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

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Date:	Sat, 23 Jul 2016 16:56:44 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"W. Trevor King" <wking@...mily.us>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>, criu@...nvz.org,
	linux-fsdevel@...r.kernel.org,
	"Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

"W. Trevor King" <wking@...mily.us> writes:

2> 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.

Eric