linux-ext4 - Re: [PATCH] jbd: correctly unescape journal data blocks

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Date:	Sun, 16 Mar 2008 08:29:20 +0800
From:	Andreas Dilger <adilger@....com>
To:	Duane Griffin <duaneg@...da.com>
Cc:	sct@...hat.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] jbd: correctly unescape journal data blocks

On Mar 15, 2008  18:49 +0000, Duane Griffin wrote:
> Fix a long-standing typo (predating git) that will cause data corruption
> if a journal data block needs unescaping. At the moment the wrong buffer
> head's data is being unescaped.
> 
> To test this case mount a filesystem with data=journal, start creating
> and deleting a bunch of files containing only JFS_MAGIC_NUMBER (0xc03b3998),
> then pull the plug on the device. Without this patch the files will contain
> zeros instead of the correct data after recovery.
> 
> Signed-off-by: Duane Griffin <duaneg@...da.com>
> ---
>  fs/jbd/recovery.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/jbd/recovery.c b/fs/jbd/recovery.c
> index 2b8edf4..43bc5e5 100644
> --- a/fs/jbd/recovery.c
> +++ b/fs/jbd/recovery.c
> @@ -478,7 +478,7 @@ static int do_one_pass(journal_t *journal,
>  					memcpy(nbh->b_data, obh->b_data,
>  							journal->j_blocksize);
>  					if (flags & JFS_FLAG_ESCAPE) {
> -						*((__be32 *)bh->b_data) =
> +						*((__be32 *)nbh->b_data) =
>  						cpu_to_be32(JFS_MAGIC_NUMBER);
>  					}

Note that this would also affect filesystems larger than ~12TB, where
JFS_MAGIC_NUMBER might be the first block number in an ext2/3 indirect
block.  That would cause the indirect block to be zapped and file data
in the whole block would suddenly become zero.  Even worse if this is
the first block in a double-indirect or triple-indirect block, where
4MB or 16GB of the file data would suddenly become a hole.  Unlikely,
but with enough monkeys it would be hit.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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