linux-ext4 - Re: [PATCH] ext4: Fix comparison endianness problem in MMP initialization

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Date:	Fri, 30 Sep 2011 15:12:02 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	djwong@...ibm.com
Cc:	"Theodore Ts'o" <tytso@....edu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: Fix comparison endianness problem in MMP initialization

On 2011-09-30, at 1:49 PM, Darrick J. Wong wrote:
> As part of startup, the MMP initialization code does this:
> 
> mmp->mmp_seq = seq = cpu_to_le32(mmp_new_seq());
> 
> Next, mmp->mmp_seq is written out to disk, a delay happens, and then
> the MMP block is read back in and the sequence value is tested:
> 
> if (seq != le32_to_cpu(mmp->mmp_seq)) {
> 	/* fail the mount */
> 
> On a LE system such as x86, the *le32* functions do nothing and this
> works.
> Unfortunately, on a BE system such as ppc64, this comparison becomes:
> 
> if (cpu_to_le32(new_seq) != le32_to_cpu(cpu_to_le32(new_seq)) {
> 	/* fail the mount */
> 
> Except for a few palindromic sequence numbers, this test always causes
> the mount to fail, which makes MMP filesystems generally unmountable
> on ppc64.  The attached patch fixes this situation.
> 
> (This fix came up while testing the metadata checksumming patchset)

Thanks also for the catch, you are completely right.  We've never run
this code on a big-endian system in all the years we had it.

> Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> ---
> 
> fs/ext4/mmp.c |    3 ++-
> 1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index 9bdef3f..a7a4986 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -295,7 +295,8 @@ skip:
> 	/*
> 	 * write a new random sequence number.
> 	 */
> -	mmp->mmp_seq = seq = cpu_to_le32(mmp_new_seq());
> +	seq = mmp_new_seq();
> +	mmp->mmp_seq = cpu_to_le32(seq);
> 
> 	retval = write_mmp_block(bh);
> 	if (retval)
> --
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Cheers, Andreas





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