netdev - Re: loosing netdevices with namespaces and unshare?

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Date:   Sat, 03 Jun 2017 05:53:48 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Harald Welte <laforge@...monks.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: loosing netdevices with namespaces and unshare?

Cong Wang <xiyou.wangcong@...il.com> writes:

> On Wed, May 31, 2017 at 11:32 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Cong Wang <xiyou.wangcong@...il.com> writes:
>>> Network namespace does not special-case the physical devices,
>>> it treats them all equally as abstract net devices.
>>
>> Absolutely not true.
>>
>> The relevant code is in net/core/dev.c:default_device_exit
>>
>> If a network device does not implement rntl_link_ops it is returned to
>> the initial network namespace.   Anything else will loose physical
>> devices.
>
> Hmm, I never noticed that if check...
>
>>
>> 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.
>>
>
> But this simply sucks:
>
>                 snprintf(fb_name, IFNAMSIZ, "dev%d", dev->ifindex);
>                 err = dev_change_net_namespace(dev, &init_net, fb_name);
>                 if (err) {
>                         pr_emerg("%s: failed to move %s to init_net: %d\n",
>                                  __func__, dev->name, err);
>                         BUG();
>                 }
>
> It is essentially hard to handle the error here, but it is quite easy to
> trigger such BUG() by naming other device devX, it is no better
> than just losing it.

The rename only happens if there is a conflicting device name.

Beyond that there is the entire hotplug functionality so it should be
possible to automatically detect a new device in your network namespace
and do something with it.

Eric