Subject: Re: loosing netdevices with namespaces and unshare?

From: [email-masked] (Eric W. Biederman)
Date: Thu, 01 Jun 2017 02:48:01 -0500

Harald Welte <[email-masked]> writes:

> Hi Eric,
>
> On Thu, Jun 01, 2017 at 01:32:49AM -0500, Eric W. Biederman wrote:
>
>> If a network device does not implement rntl_link_ops it is returned to
>> the initial network namespace.   Anything else will loose physical
>> devices.
>
> Thanks a lot for your statement.  This is a big relief, my line of
> thinking thus is confirmed:  We shall not loose physical devices.

Rereading that I should have said:
    We shall not lose physical devices.
We should let the loose to talk and say interesting things to the world.

>> Only for pure software based devices do we delete them.  Perhaps your
>> sub interface implements rtnl_link_ops?  Either that or something is
>> still holding a reference to your network namespace, which would prevent
>> the network device from being returned.
>
> My question is how to debug this further?  Monitoring
> /proc/*/ns/net* showed that the ID of the namespace is gone after
> terminating my processes in the namespace.  Short of adding printk() or
> playing with kprobes: to the related kernel code, how can I track the
> reference count or get an idea who might hold references?

You mentioned sub-interface.  I would first look to see if your
sub-interface might possibly implement rtnl_link_ops.

For testing I would toss in a full fledged physical interface and
see if that pops back.  Just to verify what you are seeing happening is
happening.

In your minimal test case of "unshare -Urn bash -c 'sleep 1; exit 0;'" I
can't imagine there is anything holding a reference.  So it may come
down to adding some printks or playing with kprobes.

All of macvlans and vlans and anything I can think of as sub-interface
all implement rtnl_link_ops and will get deleted when a network
namespace exits.  Which generally is what you want as it gives a very
nice cleanup.

Eric