From: "Theo de Raadt" <deraadt@openbsd.org>
Subject: Re: interface ioctl loopback flags
To: Alexander Bluhm <bluhm@openbsd.org>, tech@openbsd.org
Date: Thu, 11 Apr 2024 08:41:55 -0600

Claudio Jeker <cjeker@diehard.n-r-g.com> wrote:

> > Is there any usecase that userland can modify the IFF_LOOPBACK flag?
> > If I add IFF_LOOPBACK to the unchangeable flags, kernel does not crash.
> > 
> > ok?
> 
> OK claudio@. I see no reason why userland should be able to toggle
> IFF_LOOPBACK. The drivers don't expect that change.

I also think that is an accident.  IFF_LOOPBACK is telling userland
the behaviour of a specific driver, it's supposed to be static & permanent.