Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Feb 08, 2017 at 07:22:45AM -0800, James Bottomley wrote:
> On Tue, 2017-02-07 at 17:54 -0800, Josh Triplett wrote:
> > On Tue, Feb 07, 2017 at 11:49:33AM -0800, Christoph Hellwig wrote:
> > > On Tue, Feb 07, 2017 at 11:02:03AM -0800, James Bottomley wrote:
> > > > >   Another option would be to require something like a project
> > > > > as used 
> > > > > for project quotas as the root.  This would also be conveniant
> > > > > as it 
> > > > > could storge the used remapping tables.
> > > > 
> > > > So this would be like the current project quota except set on a
> > > > subtree?  I could see it being done that way but I don't see what
> > > > advantage it has over using flags in the subtree itself (the
> > > > mapping is
> > > > known based on the mount namespace, so there's really only a
> > > > single bit
> > > > of information to store).
> > > 
> > > projects (which are the underling concept for project quotas) are
> > > per-subtree in practice - the flag is set on an inode and then
> > > all directories and files underneath inherit the project ID,
> > > hardlinking outside a project is prohinited.
> > 
> > I'm interested in having a VFS-level way to do more than just a 
> > shift; I'd like to be able to arbitrarily remap IDs between what's on 
> > disk and the system IDs.
> 
> OK, so the shift is effectively an arbitrary remap because it allows
> multiple ranges to be mapped (althought the userns currently imposes a
> maximum number of five extents but that limit is a bit arbitrary just
> to try to limit the amount of space the parametrisation takes).  See
> kernel/user_namespace.c:map_id_up/down()
> 
> >   If we're talking about developing a VFS-level solution for this, 
> > I'd like to avoid limiting it to just a shift.  (A shift/range
> > would definitely be the simplest solution for many common container
> > cases, but not all.)
> 
> I assume the above satisfies you on this point, but raises the
> question: do you want an arbitrary shift not parametrised by a user
> namespace?  If so how many such shifts do you want ... giving some
> details of the use case would be helpful.

The limit of five extents means this may not work in the most general
case, no.

One use case: given an on-disk filesystem, its name-to-number mapping,
and your host name-to-number mapping, mount the filesystem with all the
UIDs bidirectionally mapped to those on your host system.

Another use case: given an on-disk filesystem with potentially arbitrary
UIDs (not necessarily in a clean contiguous block), and a pile of
unprivileged UIDs, mount the filesystem such that every on-disk UID gets
a unique unprivileged UID.

(I have some additional use cases, but they would require the ability to
extend the mapping on the fly without remounting.)