linux-ext4 - Re: [Bug 11175] New: ext3 BUG in add_dirent_to

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Date:	Wed, 30 Jul 2008 08:01:11 -0700 (PDT)
From:	Asheesh Laroia <kernel.bugzilla@...eesh.org>
To:	Theodore Tso <tytso@....edu>
cc:	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org, bugme-daemon@...zilla.kernel.org
Subject: Re: [Bug 11175] New: ext3 BUG in add_dirent_to_buf+0x6c/0x269

On Wed, 30 Jul 2008, Theodore Tso wrote:

> On Tue, Jul 29, 2008 at 10:03:38PM -0500, Eric Sandeen wrote:
>> Theodore Tso wrote:
>>> Hmm... disassembling the code, it's pretty clear the problem is here
>>> in do_split(), around line 1208:
>>>
>>> 	map = (struct dx_map_entry *) (data2 + blocksize);
>>> 	count = dx_make_map ((struct ext3_dir_entry_2 *) data1,
>>> 			     blocksize, hinfo, map);
>>> 	map -= count;
>>> 	dx_sort_map (map, count);
>>> 	/* Split the existing block in the middle, size-wise */
>>> 	size = 0;
>>> 	move = 0;
>>> 	for (i = count-1; i >= 0; i--) {
>>> 		/* is more than half of this entry in 2nd half of the block? */
>>> 		if (size + map[i].size/2 > blocksize/2)  <====
>>
>> You sure this isn't our old friend
>> https://bugzilla.redhat.com/show_bug.cgi?id=451068 ?
>>
>> which version of gcc compiled this?
>
> As we discussed on IRC, I think you're theory is dead on.  %ecx is at
> the very end of the page-2, which would correspond to
> map[count-1].size.  And size (%esi) is zero, which rules out my scenario.
>
> This very much looks like a GCC bug.  Asheesh, can you confirm which
> version of GCC you used to build your kernel?

gcc --version indicates:

gcc (Debian 4.3.1-2) 4.3.1

dpkg -l gcc reports:

ii  gcc            4:4.3.1-1      The GNU C compiler

> Longer term, do_split() was coded in a very non-robust fashion.
> Looking at do_split(), it was pretty easy to imagine corrupted
> directory blocks that might force count to be 0 (causing the for loop
> to do something insane, since i is unsigned), and adding some checks
> to make sure that the split variable is neither 0 nor equal to count
> might also be a really good idea.

Thanks for the speedy replies, all.  I guess then you're not interested in 
those e2image dumps I took, then.

I'm recompiling with GCC 4.2 now; is there a straightforward(ish) test 
you've seen that can indicate if the GCC 4.3 in Debian unstable or Debian 
testing still has this bug?  FWIW their changelogs are at 
http://packages.debian.org/changelogs/pool/main/g/gcc-4.3/gcc-4.3_4.3.1-8/changelog 
.

-- Asheesh.

-- 
He is a man capable of turning any colour into grey.
 		-- John LeCarre
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