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

[yebin <yebin@huaweicloud.com>]

On 2025/10/29 16:45, Zhang Yi wrote:
> Hi Bin!
>
> On 10/28/2025 2:47 PM, 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
>>
>
> I suppose we need to explain why we have this particular use case.
>
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.
>
> What about checking sb_rdonly(sb) before setting the journal feature, and
> skipping the setting if the filesystem is mounted as read-only?

Yes, I've thought about doing that too, but that would require us to set 
the journal's features again when remounting as rw.

>
> Regards,
> Yi.
>
>>
>> Fixes: 4fd5ea43bc11 ("jbd2: checksum journal superblock")
>> Signed-off-by: Ye Bin <yebin10@huawei.com>
>> ---
>>   fs/jbd2/journal.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
>> index d480b94117cd..5b6e8c1a5e6a 100644
>> --- a/fs/jbd2/journal.c
>> +++ b/fs/jbd2/journal.c
>> @@ -2349,6 +2349,8 @@ 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);
>> +	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 +2380,13 @@ 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);
>> +	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);
>