linux-kernel - Re: [RFC] Union mount readdir support in glibc

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Date:	Fri, 14 Mar 2008 16:58:22 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Peter Staubach <staubach@...hat.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, drepper@...hat.com,
	viro@...IV.linux.org.uk, bharata@...ux.vnet.ibm.com,
	libc-alpha@...rceware.org, jblunck@...e.de, ezk@...sunysb.edu,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	hch@....de, cmm@...ibm.com, haveblue@...ibm.com
Subject: Re: [RFC] Union mount readdir support in glibc


On Fri, 2008-03-14 at 13:53 -0400, Peter Staubach wrote:
> Miklos Szeredi wrote:
> >>> Actually, do we really need it other than to 0 and to current position
> >>> (i.e. full rewind and a no-op)?
> >>>       
> >> Ever heard of the little function "telldir"?
> >>     
> >
> > Actually, telldir/seekdir is already broken for some filesystems (NFS
> > comes to mind).  POSIX was really crazy to require a working seekdir
> > implementation, and userspace should quickly start _not_ using it.

POSIX never did require a working seekdir implementation. That
requirement came from our friends in the "Open Group":

   http://www.opengroup.org/onlinepubs/009695399/functions/seekdir.html

> What makes you think that telldir/seekdir don't work for NFS?  The over the
> wire protocols clearly take values which could be retrieved and stored via
> those interfaces.
> 
>        ps

Except for the fact that the NFS cookies are unsigned (and in the case
of NFSv3/v4 are 64-bit wide), whereas glibc gets confused when
confronted with 'negative' telldir values.

Hence the current Linux client's wrapping of the on-the-wire cookies. As
far as I can see, it is fully conformant with the spec, which has the
perfect "get out of jail free" card:

        "The definition of seekdir() and telldir() does not specify
        whether, when using these interfaces, a given directory entry
        will be seen at all, or more than once."

  Trond

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