Date: Mon, 3 Mar 2008 09:28:55 -0600
From: "Serge E. Hallyn" <serue@us.ibm.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Serge E. Hallyn" <serue@us.ibm.com>, Ian Kent <raven@themaw.net>,
       Jeff Moyer <jmoyer@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Kernel Mailing List <linux-kernel@vger.kernel.org>,
       autofs mailing list <autofs@linux.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       Pavel Emelyanov <xemul@openvz.org>
Subject: Re: [PATCH 3/4] autofs4 - track uid and gid of last mount requestor
Message-ID: <20080303152855.GB25643@sergelap.austin.ibm.com>
References: <1204179747.3501.21.camel@raven.themaw.net> <20080227223734.caab0165.akpm@linux-foundation.org> <1204182500.3501.49.camel@raven.themaw.net> <20080227232339.af6e904a.akpm@linux-foundation.org> <1204185623.3501.84.camel@raven.themaw.net> <x494pbtxaup.fsf@segfault.boston.devel.redhat.com> <20080228195118.GA16634@sergelap.austin.ibm.com> <1204255932.3969.86.camel@raven.themaw.net> <20080229160921.GA24296@sergelap.ibm.com> <m1bq5y3p2o.fsf@ebiederm.dsl.xmission.com>
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Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serue@us.ibm.com> writes:
> >
> > The way the user namespaces work right now is similar to say the IPC
> > namespace - a task belongs to one user, that user belongs to precisely
> > one user namespace.
> >
> > Even in my additional userns patches, I was changing uid to store the
> > (uid, userns) so a struct user still belonged to just one user
> > namespace.
> >
> > In contrast, with pid namespaces a task is associated with a 'struct
> > pid' which links it to multiple process ids, one in each pid namespace
> > to which it belongs.
> >
> > Perhaps we should be treating user namespaces like pid namespaces?
> >
> > For autofs this would mean that when autofs wants a uid for some task,
> > it would be given the uid in the user namespace which autofs 'knows'.
> >
> > It would also help me fix the siginfo problems I haven't solved yet -
> > rather than having to worry about user namespace lifetimes with siginfos
> > (which last a little while but have no clearly defined lifespan) we
> > could send the uid in an init user namespace or the uid in the target
> > uid namespace, or just a lightweight user struct proxy akin to 'struct
> > pid'.
> >
> > And it also obviates the need for any sort of delegation.
> >
> > So if I'm user 500 in what I think is the initial user namespace, I can
> > create a container with a new user namespace, the init task of which is
> > both uid 0 in the child userns, and uid 500 in the higher level,
> > automatically giving the container access to any files I own.
> >
> > Eric, when you get a chance (I know you're overloaded atm) I'd love to
> > hear your thoughts on this...
> 
> Succinctly.
> 
> I think the concept of mapping uids between user namespaces is
> fundamental to properly describing and thinking about the semantics of
> user namespaces correct.

Earlier I had thought this could just be done using a special keyring,
but atm I'm thinking that would be far uglier than just having a
struct pid-like credential proxy in the kernel to pass around in place
of uids.

> We don't have to start out anything except handling the case when
> no mapping exists, but asking the question how does this uid map
> between from one namespace to another is fundamental.

True.

But in any case I'm happy letting other things like netns and related
sys be completed before prototyping this.

thanks,
-serge
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