Re: Introspecting userns relationships to other namespaces?

[Michael Kerrisk (man-pages)]

On 07/07/2016 09:17 PM, James Bottomley wrote:
> On Thu, 2016-07-07 at 20:21 +0200, Michael Kerrisk (man-pages) wrote:
> > On 7 July 2016 at 17:01, James Bottomley
> > <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> [Serge already answered the parenting issue]
> > > On Thu, 2016-07-07 at 08:36 -0500, Serge E. Hallyn wrote:
> > > > Hm.  Probably best-effort based on the process hierarchy.  So
> > > > yeah you could probably get a tree into a state that would be
> > > > wrongly recreated. Create a new netns, bind mount it, exit;  Have
> > > > another task create a new user_ns, bind mount it, exit;  Third
> > > > task setns()s first to the new netns then to the new user_ns.  I
> > > > suspect criu will recreate that wrongly.
> > >
> > > This is a bit pathological, and you have to be root to do it: so
> > > root can set up a nesting hierarchy, bind it and destroy the pids
> > > but I know of no current orchestration system which does this.
> > >
> > > Actually, I have to back pedal a bit: the way I currently set up
> > > architecture emulation containers does precisely this: I set up the
> > > namespaces unprivileged with child mount namespaces, but then I ask
> > > root to bind the userns and kill the process that created it so I
> > > have a permanent handle to enter the namespace by, so I suspect
> > > that when our current orchestration systems get more sophisticated,
> > > they might eventually want to do something like this as well.
> > >
> > > In theory, we could get nsfs to show this information as an option
> > > (just add a show_options entry to the superblock ops), but the
> > > problem is that although each namespace has a parent user_ns,
> > > there's no way to get it without digging in the namespace specific
> > > structure.  Probably we should restructure to move it into
> > > ns_common, then we could display it (and enforce all namespaces
> > > having owning user_ns) but it would be a
> >
> > I'm missing something here. Is it not already the case that all
> > namespaces have an owning user_ns?
>
> Um, yes, I don't believe I said they don't.  The problem I thought you
> were having is that there's no way of seeing what it is.

Your words "and enforce all namespaces having owning user_ns" were
what left me puzzled--it sounded to me that the implication was
that this is not "enforced" right now.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/