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Subject: Re: About read-only feature EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS
To:     Andreas Dilger <adilger@dilger.ca>
Cc:     linux-ext4@vger.kernel.org,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>
References: <4697ab8d-f9cf-07cc-0ce9-db92e9381492@gmx.com>
 <F933761F-D748-4FD9-9FC3-2C52D7CA205D@dilger.ca>
From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
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Date:   Sun, 2 Feb 2020 10:23:57 +0800
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On 2020/2/2 =E4=B8=8A=E5=8D=8810:16, Andreas Dilger wrote:
> On Feb 1, 2020, at 7:02 PM, Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>>
>> Hi ext4 guys,
>>
>> Recently I found an image from android (vendor.img) has this RO featur=
e
>> 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?
>=20
> I believe that this feature was only implemented inside Google.

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

>=20
> 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=
=2E
>   use "debugfs -c -R 'stat /path/to/file' vendor.img" for block address=
es

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
>=20
> Cheers, Andreas
>=20
>=20
>=20
>=20
>=20


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