Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

["W. Trevor King" <wking@tremily.us>]

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On Sat, Jul 23, 2016 at 04:56:44PM -0500, Eric W. Biederman wrote:
> "W. Trevor King" <wking@tremily.us> writes:
> > On Sat, Jul 23, 2016 at 02:38:56PM -0700, James Bottomley wrote:
> >> On Sat, 2016-07-23 at 14:14 -0700, W. Trevor King wrote:
> >> > namespaces(7) and clone(2) both have:
> >> > 
> >> >   When a network namespace is freed (i.e., when the last
> >> >   process in the namespace terminates), its physical network
> >> >   devices are moved back to the initial network namespace (not
> >> >   to the parent of the process).
> >> > 
> >> > So the initial network namespace (the head of
> >> > net_namespace_list?)  is special [1].  To understand how
> >> > physical network devices will be handled, it seems like we want
> >> > to treat network devices as a depth-1 tree, with all
> >> > non-initial net namespaces as children of the initial net
> >> > namespace.  Can we extend this series' NS_GET_PARENT to return:
> >> > 
> >> > * EPERM for an unprivileged caller (like this series currently
> >> >   does for PID namespaces),
> >> > * ENOENT when called on net_namespace_list, and
> >> > * net_namespace_list when called on any other net namespace.
> >> 
> >> What's the practical application of this?  independent net
> >> namespaces are managed by the ip netns command.  It pins them by
> >> a bind mount in a flat fashion; if we make them hierarchical the
> >> tool would probably need updating to reflect this, so we're going
> >> to need a reason to give the network people.  Just having the
> >> interfaces not go back to root when you do an ip netns delete
> >> doesn't seem very compelling.
> >
> > I'm not suggesting we add support for deeper nesting, I'm suggesting
> > we use NS_GET_PARENT to allow sufficiently privileged users to
> > determine if a given net namespace is the initial net namespace.  You
> > could do this already with something like:
> >
> > 1. Create a new net namespace.
> > 2. Add a physical network device to that namespace.
> > 3. Delete that namespace.
> > 4. See if the physical network device shows up in your
> >    initial-net-namespace candidate.
> > 5. Delete the physical network device (hopefully it ended up
> >    somewhere you can find it ;).
> >
> > But using an NS_GET_PARENT call seems much safer and easier.
> 
> Have you had the problem in practice where you can't tell which
> network namespace is the initial network namespace.  This all seems
> like a theoretical problem rather than a real one.

I haven't had any practical problems here, I'm just trying to wrap my
head around namespace-relationship discovery.  The special physical
network device handling seems a lot like init re-parenting (with no
PR_SET_CHILD_SUBREAPER analog in a 1-deep namespace tree), so calling
the initial network namespace a parent (and all the other namespaces
its direct children) seems natural enough.  If that doesn't sound
convincing, I'm happy to punt this idea until someone runs into a
practical problem ;).

Cheers,
Trevor

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