svn commit: r367038 - head/sbin/mount_nullfs

[Edward Tomasz Napierala]

On 1025T0717, Cy Schubert wrote:
> In message <CAPjTQNEq2ukvkNoF6bsymTai52XV1tbu3TxW4rrN4n5hASMFqQ at mail.gmail.c
> om>
> , Oliver Pinter writes:
> > On Sunday, October 25, 2020, Edward Tomasz Napierala <trasz at freebsd.org>
> > wrote:
> >
> > > Author: trasz
> > > Date: Sun Oct 25 14:09:00 2020
> > > New Revision: 367038
> > > URL: https://svnweb.freebsd.org/changeset/base/367038
> > >
> > > Log:
> > >   Remove the check that prevents creating "loops" from mount_nullfs(8).
> > >
> > >
> > The simple question is: why?
> 
> I was about to ask that myself.
> 
> The why is usually more important than the how.

Two reasons.

First, from what I can tell this is an artificial limitation which serves
no purpose.  I thought it was to prevent some kind of deadlock between
vnodes, so I asked kib@, and he mentioned removing that very limitation
from the kernel code; thus, it looks to me like a historical leftover.

Second, I've stumbled upon this when trying to nullfs-mount '/' on
'/compat/ubuntu/bsd/', which would be useful for autochroot functionality,
when (or if) it becomes a thing.