netdev - cross namespace interface notification for tun devices

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Message-ID: <CAHmME9pD_kees+xrjTeWXgi26UdhbMVgvoddbCyueTZkbguvAg@mail.gmail.com>
Date:   Mon, 18 Sep 2017 20:47:10 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Netdev <netdev@...r.kernel.org>
Cc:     Mathias <mathias@...l-andersen.dk>
Subject: 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