cross namespace interface notification for tun devices

Hey guys,

It's possible to create a tun device in a process in namespace A and
then move that interface to namespace B. The controlling process in A
needs to receive notifications on when the interface is brought up or
down. It can receive these notifications via netlink while the
interface lives in A but not when it moves to B.

Any tricks or APIs to get around this?

The best I've come up with is, in a sleep loop, writing to the tun
device's fd something with a NULL or invalid payload. If the interface
is down, the kernel returns -EIO. If the interface is up, the kernel
returns -EFAULT. This seems to be a reliable distinguisher, but is a
pretty insane way of doing it. And sleep loops are somewhat different
from events too.

If there aren't any current APIs for receiving events directly from
the fd of a tun interface, would this list be happy with a patch that
adds one?

Thanks,
Jason