[patch 3/3][netns] net: hide master/linked interface from netlink

[Eric W. Biederman]

dlezcano at fr.ibm.com writes:

> From: Daniel Lezcano <dlezcano at fr.ibm.com>
>
> Actually when a network device is linked to another, the name appears
> to be @<link>. For example, if a macvlan0 is created on top of eth0,
> the ip link show is:
>
> 6: macvlan0 at eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop 
>     link/ether 6a:d4:10:0d:a8:55 brd ff:ff:ff:ff:ff:ff
>
> But if we move macvlan0 to a network namespace, eth0 does no longer
> exist inside it and the result will be:
>
> 6: macvlan0 at if2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop 
>     link/ether 6a:d4:10:0d:a8:55 brd ff:ff:ff:ff:ff:ff
>
> if2 is, I guess, some random value. That can do invalid memory
> access or inconsistent data showing.
>
> The patchset will avoid such case, it checks if the linked device exist
> into the current network namespace and if it doesn't the result will
> be:
>
> 6: macvlan0 at NONE: <BROADCAST,MULTICAST> mtu 1500 qdisc noop 
>     link/ether 6a:d4:10:0d:a8:55 brd ff:ff:ff:ff:ff:ff

Hmm.  Currently the ifindex space is global.  Something that ultimately
needs to be fixed to handle migration so we can preserve the ifindex
on a device when we migrate it.

So the @if2 piece is harmless, as it is just reporting the ifindex
number because it can't figure out the name that corresponds to
that network device.

So the worst we get is hints that some extra data exists somewhere.
I suspect the correct fix is to not even write the additional attribute
in this case instead of setting the value to zero.

I am not yet convinced we need to handle this case yet, at most this
is a cosmetic issue.

Eric