Re: [PATCH 3/4] autofs4 - track uid and gid of last mount requestor

["Serge E. Hallyn" <serue@us.ibm.com>]

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