Re: [RFC] Null Namespaces

On Thu, Jun 25, 2026 at 2:53 PM John Ericson <[email protected]> wrote:
>
> On Thu, Jun 25, 2026, at 5:00 PM, H. Peter Anvin wrote:
> > On 2026-06-24 16:12, Al Viro wrote:
> > > On Wed, Jun 24, 2026 at 06:51:47PM -0400, John Ericson wrote:
> > >
> > >> #### Null mount namespace
> > >>
> > >> - requires:
> > >>
> > >>   - null root file system: absolute paths don't work.
> > >>
> > >>   - null current working directory: relative paths with traditional,
> > >>     non-`*at` system calls (and `*at` ones using `AT_FDCWD`) don't work.
> > >>
> > >> - All operations relating to the "ambient" mount tree don't work.
> > >>
> > >> - `*at` operations with a file descriptor do work.
> > >
> > > Huh?  The last bit looks contradicts the previous one - if you have
> > > an opened directory in a mount from some namespace, those `*at` operations
> > > with that descriptor *will* be seeing the mount tree of that namespace,
> > > whatever the hell is "ambient" supposed to mean.  Either that, or you
> > > will be exposing whatever's overmounted in that mount, which is a huge
> > > can of worms.
> >
> > It seems to me that this is really no different *in practice* to having an
> > empty mount namespace, no? You might still be able to stat("/") and get a
> > d--------- result, but how does that actually affect anything?
>
> The argument against just having an empty, immutable root directory and
> calling it a day is the tie-in with a new process-spawning API discussed
> near the bottom of my original email. I want to have nice secure
> defaults, rather than forcing the programmer to remember to unshare, but
> I also don't want to degrade performance by speculatively creating new
> empty mount namespaces that might just be thrown away. Null fields alone
> get us both --- security and good performance.

This seems like a false dichotomy.  There's such thing as a singleton.

In fact, we have this spiffy nullfs_fs_get_tree.  It seems relatively
straightforward to have an API to get an fd to the singleton nullfs,
and the default for a newly spawned process could even be to have cwd
pointing at nullfs.

root is still harder, because of the shadowing issue.  I think I
proposed, ages ago, relaxing the chroot rules so that, at least under
certain circumstances (e.g. the task is not already chrooted) an
unprivileged task could chroot.  chrooting to nullfs seems like a
somewhat useful operation.

I can imagine more complex schemes to allow even a chrooted process to
safely start acting as though their root is nullfs, but that would be
potentially fairly nasty.  *Maybe* everything would work if there was
a root-for-dotdot and a separate root-for-absolute-paths, and
nameidata->root could point to the former, but I'm certainly not
willing to say that I think this would work with any confidence at
all.

--Andy