linux-ext4 - Re: About read-only feature EXT4_FEATURE_RO_COMPAT_SHARED

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Date:   Sun, 2 Feb 2020 10:23:57 +0800
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     linux-ext4@...r.kernel.org,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: About read-only feature EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS



On 2020/2/2 上午10:16, Andreas Dilger wrote:
> On Feb 1, 2020, at 7:02 PM, Qu Wenruo <quwenruo.btrfs@....com> wrote:
>>
>> Hi ext4 guys,
>>
>> Recently I found an image from android (vendor.img) has this RO feature
>> set, but kernel doesn't support it, thus no easy way to modify it.
>> (Although I can just modify the underlying block for my purpose, it's
>> just one line change, I still want a more elegant way).
>>
>> Thus it can only be mounted RO. So far so good, as from its name, it's
>> kinda of deduped (BTW, both XFS and Btrfs supports RW mount for
>> reflinked/deduped fs).
>>
>> But the problem is, how to create such image?
>>
>> Man page of mke2fs has no mention of such thing at all, and obviously
>> for whoever comes up with such "brilliant" way to block users from
>> modifying things, the "-E unshare_blocks" will just make the image too
>> large for the device.
>>
>> Or we must go the Android rabbit hole to find an exotic tool to modify
>> even one line of a config file?
> 
> I believe that this feature was only implemented inside Google.

Well, "Don't be evil" is a joke now, right?

> 
> However, if you want to make changes to some files in this filesystem
> there should be a number of ways to do it:
> - use "dd" to dump file block(s) from image, edit them, then write back.
>   use "debugfs -c -R 'stat /path/to/file' vendor.img" for block addresses

Exactly what I'm going to try.

(Thankfully EXT4 hasn't implemented data checksum)

Thanks for the info,
Qu

> - use debugfs to clear the flag, mount the filesystem normally, then
>   overwrite the file *in place* (using "dd" or similar) so that the
>   blocks for the shared file are not reallocated due to unlink, write
> - make a simple patch for the kernel to "support" this feature, then
>   mount it and modify the file in a similar manner
> 
> Cheers, Andreas
> 
> 
> 
> 
> 



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