Re: [RFC] Null Namespaces

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

On Wed, Jun 24, 2026 at 07:53:53PM -0400, John Ericson wrote:
> I wanted to discuss a bit about each type of namespace to indicate that
> this is a concept I think works across the board --- it wouldn't be such
> a good solution for the process spawning API if it was only applicable
> to some but not all namespace types. But the truth is that I have
> thought about the FS cases the most, as I think you have picked up on.
> 
> If there is interest in landing
> 
>   1. null CWD
>   2. null root fs
>   3. null mount namespace
> 
> in isolation, and then returning to the other namespaces to iron out
> their details, that would be fantastic. It would be much nicer for me to
> get some momentum that way, without having to design everything all at
> once first before getting to implement anything.

Please, start with explaining what, in your opinion, a mount namespace _is_,
and where does "mount X is attached at path P relative to mount Y" belong.

What's the fundamental difference between CWD and any open descriptor for
a directory?  Why does it make sense to ban the former, but allow the
equivalents done via the latter?