How to pack an extracted initrd (x-cpio archive)?

Question

I am customizing an Ubuntu 20.04 ISO, in this process, I have to edit the files inside initrd file which is inside the casper directory Ubuntu20.04-ISO/casper/initrd

How to recompile this file type of CPIO-archive (application/x-cpio)

I am able to unpack its contents with below command.

unmkinitramfs initrd .

But I am not able to recompile it, How can I Achieve this?

Looking for a practical step by step answer, I mean copy pasting the initrd file to new directory and recompiling it and proven to boot with this newly created say myinitrd file.

Answer 1

Depending on the targeted device/config initrd may have different format, you can check its format with:

binwalk ./initrd

According to your cross posts the required format for your initrd is cpio formatted with newc along with lzma compression, and thus a method of identification would be the following... note that this is just to explain how to identify an initrd:

binwalk initrd

#DECIMAL    HEXADECIMAL     DESCRIPTION
#------------------------------------------
# 0         0x0             LZMA compressed data

cp initrd compressedinit.xz
xz -d compressedinit.xz
binwalk compressedinit

#DECIMAL    HEXADECIMAL     DESCRIPTION
#------------------------------------------
# 0         0x0             ASCII cpio archive...
# 112       0x70            ASCII cpio archive...
# ...

This permit to identify the initrd as cpio formatted with newc along with lzma compression. Packing/Repacking an initrd depend on it's format. With binwalk, xz, gzip etc. you should be able to discover the used format and thus recreate it after extraction according to the used compression.

Note that cpio -H option determine the archive format, this can be one of the following crc newc odc bin ustar tar depending on the targeted format.

As your extracted initrd contains:

~/test$ ls
early  early2  main

You can use the following steps to unpack and repack it (source: your superuser's QA):

# Extract and prepare the working directories
# -------------------------------------------

mkdir /tmp/tmp/
mkdir /tmp/tmp/extracted/
cp initrd /tmp/tmp/
cd /tmp/tmp
unmkinitramfs initrd ./extracted
cd extracted

# Add the first microcode firmware
# --------------------------------

cd early
find . -print0 | cpio --null --create --format=newc > /tmp/tmp/newinitrd

# Add the second microcode firmware
# ---------------------------------

cd ../early2
find kernel -print0 | cpio --null --create --format=newc >> /tmp/tmp/newinitrd

# Add the ram fs file system
# --------------------------

cd ../main
find . | cpio --create --format=newc | xz --format=lzma >> /tmp/tmp/newinitrd

# Verify both initrds 
# -------------------

cd /tmp/tmp
binwalk initrd
binwalk newinitrd

Note that this method is adapted to your situation, on other case where an initrd is a cpio formatted with newc along with lzma compression we may use:

cd /extracted/initrd
find . | cpio -o -H newc | xz --format=lzma > ../newinitrd

Answer 2

Here is what i use to repack an xz initramfs for syslinux 4.06 (live boot)

#!/bin/bash

find . 2>/dev/null | cpio --create --format='newc' > ../initrfs.img.cpio
xz --check=crc32 --lzma2=,dict=2048kiB ../initrfs.img.cpio
mv ../initrfs.img.cpio.xz ../initrfs.img

Answer 3

cpio: option requires an argument -- 'H'

It really needs that -H newc option. I tried once without and it did not work. It works without compression.