Re: Introspecting userns relationships to other namespaces?

[Serge E. Hallyn]

On Wed, Jul 06, 2016 at 10:41:48AM +0200, Michael Kerrisk (man-pages) wrote:
> [Rats! Doing now what I should have down to start with. Looping some
> lists and CRIU and other possibly relevant people into this
> conversation]
> 
> Hi Eric,
> 
> On 5 July 2016 at 23:47, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> > "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes:
> >
> >> Hi Eric,
> >>
> >> I have a question. Is there any way currently to discover which
> >> user namespace a particular nonuser namespace is governed by?
> >> Maybe I am missing something, but there does not seem to be a
> >> way to do this. Also, can one discover which userns is the
> >> parent of a given userns? Again, I can't see a way to do this.
> >>
> >> The point here is introspecting so that a process might determine
> >> what its capabilities are when operating on some resource governed
> >> by a (nonuser) namespace.
> >
> > To the best of my knowledge that there is not an interface to get that
> > information.  It would be good to have such an interface for no other
> > reason than the CRIU folks are going to need it at some point.  I am a
> > bit surprised they have not complained yet.

I don't think they need it.  They do in fact have what they need.  Assume
you have tasks T1, T2, T1_1 and T2_1;  T1 and T2 are in init_user_ns;  T1
spawned T1_1 in a new userns;  T2 spawned T2_1 which setns()d to T1_1's ns.
There's some {handwave} uid mapping, does not matter.

At restart, it doesn't matter which task originally created the new userns.
criu knows T1_1 and T2_1 are in the same userns;  it creates the userns, sets
up the mapping, and T1_1 and T2_1 setns() to it.

> > That said in a normal use scenario I don't think that information is
> > needed.
> >
> > Do you have a particular use case besides checkpoint/restart where this
> > is useful?  That might help in coming up with a good userspace interface
> > for this information.
> 
> So, I spend a moderate amount of time working with people to introduce
> them to the namespaces infrastructure, and one topic that comes up now
> and this introspection/visualization tools. For example,
> nowadays--thanks to the (bizarrely misnamed) NStgid and NSpid fields
> in /proc/PID--it's possible to (and someone I was working with did)
> write tools that introspect the PID namespace hierarchy to show all of
> process's and their PIDs in the various namespace instance. It's a
> natural enough thing to want to do, when confronted with the
> complexity of the namespaces.
> 
> Someone else then asked me a question that led me to wonder about
> generally introspecting on the parental relationships between user
> namespaces and the association of other namespaces types with user
> namespaces. One use would be visualization, in order to understand the
> running system. Another would be to answer the question I already
> mentioned: what capability does process X have to perform operations
> on a resource governed by namespace Y?

I agree they'll probably want it, but if we want for a real need and
use case we can do a better job of providing what's needed.

-serge