[patch 1/1][NETNS][IPV6] protect addrconf from loopback registration

[Denis V. Lunev]

>> why should we care on down? we are destroying the device. It should
>> gone. All references to it should also gone. So, we should perform the
>> cleaning and remove all IPv6 addresses, so notifier should also work.
> 
> We need to take care of netdev down, someone can put the loopback down
> if he wants.
> 
>> The code relies on the "persistent" loopback and this is a _bad_ thing.
>> This is longstanding bug in the code, that the dst_entry should have a
>> valid reference to a device. This is the only purpose for a loopback
>> persistence. Though, at the namespace death no such entries must be and
>> this will be checked during unregister process. This patch definitely
>> breaks this assumption :(
>>
>> Namespaces are good to catch leakage using standard codepaths, so they
>> should be preserved as much as possible. So, _all_ normal down code
>> should be called for a loopback device in other than init_net context.
> 
> I agree with you, this is a bug in ipv6 and the loopback; when playing
> with ipv6 we found that the loopback is still referenced 9 times when
> the system is shutdown.

Pff...  I can't guess right now where the error can be :( We have
correct behavior of loopback even for IPv6 within OpenVZ. On down the
count is 0 and device is destroyed correctly without these kludges. So,
something is definitely wrong.

> The purpose of this patch is to protect the __actual__ code from the new
> loopback behavior. We are looking at a more generic approach with the
> namespace for ipv6, as you mentioned, namespaces are good for network
> leakage detection as we create several instances of the network stack.

The only kludge required is already in place. addrconf_ifdown has a
protection for init_net.
        if (dev == init_net.loopback_dev && how == 1)
                how = 0;
Other places should be untouched.

Unregister for a loopback in !init_net is a _valid_ operation and should
be clean, i.e. without kludges in the path. This is the only way to
check the ref-counting.