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

From: Miao Xie
Date: Mon Aug 19 2019 - 23:34:16 EST

on 2019/8/20 at 8:55, Qu Wenruo wrote:
> [...]
>>>> I have made a simple fuzzer to inject messy in inode metadata,
>>>> dir data, compressed indexes and super block,
>>>> 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.

It is a good idea. In fact, we already have a verification layer which was implemented
as a device mapper sub-module. I think it is enough for a read-only filesystem because
it is simple, flexible and independent(we can modify the filesystem layout without
verification module modification).

>> 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.

Agree, Xiang and I have no time to developing this feature now, we are glad very much if you could help
us to do it ;)


> 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