decompress and recompress the initrd image of Ubuntu Cosmic

de-re-compress-cosmic-initrd.sh

#!/bin/bash

#sudo apt-get install binwalk

# $ binwalk initrd
#
#DECIMAL       HEXADECIMAL     DESCRIPTION
#--------------------------------------------------------------------------------
#0             0x0             ASCII cpio archive (SVR4 with no CRC), file name: ".", file name length: "0x00000002", file size: "0x00000000"
#112           0x70            ASCII cpio archive (SVR4 with no CRC), file name: "kernel", file name length: "0x00000007", file size: "0x00000000"
#232           0xE8            ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86", file name length: "0x0000000B", file size: "0x00000000"
#356           0x164           ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode", file name length: "0x00000015", file size: "0x00000000"
#488           0x1E8           ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode/AuthenticAMD.bin", file name length: "0x00000026", file size: "0x00006B2A"
#28072         0x6DA8          ASCII cpio archive (SVR4 with no CRC), file name: "TRAILER!!!", file name length: "0x0000000B", file size: "0x00000000"
#28672         0x7000          ASCII cpio archive (SVR4 with no CRC), file name: "kernel", file name length: "0x00000007", file size: "0x00000000"
#28792         0x7078          ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86", file name length: "0x0000000B", file size: "0x00000000"
#28916         0x70F4          ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode", file name length: "0x00000015", file size: "0x00000000"
#29048         0x7178          ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode/.enuineIntel.align.0123456789abc", file name length: "0x00000036", file size: "0x00000000"
#29212         0x721C          ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode/GenuineIntel.bin", file name length: "0x00000026", file size: "0x00180C00"
#1605296       0x187EB0        ASCII cpio archive (SVR4 with no CRC), file name: "TRAILER!!!", file name length: "0x0000000B", file size: "0x00000000"
#1605632       0x188000        LZMA compressed data, properties: 0x5D, dictionary size: 8388608 bytes, uncompressed size: -1 bytes

mkdir working
cd working
# firstly get the amd and intel microcode
cpio -id < ../initrd
dd if=../initrd bs=28672 skip=1 | cpio -id

# and then get the root filesystem
#
# if you don't care about the file size you can archive in the following method
#
#dd if=../initrd bs=1605632 skip=1 | unlzma -c | cpio -id
#
#find | cpio -H newc -o > ../initrd.new
#
#-rw-rw-r-- 1 tai271828 tai271828  39M Nov 21 08:37 initrd
#-rw-rw-r-- 1 tai271828 tai271828 167M Nov 21 08:56 initrd.new

# or if you want to lz compress the archive you can
( mkdir rfs; cd rfs; dd if=../../initrd bs=1605632 skip=1 | unlzma -c | cpio -id  )

# edit the microcode and the root filesystem

( cd kernel; find | cpio -H newc -o > ../kernel.cpio)
#
( cd rfs; find | cpio -H newc -o > ../rfs.cpio)
lzma -c ./rfs.cpio > ./rfs.cpio.lz

#concatenate these two cpio archives
cat ./kernel.cpio ./rfs.cpio.lz > ../initrd.new
# then it is mucher smaller
#
#-rw-rw-r-- 1 tai271828 tai271828 39M Nov 21 08:37 initrd
#-rw-rw-r-- 1 tai271828 tai271828 39M Nov 21 09:14 initrd.new
#
# $ binwalk initrd.new 
#
#DECIMAL       HEXADECIMAL     DESCRIPTION
#--------------------------------------------------------------------------------
#0             0x0             ASCII cpio archive (SVR4 with no CRC), file name: ".", file name length: "0x00000002", file size: "0x00000000"
#112           0x70            ASCII cpio archive (SVR4 with no CRC), file name: "x86", file name length: "0x00000004", file size: "0x00000000"
#228           0xE4            ASCII cpio archive (SVR4 with no CRC), file name: "x86/microcode", file name length: "0x0000000E", file size: "0x00000000"
#352           0x160           ASCII cpio archive (SVR4 with no CRC), file name: "x86/microcode/GenuineIntel.bin", file name length: "0x0000001F", file size: "0x00180C00"
#1576432       0x180DF0        ASCII cpio archive (SVR4 with no CRC), file name: "x86/microcode/AuthenticAMD.bin", file name length: "0x0000001F", file size: "0x00006B2A"
#1604012       0x1879AC        ASCII cpio archive (SVR4 with no CRC), file name: "x86/microcode/.enuineIntel.align.0123456789abc", file name length: "0x0000002F", file size: "0x00000000"
#1604172       0x187A4C        ASCII cpio archive (SVR4 with no CRC), file name: "TRAILER!!!", file name length: "0x0000000B", file size: "0x00000000"
#1604608       0x187C00        LZMA compressed data, properties: 0x5D, dictionary size: 8388608 bytes, uncompressed size: -1 bytes
#