Re: [PATCH] ext4: add inode to ordered data list when extending file without block allocation

["zhangyi (F)" <yi.zhang@huawei.com>]

Thanks a lot for your indepth explanation, I get it now.

Thanks
Yi.

On 2019/4/5 17:12, Jan Kara Wrote:
> On Thu 04-04-19 20:46:47, zhangyi (F) wrote:
>> On 2019/4/4 18:18, Jan Kara Wrote:
>>> On Thu 04-04-19 17:29:52, zhangyi (F) wrote:
>>>> Currently we capture a NULL data exposure problem after a crash or
>>>> poweroff when append writing a file in the data=ordered mode. The
>>>> problem is that we were not add inode to the transaction's order data
>>>> list when updating i_disksize without new block allocation no matter
>>>> the delay allocated block feature is enabled or not.
>>>>
>>>> write                           jbd2                    writeback
>>>> append write in allocated block
>>>> mark buffer dirty
>>>> update i_disksize
>>>> mark inode dirty
>>>>                           commit transaction
>>>>                           write inode
>>>>                           (data exposure after a crash)
>>>>                                                     write dirty buffer
>>>>
>>>> It's fine in the case of new block allocation because we do this job in
>>>> ext4_map_blocks(). To fix this problem, this patch add inode to current
>>>> transaction's order data list after new data is copied and needing
>>>> update i_disksize in the case of no block allocation.
>>>>
>>>> Fixes: 06bd3c36a733ac ("ext4: fix data exposure after a crash")
>>>> Fixes: f3b59291a69d0b ("ext4: remove calls to ext4_jbd2_file_inode() from delalloc write path")
>>>> Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
>>>
>>> Thanks for the patch. The current behavior is a deliberate decision.
>>> data=ordered mode does guarantee there is no stale data visible in case of
>>> crash. However it does not guarantee you cannot see zeros where data was
>>> written. 
>>>
>>
>> Hi Jan,
>> Thanks a lot for your explanation. I read the
>> Documentation/admin-guide/ext4.rst, which said about the ordered mode:
>>
>>> ... When it's time to write the new metadata out to disk, the associated data
>>> blocks are written first...
>>
>> So I reckon that the dirty data block should be written to disk before
>> committing i_disksize and we cannot see the zero data. Now, I don't find
>> any offical docs to record the behavior you mentioned, do you have some
>> links talk about this behavior or am I miss something ?
> 
> Yeah, I agree that paragraph in Documentation/admin-guide/ext4.rst could be
> interpretted the way you understood it. Ext4 Wiki [1] has this comment:
> 
> (Note, however, that Ordered Mode does not guarantee that the file will be
> consistent at an application level; the application must use fsync() at
> appropriate commit points in order to guarantee application-level
> consistency.)
> 
> In addition, there are some applications which depend on data=ordered to
> automatically force data blocks to be written to disk soon after the file
> is written. Using Writeback Mode extends the time from when a file is
> written to when it is pushed out to disk to 30 seconds. This can be
> surprising for some users; however, it should be noted that such problems
> can still be an issue with Ordered Mode (although they are much rarer).
> Again, a careful application or library should always use fsync() at points
> where the application is at a stable commit point. 
> 
> ---
> 
> And we generally always made it clear that data=ordered mode is a "security
> feature" - don't expose potentialy sensitive data after crash. Not app data
> consistency feature (fsync(2) is for that). I don't have a good reference
> for that except perhaps bugzilla comment from Ted [2] where he explains
> some rationale behind data=ordered mode.
> 
> So overall no, data=ordered mode is not designed to provide the data
> integrity guarantees you'd like to see. And providing more guarantees has
> performance (and maintenance) costs so it should be better justified than
> just "it would be nice for applications".
> 
> Also when you'd like to provide some guarantees about data integrity, you
> need to specify your "promise" (currently our promise is: "no uninitialized
> data visible"). Because full "writes are atomic operation wrt crash" kind
> of guarantee is very hard (expensive) to achieve. I hope this clears things
> out a bit.
> 
> 								Honza
> 
> [1] https://ext4.wiki.kernel.org/index.php/Ext3_Data=Ordered_vs_Data=Writeback_mode
> [2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/317781/comments/45
>