierachies are no supported.
> >> > 
> >> 
> >> Should read "now"? :-)
> >> If so, *awesome*!
> >
> > D'oh!  Yes, now :-)
> 
> I am glad to see multiple hierarchy support, that is something people
> can use today.
> 
> A couple of quick questions before I delve into a review.
> 
> Does this allow mixing of cgroupfs and cgroupfs2?  That is can I: "mount
> -t cgroupfs" inside a container and "mount -t cgroupfs2" outside a
> container? and still have reasonable things happen?  I suspect the
> semantics of cgroups prevent this but I am interested to know what happens.

As Tejun said, this is not an issue.  There's not an actual separate cgroupfs2
filesystem, it's just a separate hierarchy which controllers can be bound to
or not, which has its own set of semantics (like no tasks on leafnodes).  So
a legacy application would never be able to run on the unified hierarchy, but
this does not change that.

> Similary have you considered what it required to be able to safely set
> FS_USERNS_MOUNT?

I think the only thing we need to do is

1. go through and make sure that any ability to change mount flags is under
capable() (which I have not yet done).  The cgroup_mount() itself checks that
flags are not changed, but there may be some subtle way to effect a change
that I'm not aware of yet.

2. Make sure that to bind a new controller you must be true root.  It's
possible that a patch like the one below would suffice.

-serge