linux-kernel - Re: [PATCH -V7 21/26] richacl: xattr mapping functions

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Date:	Fri, 21 Oct 2011 01:46:29 +0200
From:	Andreas Gruenbacher <agruen@...nel.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
	dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V7 21/26] richacl: xattr mapping functions

On Thu, 2011-10-20 at 06:25 -0400, J. Bruce Fields wrote:
> On Thu, Oct 20, 2011 at 05:19:46AM -0400, Christoph Hellwig wrote:
> > On Thu, Oct 20, 2011 at 05:14:34AM -0400, J. Bruce Fields wrote:
> > > > > Does it really make sense to use a string here just to pick between the
> > > > > three choices OWNER@, GROUP@, and EVERYONE@?  Why not just another small
> > > > > integer?  Is the goal to expand this somehow eventually?
> > > > 
> > 
> > > > I guess Andreas wanted the disk layout to be able to store user@...ain
> > > > format if needed.

Yep.  On the other hand, none of the code won't actually allow to use
user@...ain identifiers, it won't help with other identifier types like
Windows SIDs, and it doesn't make the code any prettier, so this should
probably go away.

> > > Is that likely?  For that to be useful, tasks would need to be able to
> > > run as user@...ain strings.  And we'd probably want owners and groups to
> > > also be user@...ain strings.

I really don't see this happen anytime soon, and likely not at all.

> > > The container people seem to eventually want to add some kind of
> > > namespace identifier everywhere:
> > > 
> > > 	http://marc.info/?l=linux-kernel&m=131836778427871&w=2
> > > 
> > > in which case I guess we'd likely end up with (uid, user namespace id)
> > > instead of user@...ain?

The filesystem still wouldn't have namespace ids for the owner and
owning group, which is a much bigger issue.  I think we're safe not to
worry about namespace ids at this point; they also might never happen.

> > Storing strings is an extremly stupid idea.  The only thing that would
> > make sense would be storing a windows-style 128-bit GUID.
> > 
> 
> So if we want to do this without strings:
> 
> > > > +struct richace_xattr {
> > > > + __le16          e_type;
> > > > + __le16          e_flags;
> > > > + __le32          e_mask;
> > > > + __le32          e_id;
> > > > + char            e_who[0];
> 
> We could drop that last field and use some predefined values for e_id to
> represent owner/group/everyone in the e_type == ACE4_SPECIAL_WHO case.

That makes sense to me.

There seems to be a WELL_KNOWN_SID_TYPE enumeration which maps those
kinds of special identifiers to small integers in Windows; maybe it
makes sense to use the same numbers for OWNER@, GROUP@, and EVERYONE@.

> Then I'm not sure how you'd extend it if you later decided to add
> Windows GUID's or whatever.
> 
> But maybe it's not realistic to expect to be able to do that without a
> new interface and on-disk format: how could old software be expected to
> deal with acls that didn't use uid's?

The acl itself has a version field, so new formats could be introduced
in the future with a new version.

Thanks,
Andreas

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