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Subject: Re: [PATCH] erofs: move erofs out of staging
To: Qu Wenruo <quwenruo.btrfs@gmx.com>, Gao Xiang <hsiangkao@aol.com>,
 "Darrick J. Wong" <darrick.wong@oracle.com>
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On 2019/8/20 8:55, Qu Wenruo wrote:
> [...]
>>>> I have made a simple fuzzer to inject messy in inode metadata,
>>>> dir data, compressed indexes and super block,
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/commit/?h=experimental-fuzzer
>>>>
>>>> I am testing with some given dirs and the following script.
>>>> Does it look reasonable?
>>>>
>>>> # !/bin/bash
>>>>
>>>> mkdir -p mntdir
>>>>
>>>> for ((i=0; i<1000; ++i)); do
>>>> 	mkfs/mkfs.erofs -F$i testdir_fsl.fuzz.img testdir_fsl > /dev/null 2>&1
>>>
>>> mkfs fuzzes the image? Er....
>>
>> Thanks for your reply.
>>
>> First, This is just the first step of erofs fuzzer I wrote yesterday night...
>>
>>>
>>> Over in XFS land we have an xfs debugging tool (xfs_db) that knows how
>>> to dump (and write!) most every field of every metadata type.  This
>>> makes it fairly easy to write systematic level 0 fuzzing tests that
>>> check how well the filesystem reacts to garbage data (zeroing,
>>> randomizing, oneing, adding and subtracting small integers) in a field.
>>> (It also knows how to trash entire blocks.)
> 
> The same tool exists for btrfs, although lacks the write ability, but
> that dump is more comprehensive and a great tool to learn the on-disk
> format.
> 
> 
> And for the fuzzing defending part, just a few kernel releases ago,
> there is none for btrfs, and now we have a full static verification
> layer to cover (almost) all on-disk data at read and write time.
> (Along with enhanced runtime check)
> 
> We have covered from vague values inside tree blocks and invalid/missing
> cross-ref find at runtime.
> 
> Currently the two layered check works pretty fine (well, sometimes too
> good to detect older, improper behaved kernel).
> - Tree blocks with vague data just get rejected by verification layer
>   So that all members should fit on-disk format, from alignment to
>   generation to inode mode.
> 
>   The error will trigger a good enough (TM) error message for developer
>   to read, and if we have other copies, we retry other copies just as
>   we hit a bad copy.
> 
> - At runtime, we have much less to check
>   Only cross-ref related things can be wrong now. since everything
>   inside a single tree block has already be checked.
> 
> In fact, from my respect of view, such read time check should be there
> from the very beginning.
> It acts kinda of a on-disk format spec. (In fact, by implementing the
> verification layer itself, it already exposes a lot of btrfs design
> trade-offs)
> 
> Even for a fs as complex (buggy) as btrfs, we only take 1K lines to
> implement the verification layer.
> So I'd like to see every new mainlined fs to have such ability.

Out of curiosity, it looks like every mainstream filesystem has its own
fuzz/injection tool in their tool-set, if it's really such a generic
requirement, why shouldn't there be a common tool to handle that, let specified
filesystem fill the tool's callback to seek a node/block and supported fields
can be fuzzed in inode. It can help to avoid redundant work whenever Linux
welcomes a new filesystem....

Thanks,

> 
>>
>> Actually, compared with XFS, EROFS has rather simple on-disk format.
>> What we inject one time is quite deterministic.
>>
>> The first step just purposely writes some random fuzzed data to
>> the base inode metadata, compressed indexes, or dir data field
>> (one round one field) to make it validity and coverability.
>>
>>>
>>> You might want to write such a debugging tool for erofs so that you can
>>> take apart crashed images to get a better idea of what went wrong, and
>>> to write easy fuzzing tests.
>>
>> Yes, we will do such a debugging tool of course. Actually Li Guifu is now
>> developping a erofs-fuse to support old linux versions or other OSes for
>> archiveing only use, we will base on that code to develop a better fuzzer
>> tool as well.
> 
> Personally speaking, debugging tool is way more important than a running
> kernel module/fuse.
> It's human trying to write the code, most of time is spent educating
> code readers, thus debugging tool is way more important than dead cold code.
> 
> Thanks,
> Qu
>>
>> Thanks,
>> Gao Xiang
>>
>>>
>>> --D
>>>
>>>> 	umount mntdir
>>>> 	mount -t erofs -o loop testdir_fsl.fuzz.img mntdir
>>>> 	for j in `find mntdir -type f`; do
>>>> 		md5sum $j > /dev/null
>>>> 	done
>>>> done
>>>>
>>>> Thanks,
>>>> Gao Xiang
>>>>
>>>>>
>>>>> Thanks,
>>>>> Gao Xiang
>>>>>
>