linux-kernel - Re: [PATCH 1/2] autofs4: allow autofs to work outside the initial PID namespace

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Date:	Mon, 26 Nov 2012 10:29:28 +0800
From:	Ian Kent <raven@...maw.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, autofs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	sukadev@...ux.vnet.ibm.com, serge.hallyn@...onical.com
Subject: Re: [PATCH 1/2] autofs4: allow autofs to work outside the initial
 PID namespace

On Mon, 2012-11-26 at 07:25 +0800, Ian Kent wrote:
> On Sat, 2012-11-24 at 14:35 -0800, Eric W. Biederman wrote:
> > Miklos Szeredi <miklos@...redi.hu> writes:
> > 
> > > On Sat, Nov 24, 2012 at 1:07 PM, Eric W. Biederman
> > > <ebiederm@...ssion.com> wrote:
> > >> Ian Kent <raven@...maw.net> writes:
> > >>
> > >>> On Sat, 2012-11-24 at 10:23 +0800, Ian Kent wrote:
> > >>>> On Fri, 2012-11-23 at 15:30 +0100, Miklos Szeredi wrote:
> > >
> > >>>> AFAICS autofs mounts mounted with MS_PRIVATE in the initial namespace do
> > >>>> propagate to the clone when it's created so I'm assuming subsequent
> > >>>> mounts would also. If these mounts are busy in some way they can't be
> > >>>> umounted in the clone unless "/" is marked private before attempting the
> > >>>> umount.
> > 
> > Subsequent mounts after the clone do not have a mechanism to propogate
> > with MS_PRIVATE.  As creating a new mount namespaces is essentially
> > an instance of mount --bind.  Those semantics are a little unintuitive
> > I have to admit.
> > 
> > >>> This may sound stupid but if there something like, say, MS_NOPROPAGATE
> > >>> then the problem I see would pretty much just go away. No more need to
> > >>> umount existing mounts and container instances would be isolated. But, I
> > >>> guess, I'm not considering the possibility of cloned of processes as
> > >>> well .... if that makes sense, ;)
> > >>
> > >> Something is very weird is going on.  MS_PRIVATE should be the
> > >> MS_NOPROPOGATE you are looking for.  There is also MS_UNBINDABLE.
> > >> which is a stronger form of MS_PRIVATE and probably worth play with.
> > >>
> > >
> > > MS_UNBINDABLE says:  skip this mount when copying a mount tree, such
> > > as when the mount namespace is cloned.
> > >
> > > If you set MS_UNBINDABLE on autofs mounts then they will simply not
> > > appear in a cloned namespace.  Which sounds like a good idea,  no?
> > 
> > Good point.  If the desire is for a mount to be managed by autofs
> > setting MS_UNBINDABLE seems required.
> 
> Arrgh, I know that's something I should have looked into long ago.
> The fact is that autofs mounts are directly related to a specific path
> defined by automount maps that are associated with the daemon so bind
> mounting them elsewhere makes no sense.

Except, AFAICS, they do appear in the clone.

> 
> Is it necessary (or sensible) to use MS_PRIVATE with MS_UNBINDABLE?

and specifying MS_PRIVATE as well gives an EINVAL return.

> 
> > 
> > Eric
> > 
> 
> 
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