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

["'J. Bruce Fields'" <bfields@fieldses.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?
> 
> I just had a question though...
> 
> Can a process that is in both groups open two file descriptors, one
> read-only and one write-only? I think so.

Yep.

> Assuming so, what happens with NFS v4 where the 2nd open results in an
> open-upgrade over the wire to read-write?

If anyone cares: currently knfsd will attempt a single read-write open
if a client does a read-write open, but if a client (for example) opens
for read and then upgrades for write, nfsd will do a read-only open and
then a write-only open.  So given the above acl that open+upgrade
sequence would succeed where the read-write open would fail.

> > > If we modified the behavior to permit O_RDWR in this case, would that
> > > cause anyone a problem?
> > 
> > Hmmmm. It changes userspace visible behavior. I can't think of any reason
> > anyone would be relying on this (other than bugs :-) but still...
> 
> Yea, I would be wary of changing user space behavior. At the least, it MIGHT
> cause someone's conformance test to fail. On the other hand, the POSIX ACL
> draft never become a standard so no one would really have a complaint if
> Linux's implementation were slightly different...

I doubt anyone worries about that so much as about breaking something on
upgrade due to someone expecting the older behavior.

That sounds unlikely, though.

--b.