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Date: Tue, 20 Aug 2019 11:46:01 +0800
From: Gao Xiang <gaoxiang25@huawei.com>
To: Miao Xie <miaoxie@huawei.com>
Subject: Re: [PATCH] erofs: move erofs out of staging
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Cc: Jan Kara <jack@suse.cz>, Dave Chinner <david@fromorbit.com>,
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On Tue, Aug 20, 2019 at 11:33:51AM +0800, Miao Xie wrote:
> 
> 
> 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,
> >>>> 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.
> 
> 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 ;)

I can speed all my spare time for this...

As I said before, All HUAWEI smartphone products will continue using
this filesystem, and maintaining this filesystem is one of our paid
jobs, but since our Android products is based on dm-verity + EROFS,
it's only on my personal time schedule (bosses care more about Android
and money) and I will do that in my spare time of course.

Thanks,
Gao Xiang

> 
> Thanks
> Miao
> 
> > 
> > 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
> >>>>>
> >