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:

quoted

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.

Cheers,
Trevor

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