linux-kernel - Re: [PATCH] nfsd: wrong index used in inner loop

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Message-ID: <20110316225524.GC17004@fieldses.org>
Date:	Wed, 16 Mar 2011 18:55:25 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	Mi Jinlong <mijinlong@...fujitsu.com>, roel <roel.kluin@...il.com>,
	Neil Brown <neilb@...e.de>, linux-nfs@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nfsd: wrong index used in inner loop

On Mon, Mar 14, 2011 at 07:52:11PM -0400, Trond Myklebust wrote:
> On Mon, 2011-03-14 at 18:22 -0400, J. Bruce Fields wrote:
> > On Fri, Mar 11, 2011 at 12:13:55PM +0800, Mi Jinlong wrote:
> > > 
> > > 
> > > J. Bruce Fields:
> > > > On Tue, Mar 08, 2011 at 10:32:26PM +0100, roel wrote:
> > > >> @@ -1215,7 +1215,7 @@ nfsd4_decode_create_session(struct nfsd4_compoundargs *argp,
> > > >>  			READ_BUF(4);
> > > >>  			READ32(dummy);
> > > >>  			READ_BUF(dummy * 4);
> > > >> -			for (i = 0; i < dummy; ++i)
> > > >> +			for (j = 0; j < dummy; ++j)
> > > >>  				READ32(dummy);
> > > 
> > >   We must not use dummy for index here.
> > >   After the first index, READ32(dummy) will change dummy!!!!
> > 
> > Actually, wait, this is kind of silly.  I don't see why we couldn't just
> > skip the loop and do
> > 
> > 	p += dummy;
> 
> This is exactly why I _hate_ the READ*() macros and their ilk, and am
> really happy we got rid of them in the client.

Agreed, I'm all for getting rid of them completely.

> READ_BUF() _sets_ p to whatever the value of argp->p is, and then
> updates argp->p. It is just very very very hard to see that due to the
> lack of transparency.
>
> IOW: You don't need the "p += dummy" either. That happens automatically
> when you next invoke READ_BUF().

Yes, you're right, we could remove that silly loop entirely.

--b.
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