Re: [PATCH] erofs: move erofs out of staging

[Chao Yu <yuchao0@huawei.com>]

On 2019/8/20 10:38, Qu Wenruo wrote:
> 
> 
> On 2019/8/20 上午10:24, Chao Yu wrote:
>> 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 could be possible for XFS/EXT* to share the same infrastructure
> without much hassle.
> (If not considering external journal)
> 
> But for btrfs, it's like a regular fs on a super large dm-linear, which
> further builds its chunks on different dm-raid1/dm-linear/dm-raid56.
> 
> So not sure if it's possible for btrfs, as it contains its logical
> address layer bytenr (the most common one) along with per-chunk physical
> mapping bytenr (in another tree).

Yeah, it looks like we need searching more levels mapping to find the final
physical block address of inode/node/data in btrfs.

IMO, in a little lazy way, we can reform and reuse existed function in
btrfs-progs which can find the mapping info of inode/node/data according to
specified ino or ino+pg_no.

> 
> It may depends on the granularity. But definitely a good idea to do so
> in a generic way.
> Currently we depend on super kind student developers/reporters on such

Yup, I just guess Wen Xu may be interested in working on a generic way to fuzz
filesystem, as I know they dig deep in filesystem code when doing fuzz. BTW,
which impresses me is, constructing checkpoint by injecting one byte, and then
write a correct recalculated checksum value on that checkpoint, making that
checkpoint looks valid...

Thanks,

> fuzzed images, and developers sometimes get inspired by real world
> corruption (or his/her mood) to add some valid but hard-to-hit corner
> case check.
> 
> Thanks,
> Qu
> 
>> 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
>>>>>>>
>>>
>