netdev - Re: loosing netdevices with namespaces and unshare?

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Message-ID: <CAM_iQpWaJ7WKOEQe=00AX35x_gCKHoHikVTGH6UQTPw+3oD_pw@mail.gmail.com>
Date:   Fri, 2 Jun 2017 16:25:44 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Harald Welte <laforge@...monks.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: loosing netdevices with namespaces and unshare?

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.