linux-kernel - Re: [patch 0/8] unprivileged mount syscall

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Date:	Fri, 13 Apr 2007 16:44:15 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	serue@...ibm.com, linuxram@...ibm.com, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, containers@...ts.osdl.org,
	util-linux-ng@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 0/8] unprivileged mount syscall

Quoting Miklos Szeredi (miklos@...redi.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
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