Re: [PATCH v3 2/3] fs: introduce uid/gid shifting bind mount

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2020-02-18 at 14:33 -0800, Christoph Hellwig wrote:
> On Mon, Feb 17, 2020 at 12:53:06PM -0800, James Bottomley wrote:

[...]
> > diff --git a/include/linux/cred.h b/include/linux/cred.h
> > index 18639c069263..d29638617844 100644
> > --- a/include/linux/cred.h
> > +++ b/include/linux/cred.h
> > @@ -59,6 +59,7 @@ extern struct group_info *groups_alloc(int);
> >  extern void groups_free(struct group_info *);
> >  
> >  extern int in_group_p(kgid_t);
> > +extern int in_group_p_shifted(kgid_t);
> 
> How do I know when to use in_group_p_shifted vs in_group_p?
> What about the various other fs callers?

So this is one I wondered about too.  The problem is that the shifted
credential (the one representing the fsuid/fsgid the filesystem will
see) still has cred->group_info representing the kuid/kgid which are
unshifted from the filesystem perspective.  The solution was to use
in_group_p_shifted when you're comparing a filesystem view fsgid and
use in_group_p when you're comparing a kernel kgid.

However, I'm now thinking that's way too complex and what should happen
is that I should shift every member of cred->group_info so that all
searches happen on the fs view, meaning the fs always uses in_group_p
like it does today and only the corner cases that compare a kgid need
shifting.

James