linux-ext4 - Re: [PATCH] ext4: don't cache out of order extents

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Message-ID: <20131017150630.GC11404@dhcp-13-216.nay.redhat.com>
Date:	Thu, 17 Oct 2013 23:06:30 +0800
From:	Eryu Guan <guaneryu@...il.com>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] ext4: don't cache out of order extents

On Thu, Oct 17, 2013 at 03:44:35PM +0200, Lukáš Czerner wrote:
> On Thu, 17 Oct 2013, Eryu Guan wrote:
> 
> > Date: Thu, 17 Oct 2013 17:27:53 +0800
> > From: Eryu Guan <guaneryu@...il.com>
> > To: linux-ext4@...r.kernel.org
> > Cc: Eryu Guan <guaneryu@...il.com>, Theodore Ts'o <tytso@....edu>
> > Subject: [PATCH] ext4: don't cache out of order extents
> > 
> > A corrupted ext4 may have out of order leaf extents, i.e.
> > 
> > extent: lblk 0--1023, len 1024, pblk 9217, flags: LEAF UNINIT
> > extent: lblk 1000--2047, len 1024, pblk 10241, flags: LEAF UNINIT
> >              ^^^^ overlap with previous extent
> > 
> > Reading such extent could hit BUG_ON() in ext4_es_cache_extent().
> > 
> > 	BUG_ON(end < lblk);
> > 
> > The problem is that __read_extent_tree_block() tries to cache holes as
> > well but assumes 'lblk' is greater than 'prev' and passes underflowed
> > length to ext4_es_cache_extent().
> > 
> > I hit this when fuzz testing ext4, and am able to reproduce it by
> > modifying the on-disk extent by hand.
> > 
> > Ran xfstests on patched ext4 and no regression.
> 
> So what will happen with the file system with this patch when
> presented with such corruption ?
> 
> It seems to me that ext4_es_cache_extent() will happily skip this
> extent because it will find that this particular offset is already
> in the tree. Hence we'll have a gap in the status tree which really
> should not be there and I suspect that something bad will happen.
> 
> I think that we should deal with this corruption immediately when we
> spot it there, not just hide it.

Yes, agreed, we should validate the extent not cover the corruption as
Ted pointed out. Don't know why I didn't think about it more in the
first place..

Thanks!

Eryu Guan
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