Re: [patch 0/8] unprivileged mount syscall

Serge E. Hallyn
2007-04-13T21:44:17Z

Quoting Miklos Szeredi (miklos@szeredi.hu):
> > > Thinking a bit more about this, I'm quite sure most users wouldn't
> > > even want private namespaces. It would be enough to
> > >
> > > chroot /share/$USER
> > >
> > > and be done with it.
> > >
> > > Private namespaces are only good for keeping a bunch of mounts
> > > referenced by a group of processes. But my guess is, that the natural
> > > behavior for users is to see a persistent set of mounts.
> > >
> > > If for example they mount something on a remote machine, then log out
> > > from the ssh session and later log back in, they would want to see
> > > their previous mount still there.
> > >
> > > Miklos
> >
> > Agreed on desired behavior, but not on chroot sufficing. It actually
> > sounds like you want exactly what was outlined in the OLS paper.
> >
> > Users still need to be in a different mounts namespace from the admin
> > user so long as we consider the deluser and backup problems
>
> I don't think it matters, because /share/$USER duplicates a part or
> the whole of the user's namespace.
>
> So backup would have to be taught about /share anyway, and deluser
> operates on /home/$USER and not on /share/*, so there shouldn't be any
> problem.

In what I was thinking of, /share/$USER is bind mounted to
~$USER/share, so it would have to be done in a private namespace in
order for deluser to not be tricked.

> There's actually very little difference between rbind+chroot, and
> CLONE_NEWNS. In a private namespace:
>
> 1) when no more processes reference the namespace, the tree will be
> disbanded
>
> 2) the mount tree won't be accessible from outside the namespace

But it *can* be, if properly set up. That's part of the point of the
example in the OLS paper. When a user logs in, sshd clones a new
namespace, then bind-mounts /share/$USER into ~$USER/share. So assuming
that /share/$USER was --make-shared'd, it and ~$USER are now in the
same peer group, and any changes made by the user under ~$USER will
be reflected back into /share/$USER.

> Wanting a persistent namespace contradicts 1).

Not necessarily, see above.

> Wanting a per-user (as opposed to per-session) namespace contradicts
> 2). The namespace _has_ to be accessible from outside, so that a new
> session can access/copy it.

Again, I *think* you are wrong that private namespace contradicts this
requirement.

> So both requirements point to the rbind/chroot solution.

It all points to a combination of the two :-)

-serge