desktop Digest, Vol 76, Issue 13

[Hristo Petkov]

Hi,
A Question:
The present linux desktop (at least mine on fc11) has NFS support in the Services menu. I doubt anyone uses this (I am not using it for sure). I don't know even what it is doing (is it supposed to compete Facebook and Google Me, or something).

Regards

--- On Tue, 6/29/10, desktop-request at lists.fedoraproject.org <desktop-request at lists.fedoraproject.org> wrote:

From: desktop-request at lists.fedoraproject.org <desktop-request at lists.fedoraproject.org>
Subject: desktop Digest, Vol 76, Issue 13
To: desktop at lists.fedoraproject.org
Date: Tuesday, June 29, 2010, 12:00 PM

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Today's Topics:

   1. Re: Usefulness of extended attributes over NFS
      (Lennart Poettering)


----------------------------------------------------------------------

Message: 1
Date: Mon, 28 Jun 2010 16:07:05 +0200
From: Lennart Poettering <mzerqung at 0pointer.de>
Subject: Re: Usefulness of extended attributes over NFS
To: desktop at lists.fedoraproject.org
Message-ID: <20100628140702.GA6527 at tango.0pointer.de>
Content-Type: text/plain; charset=us-ascii

On Mon, 28.06.10 09:18, James Morris (jmorris at namei.org) wrote:

> I've been working on an implementation of extended attributes for NFSv3, 
> which would enable getxattr(2) and friends over the network.
> 
> Here's a link to the latest patchset:
> http://lwn.net/Articles/392944/
> 
> and my LinuxCon slides on the topic from last year:
> http://namei.org/presentations/linuxcon09_nfsv3xattrs.pdf
> 
> I've been asked by the upstream NFS maintainers about general use-cases 
> for this feature.  We have a concrete requirement to convey security
> labels over NFS, although it's not clear how much need there is for 
> user-managed extended attributes (e.g. user.foo)
> 
> I was wondering if anyone could offer examples or insights into whether 
> Linux desktop apps are making much use of extended attributes, or whether 
> they might if NFS support were available.  (I'm aware of Beagle, which is 
> probably not great over NFS in any case).

Well, I think there's a bit of a chicken and egg problem. Since XATTRs
are not available on so many file systems (not even tmpfs has them) and
there is no nice way to test whether they work (except by creating an
attribute), nobody uses them. That said GLib's GIO framework (i..e the
API modern gtk programs use to access files) has complete coverage for
extended attributes, but probably not many folks actually use those
APIs. Also, the fact that they need to be enabled manually (via the
user_xattr mount option) is problematic.

Samba uses user xattrs. Then, there's a somewhat standardized scheme for
storing the MIME type of a file in an extended attribute, as an optional
implementation feature fo rthe XDG MIME info spec:

http://standards.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-0.11.html#id2506733

However, at least Gtk/GLib/GIO do not implement that optional part. I
once wrote an Apache module that added support for that user.mime_type
xattr to Apache: http://0pointer.de/lennart/projects/mod_mime_xattr/

However, that's mostly where he story ends I think, it is indeed not
widely used. Generally I believe they are useful however, and if they
would be ubiquitiously available they'd probably be used more
often. However, for that to happen we'd also need something like a
fpathconf() check or so to figure out whether user xattrs are allowed or
not.

I'd be particularly interested in user xattrs support on the virtual
file systems such as /proc, /sys and tmpfs. Neither of those file systems
have support for this right now, but especially for /proc it could become
very handy to export additional meta information about processes on the
/proc/$PID directory.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.


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