linux-ext4 - Is it possible to make ext4 images reproducible even after filesystem operations ?

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Message-ID: 
 <TYCPR01MB966943691EBB5DA3F5F85621C4E62@TYCPR01MB9669.jpnprd01.prod.outlook.com>
Date: Tue, 21 Jan 2025 12:29:16 +0000
From: <Adithya.Balakumar@...hiba-tsip.com>
To: <linux-ext4@...r.kernel.org>
CC: <Shivanand.Kunijadar@...hiba-tsip.com>, <dinesh.kumar@...hiba-tsip.com>,
        <kazuhiro3.hayashi@...hiba.co.jp>, <nobuhiro1.iwamatsu@...hiba.co.jp>
Subject: Is it possible to make ext4 images reproducible even after filesystem
 operations ?

Hi All,

I am working towards reproducible builds for a project that I am involved in. We use a few ext4 partitions in our disk images and I am trying to make the ext4 filesystems reproducible.

I understand that from e2fsprogs v1.47.1 onwards we can create a reproducible ext4 filesystem image. We can indeed create a reproducible ext4 filesystem image when we use the "-d" option in "mke2fs" command to pass the contents of the filesystem at the time of creation of the filesystem itself. I understand that there are a few other parameters that needs to passed to the "mke2fs" command like a deterministic UUID and hash_seed values to make the filesystem image reproducible.

In the project that I am working on, there are some mount operations done on the filesystem to copy certain files into the file system. This updates the "Last mount" and "Last write" timestamps in the filesystem metadata (confirmed this with dumpe2fs) thereby making the images generated not reproducible.

I would like to understand if its possible to make the ext4 images reproducible even after filesystem operations like mounting and unmounting the filesystem ?

Thanks and Regards,
Adithya Balakumar