From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 931B1C433DF for ; Wed, 8 Jul 2020 16:09:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 77F5E20672 for ; Wed, 8 Jul 2020 16:09:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730141AbgGHQJU (ORCPT ); Wed, 8 Jul 2020 12:09:20 -0400 Received: from mx2.suse.de ([195.135.220.15]:40340 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730075AbgGHQJU (ORCPT ); Wed, 8 Jul 2020 12:09:20 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 9BAA9ACC6; Wed, 8 Jul 2020 16:09:18 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id ED86D1E12BF; Wed, 8 Jul 2020 18:09:17 +0200 (CEST) Date: Wed, 8 Jul 2020 18:09:17 +0200 From: Jan Kara To: Eric Sandeen Cc: Lukas Czerner , Eric Sandeen , "linux-ext4@vger.kernel.org" Subject: Re: [PATCH 1/1] ext4: fix potential negative array index in do_split() Message-ID: <20200708160917.GC5288@quack2.suse.cz> References: <20200619064122.vj346xptid5viogv@work> <04a5b98c-4bb7-4861-76c3-dd0b0c6a6610@sandeen.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <04a5b98c-4bb7-4861-76c3-dd0b0c6a6610@sandeen.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Fri 19-06-20 08:39:53, Eric Sandeen wrote: > On 6/19/20 1:41 AM, Lukas Czerner wrote: > > On Wed, Jun 17, 2020 at 02:19:04PM -0500, Eric Sandeen wrote: > >> If for any reason a directory passed to do_split() does not have enough > >> active entries to exceed half the size of the block, we can end up > >> iterating over all "count" entries without finding a split point. > >> > >> In this case, count == move, and split will be zero, and we will > >> attempt a negative index into map[]. > >> > >> Guard against this by detecting this case, and falling back to > >> split-to-half-of-count instead; in this case we will still have > >> plenty of space (> half blocksize) in each split block. > > ... > > >> + /* > >> + * map index at which we will split > >> + * > >> + * If the sum of active entries didn't exceed half the block size, just > >> + * split it in half by count; each resulting block will have at least > >> + * half the space free. > >> + */ > >> + if (i > 0) > >> + split = count - move; > >> + else > >> + split = count/2; > > > > Won't we have exactly the same problem as we did before your commit > > ef2b02d3e617cb0400eedf2668f86215e1b0e6af ? Since we do not know how much > > space we actually moved we might have not made enough space for the new > > entry ? > > I don't think so - while we don't have the original reproducer, I assume that > it was the case where the block was very full, and splitting by count left us > with one of the split blocks still over half full (because ensuring that we > split in half by size seemed to fix it) > > In this case, the sum of the active entries was <= half the block size. > So if we split by count, we're guaranteed to have >= half the block size free > in each side of the split. > > > Also since we have the move == count when the problem appears then it's > > clear that we never hit the condition > > > > 1865 → → /* is more than half of this entry in 2nd half of the block? */ > > 1866 → → if (size + map[i].size/2 > blocksize/2) > > 1867 → → → break; > > > > in the loop. This is surprising but it means the the entries must have > > gaps between them that are small enough that we can't fit the entry > > right in ? Should not we try to compact it before splitting, or is it > > the case that this should have been done somewhere else ? > > Yes, that's exactly what happened - see my 0/1 cover letter. Maybe that should > be in the patch description itself. ALso, yes compaction would help but I was > unclear as to whether that should be done here, is the side effect of some other > bug, etc. In general, we do seem to do compaction elsewhere and I don't know > how we got to this point. > > > If we really want ot be fair and we want to split it right in the middle > > of the entries size-wise then we need to keep track of of sum of the > > entries and decide based on that, not blocksize/2. But maybe the problem > > could be solved by compacting the entries together because the condition > > seems to rely on that. > > I thought about that as well, but it took a bit more code to do; we could make > make_map() return both count and total size, for example. But based on my > theory above that both sides of the split will have >= half block free, it > didn't seem necessary, particularly since this seems like an edge case? This didn't seem to conclude in any way? The patch looks good to me FWIW so feel free to add: Reviewed-by: Jan Kara Ted, can you please pick this patch up? Thanks! Honza -- Jan Kara SUSE Labs, CR