Re: [PATCH v7 7/7] enable building user namespace with xfs

[Dwight Engen <dwight.engen@oracle.com>]

On Thu, 1 Aug 2013 10:06:43 -0500
Ben Myers <bpm@sgi.com> wrote:

> Dave,
> 
> On Thu, Aug 01, 2013 at 09:28:52AM +1000, Dave Chinner wrote:
> > On Wed, Jul 31, 2013 at 08:25:23AM -0500, Ben Myers wrote:
> > > Hey,
> > > 
> > > On Wed, Jul 31, 2013 at 10:21:19AM +1000, Dave Chinner wrote:
> > > > On Tue, Jul 30, 2013 at 06:40:21PM -0500, Ben Myers wrote:
> > > > > On Mon, Jul 29, 2013 at 11:07:09PM -0400, Dwight Engen wrote:
> > > > > > >From e6a9ee0cfa0ed40484f66bc1726dc19de36038b8 Mon Sep 17
> > > > > > >00:00:00 2001
> > > > > > From: Dwight Engen <dwight.engen@oracle.com>
> > > > > > Date: Tue, 2 Jul 2013 09:52:54 -0400
> > > > > > Subject: [PATCH 7/7] enable building user namespace with xfs
> > > > > > 
> > > > > > Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
> > > > > 
> > > > > Was there a patch running around to limit bulkstat to
> > > > > init_user_ns?  Any other items that needed to be addressed
> > > > > before applying this patch?
> > > > 
> > > > Bulkstat has a capable(CAP_SYS_ADMIN) check and therefore can
> > > > only be executed in the init name space. Similarly, all the
> > > > open-by-handle interfaces have the same capable() checks so
> > > > they can only be executed int he init name space, too.
> > > 
> > > Gah.  I was under the impression that you could have a process
> > > with CAP_SYS_ADMIN in a namespace other than init_user_ns.
> > 
> > Ben, until about a week and a half ago I was also working under that
> > same understanding as you.
> 
> Well huh.  According to Dwight you can have a process with
> CAP_SYS_ADMIN in a namespace other than init_user_ns.  Kinda cool,
> IMO.

Yeah, see commit 3486740a and cap_capable(). Took me a bit to grok
that for loop, but I think the semantics are that if you are in a non
init_user_ns, you can start from current_user_ns() looking for a cap
and walk back the heirarchy (userns's can be nested) but it won't ever
check in init_user_ns, and so by definition you have no capabilities
in init_user_ns.

ie. if you are in a userns, you might be nsown_capable(CAP_SYS_ADMIN)
but you cannot ever be capable(CAP_SYS_ADMIN). Thats my understanding
anyway :)

> -Ben

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