Re: [RFC] pivot_root(2) races

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Feb 13, 2026 at 12:27:46PM -0800, H. Peter Anvin wrote:
> On 2026-02-13 09:47, Askar Safin wrote:
> > "H. Peter Anvin" <hpa@zytor.com>:
> >> It would be interesting to see how much would break if pivot_root() was restricted (with kernel threads parked in nullfs safely out of the way.)
> > 
> > As well as I understand, kernel threads need to follow real root directory,
> > because they sometimes load firmware from /lib/firmware and call
> > user mode helpers, such as modprobe.
> > 
> 
> If they are parked in nullfs, which is always overmounted by the global root,
> that should Just Work[TM]. Path resolution based on that directory should
> follow the mount point unless I am mistaken (which is possible, the Linux vfs
> has changed a lot since the last time I did a deep dive.)

You are, and it had always been that way.  We do *not* follow mounts at
the starting point.  /../lib would work, /lib won't.  I'd love to deal with
that wart, but that would break early boot on unknown number of boxen and
breakage that early is really unpleasant to debug.