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

[Josh Triplett]

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.  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.)