Re: Implementing POSIX ACLs - was: Re: [PATCH] Revised extended attributes interface

From: Anton Altaparmakov (
Date: Tue Dec 11 2001 - 10:15:50 EST

Hi Stephen,

At 14:34 11/12/01, Stephen C. Tweedie wrote:
>On Tue, Dec 11, 2001 at 01:30:16PM +0000, Anton Altaparmakov wrote:
> > At 11:33 11/12/01, Stephen C. Tweedie wrote:
> > >On Tue, Dec 11, 2001 at 12:22:58PM +1100, Timothy Shimmin wrote:
> > > > Could this not be catered for independent of the proposed EA interface
> > > > for getting/setting/removing EAs ?
> > >
> > >Definitely. The whole problem I pointed out with the EA interface was
> > >that it didn't talk about ACLs at all. So, sure, it gives us an API
> > >for arbitrary EAs, but it does absolutely nothing to help us unify ACL
> > >APIs.
> >
> > And this is a Good Thing(TM). EAs are completely orthogonal to ACLs and
> the
> > API for EAs should not in any way have anything to do with ACLs.
>At the moment, however, they do.
>The user-visible APIs for ACLs and EAs are quite separate. However,
>the way that the user ACL libraries talk to the filesystem is through
>special reserved EAs, to which the kernel magically imparts ACL
>semantics. The format of that EA, its name, its semantics and so on,
>are all completely glossed over by the existing EA spec, despite the
>fact that this mechanism is right at the very core of the ACL

You are certainly right here. However, I thought the existing ACL
implementation was going to provide the detailed EA specs on top of the
generic EA spec. I can see why you would want to reserve certain EAs for
ACL use in the EA spec though. It would be just like reserving syscall
numbers and that makes a lot of sense...

> > It is up to a different API to implement ACLs. In the case of xfs, ext3,
> > etc, it can have EAs as a backend to the API but in the case of NTFS ACLs
> > would not be anything to do with EAs.
>I know, and that's part of the problem we identified over a year ago.
>My old EA proposal had the concept of "attribute families", and
>allowed us to define ACL attributes in a completely different
>namespace from EAs, so that NTFS, AFS or NFSv4 ACLs could be passed
>cleanly through the same API.
> > _Please_ do not mix the two issues. We have here a IMO nice API for EAs.
> > Lets get it into the kernel. Once that is done, one can start talking
> about
> > an API for ACLs.
>I'm not mixing them: I'm *trying* to unmix them.

Ok. (-:

>The existing EA code implements magic "handlers" for system EAs.
>Undefined magic gets called when you set such an EA. There's no
>mention about this in the spec.

That is a short coming of the EA spec. Agreed. Or rather it is a flaw in
the EA design as there shouldn't be any magic handlers. I have always seen
EAs as a means to store things in files that isn't the file data but other
random pieces of information. The fs shouldn't care what those are.

>So right now the EA code opens up what is basically a named-ioctl back
>door into the filesystems, and ACLs work on top of that. The existing
>ACL and EA code is already enormously intertwined.

Viewing it from this point of view you are of course right.

The ACL API should only use EAs as backing store, no magic handlers
attached to the EAs. Then the system would not be open to abuse. The ACL
part of the fs can of course make use of the EA backing store if it wants to.

> > IMHO a generic POSIX ACL API would never even know that ACLs are stored as
> > EAs, this should be up to the individual fs and if several fs use EAs it
> > would make sense to have vfs helpers which all fs can use (a-la generic_*
> > helpers).
>Yes, and the existing bestbits ACL API does not have anything like
>that abstraction: rather, it relies on doing an assignment to a "$acl"

So the ACL API is flawed.

>So how are we going to do ACLs on top of EAs? Even if we forget about
>the ACL API for now, the API between the ACL layer and the EA layer
>*does* matter.

Indeed. I would favour an abstraction at vfs level and fs specific methods
as I described in previous post because this really divorses the two layers.

I know code speaks but I am more interested in making NTFS work properly
read-write and only then implement ACLs in it at the moment...

Best regards,


   "I've not lost my mind. It's backed up on tape somewhere." - Unknown
Anton Altaparmakov <aia21 at> (replace at with @)
Linux NTFS Maintainer / WWW:
ICQ: 8561279 / WWW:

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