Re: [PATCH v2] jbd2: fix the inconsistency between checksum and data in memory for journal sb

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Nov 03, 2025 at 09:01:23AM +0800, Ye Bin wrote:
> From: Ye Bin <yebin10@huawei.com>
> 
> Copying the file system while it is mounted as read-only results in
> a mount failure:
> [~]# mkfs.ext4 -F /dev/sdc
> [~]# mount /dev/sdc -o ro /mnt/test
> [~]# dd if=/dev/sdc of=/dev/sda bs=1M
> [~]# mount /dev/sda /mnt/test1
> [ 1094.849826] JBD2: journal checksum error
> [ 1094.850927] EXT4-fs (sda): Could not load journal inode
> mount: mount /dev/sda on /mnt/test1 failed: Bad message
> 
> The process described above is just an abstracted way I came up with to
> reproduce the issue. In the actual scenario, the file system was mounted
> read-only and then copied while it was still mounted. It was found that
> the mount operation failed. The user intended to verify the data or use
> it as a backup, and this action was performed during a version upgrade.
> Above issue may happen as follows:
> ext4_fill_super
>  set_journal_csum_feature_set(sb)
>   if (ext4_has_metadata_csum(sb))
>    incompat = JBD2_FEATURE_INCOMPAT_CSUM_V3;
>   if (test_opt(sb, JOURNAL_CHECKSUM)
>    jbd2_journal_set_features(sbi->s_journal, compat, 0, incompat);
>     lock_buffer(journal->j_sb_buffer);
>     sb->s_feature_incompat  |= cpu_to_be32(incompat);
>     //The data in the journal sb was modified, but the checksum was not
>       updated, so the data remaining in memory has a mismatch between the
>       data and the checksum.
>     unlock_buffer(journal->j_sb_buffer);
> 
> In this case, the journal sb copied over is in a state where the checksum
> and data are inconsistent, so mounting fails.
> To solve the above issue, update the checksum in memory after modifying
> the journal sb.
> 
> Fixes: 4fd5ea43bc11 ("jbd2: checksum journal superblock")
> Signed-off-by: Ye Bin <yebin10@huawei.com>

Looks fine to me,
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  fs/jbd2/journal.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index d480b94117cd..bf255f8b5eeb 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -2349,6 +2349,12 @@ int jbd2_journal_set_features(journal_t *journal, unsigned long compat,
>  	sb->s_feature_compat    |= cpu_to_be32(compat);
>  	sb->s_feature_ro_compat |= cpu_to_be32(ro);
>  	sb->s_feature_incompat  |= cpu_to_be32(incompat);
> +	/*
> +	 * Update the checksum now so that it is valid even for read-only
> +	 * filesystems where jbd2_write_superblock() doesn't get called.
> +	 */
> +	if (jbd2_journal_has_csum_v2or3(journal))
> +		sb->s_checksum = jbd2_superblock_csum(sb);
>  	unlock_buffer(journal->j_sb_buffer);
>  	jbd2_journal_init_transaction_limits(journal);
>  
> @@ -2378,9 +2384,17 @@ void jbd2_journal_clear_features(journal_t *journal, unsigned long compat,
>  
>  	sb = journal->j_superblock;
>  
> +	lock_buffer(journal->j_sb_buffer);
>  	sb->s_feature_compat    &= ~cpu_to_be32(compat);
>  	sb->s_feature_ro_compat &= ~cpu_to_be32(ro);
>  	sb->s_feature_incompat  &= ~cpu_to_be32(incompat);
> +	/*
> +	 * Update the checksum now so that it is valid even for read-only
> +	 * filesystems where jbd2_write_superblock() doesn't get called.
> +	 */
> +	if (jbd2_journal_has_csum_v2or3(journal))
> +		sb->s_checksum = jbd2_superblock_csum(sb);
> +	unlock_buffer(journal->j_sb_buffer);
>  	jbd2_journal_init_transaction_limits(journal);
>  }
>  EXPORT_SYMBOL(jbd2_journal_clear_features);
> -- 
> 2.34.1
> 
>