linux-kernel - Re: [PATCH] NFS: nfsaclsvc_encode_getaclres() - Fix potential NULL deref and tiny optimization.

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Date:	Thu, 2 Nov 2006 16:07:46 +0100
From:	Andreas Gruenbacher <agruen@...e.de>
To:	David Rientjes <rientjes@...washington.edu>
Cc:	Jesper Juhl <jesper.juhl@...il.com>, linux-kernel@...r.kernel.org,
	Neil Brown <neilb@....unsw.edu.au>, nfs@...ts.sourceforge.net,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH] NFS: nfsaclsvc_encode_getaclres() - Fix potential NULL deref and tiny optimization.

On Tuesday 31 October 2006 21:39, David Rientjes wrote:
> On Tue, 31 Oct 2006, Andreas Gruenbacher wrote:
> > > > w should be an unsigned int.
> > >
> > > Makes sense.
> >
> > No, this breaks the while loop further below: with an unsigned int, the
> > loop counter underflows and wraps.
>
> This is not a problem with w being an unsigned int, it's a problem with
> the while loop.  nfsacl_size() returns unsigned int as it should and the
> while loop can be written to respect that since integer division in C
> truncates:
>
> 	for (n = w / PAGE_SIZE; n > 0; n--)
> 		if (!rqstp->rq_respages[rqstp->rq_resused++];

Assuming that PAGE_SIZE = 4096 and w = 100, the original loop iterates once, 
while your proposed version iterates zero times -- the current code does the 
right thing. So the proposed change is still bad, sorry.

Andreas
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