From: Theodore Tso Subject: Re: [PATCH v2] ext3, ext4: do_split() fix loop, with obvious unsigned wrap Date: Tue, 2 Dec 2008 08:24:41 -0500 Message-ID: <20081202132441.GC16172@mit.edu> References: <49343AD9.4020606@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: davidsen@tmr.com, adilger@sun.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org To: roel kluin Return-path: Received: from www.church-of-our-saviour.org ([69.25.196.31]:55991 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753320AbYLBNYo (ORCPT ); Tue, 2 Dec 2008 08:24:44 -0500 Content-Disposition: inline In-Reply-To: <49343AD9.4020606@gmail.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Dec 01, 2008 at 02:28:25PM -0500, roel kluin wrote: > Fix loop, with obvious unsigned wrap > > Signed-off-by: Roel Kluin Um, no. Sorry, I didn't have a chance to reply earlier but this is obviously wrong. > --- > diff --git a/fs/ext3/namei.c b/fs/ext3/namei.c > index 3e5edc9..b0dcfb3 100644 > --- a/fs/ext3/namei.c > +++ b/fs/ext3/namei.c > @@ -1188,7 +1188,7 @@ static struct ext3_dir_entry_2 *do_split(handle_t *handle, struct inode *dir, > /* Split the existing block in the middle, size-wise */ > size = 0; > move = 0; > - for (i = count-1; i >= 0; i--) { > + for (i = count; i--; ) { > /* is more than half of this entry in 2nd half of the block? */ > if (size + map[i].size/2 > blocksize/2) > break; Note that i is actually **used** in the loop? So changing the starting value of the counter without also adjusting all of the places where i is used will cause the code to break, and in hard to find ways... Given that there are two loop termination conditions, and in fact the one in the loop is the one that actually gets used 99% of the time (which is why we've never noticed the problem in real life), probably the best way of handling this is to recast it not as a for loop, but as a while loop. - Ted