Re: [Lsf-pc] [LSF/MM ATTEND] Richacls

[Jeremy Allison <jra@samba.org>]

On Tue, Jan 13, 2015 at 11:53:42AM -0800, Frank Filz wrote:
> > On Tue, Jan 13, 2015 at 12:40:29PM -0500, J. Bruce Fields wrote:
> > > On Tue, Jan 13, 2015 at 06:23:26PM +0100, Andreas Gruenbacher wrote:
> > > > On 01/13/2015 05:48 PM, Jeremy Allison wrote:
> > > > >My understanding of Christoph's objection (although I'm sure he can
> > > > >chime in himself :-) was that he wanted to see POSIX ACLs reworked
> > > > >as a mapping on top of RichACLs, so that ultimately RichACLs would
> > > > >be the only on-disk format of the EA.
> > > > >
> > > > >I think that is doable, as I think any POSIX ACL can be represented
> > > > >as an underlying RichACL, just not the reverse.
> > > >
> > > > On of the differences is that permissions in POSIX ACLs do
> > > > accumulate, while in NFSv4 and CIFS ACLs, and therefore also
> > > > richacls, they do not. So the two models are really not
> > > > interchangeable, however annoying that may be.
> 
> I think Andreas got do and do not reversed (though looks like everyone read
> it the right way...)
> 
> > > > For example, with the following POSIX ACL, a non-root process in
> > > > group 5001 and 5002 would not be allowed to open f with O_RDWR, only
> > > > with O_RDONLY *or* O_WRONLY.
> > > >
> > > >   # file: f
> > > >   # owner: root
> > > >   # group: root
> > > >   user::rw-
> > > >   group::rw-
> > > >   group:5001:r--
> > > >   group:5002:-w-
> > > >   mask::rw-
> > > >   other::---
> > > >
> > > > In all the other ACL models, the process would be allowed to open f
> > > > with O_RDWR.
> 
> Hasn't this been resolved in in knfsd by use of DENY ACEs in converting the
> POSIX ACL to NFS v4?

No, it can't work.

Consider a non-root process in group 5001 and 5002.

The obvious way to map the above POSIX ACL to NFSv4
is:

1) group:5001:DENY:WRITE
2) group:5001:ALLOW:READ
3) group:5002:DENY:READ
4) group:5001:ALLOW:WRITE

But NFSv4/CIFS permissions are cummulative. So
requesting O_WRONLY will always fail as it runs
into rule number 1.

So let's reorder it:

1) group:5002:DENY:READ
2) group:5001:ALLOW:WRITE
3) group:5001:DENY:WRITE
4) group:5001:ALLOW:READ

Now O_RDONLY fails due to rule #1 :-(.

If we put both DENY rules first, both
O_RDONLY and O_WRONLY fail. If we put both
ALLOW rules first, then O_RDWR is allowed,
which shouldn't be.

There's just no way to get to the "or" behavior
from here.