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

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Message-Id: <E1HcMOq-0002As-00@dorka.pomaz.szeredi.hu>
Date:	Fri, 13 Apr 2007 16:05:16 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	serue@...ibm.com
CC:	linuxram@...ibm.com, serue@...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

> > 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.

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

Wanting a persistent namespace contradicts 1).

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.

So both requirements point to the rbind/chroot solution.

Miklos
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