Re: [RFC] Null Namespaces

[John Ericson]

On Wed, Jun 24, 2026, at 7:20 PM, Andy Lutomirski wrote:
> I think I like this, but some comments:

Thanks, that's really nice to hear!

While arguably this is just the culmination of a direction Linux has
been going in for a while, it could also be seen as a very "out there"
idea. That at least one person likes the rough sound of things makes me
feel a lot better!

> On Wed, Jun 24, 2026 at 4:06 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > On Wed, Jun 24, 2026 at 3:52 PM John Ericson <mail@xxxxxxxxxxxxxx> wrote:
>
> > >   - null current working directory: relative paths with traditional,
> > >     non-`*at` system calls (and `*at` ones using `AT_FDCWD`) don't work.
> >
> > It's perfectly valid to cd to a directory that does not belong to
> > one's namespace.  We have fchdir.  What's wrong with letting it
> > continue working?
> >
> > Regardless of that, the current directory either needs to be a
> > directory or to be nothing at all, and if we support the latter, we
> > need to figure out what /proc will show.
>
> Thinking about this more: I think that handling CWD might actually be
> a prerequisite for the series and has little to do with namespaces.
> Maybe try adding, as a standalone feature, the ability to have a null
> CWD.  Define semantics and see what the implementation looks like.
>
> Then, if you add null namespaces, you could optionally make
> transitioning to a null namespace set a null CWD.  Or those features
> could be orthogonal.

Hehe, I had the same thought after working on the filesystem patches,
along with the analogous thought for the root filesystem. It had been so
long since I had done a `chroot` without also doing a mount namespace
`unshare` --- despite the former being much older --- that I had
forgotten this separation of concerns.

My apologies for forgetting to include this insight in the original
email.

> Maybe the way to go is to implement the ones that have clearer
> semantics and to defer the others.

I would much prefer this, actually.

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.

John