netlink and user namespaces

[Alexander Larsson <alexl-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Now that I'm using a non-privileged user namespace for my desktop
sandboxing system all kind of network status things are breaking. The
reason for this is that they use netlink to enumerated interfaces, and
to verify that the replies are from the kernel (apparently anyone can
send anyone netlink messages) this code is verifying that the
SCM_CREDENTIAL sender of the netlink messages is uid 0.

For instance: 
http://git.0pointer.net/avahi.git/commit/avahi-core/netlink.c?id=37b2be93e63ceff95698f24cd91cb11774eb621c
and:
https://git.gnome.org/browse/glib/tree/gio/gnetworkmonitornetlink.c#n340

This obviously breaks when uid is not mapped (as it can't be in an
unprivileged user namespace), as uid will be overflowuid.

Is there any other way to check that a netlink message is from the
kernel?

-- 
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 Alexander Larsson                                            Red Hat, Inc 
       alexl-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org            alexander.larsson-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org 
He's an uncontrollable skateboarding farmboy trapped in a world he never 
made. She's a strong-willed renegade schoolgirl married to the Mob. They 
fight crime!