I am trying to update a process used to remaster the Ubuntu 18.04 install image for 20.04 and running into a problem with extracting the initrd archive. The command used is cpio -id --no-absolute-filenames. When I use this same command to extract the initrd image from the 20.04 image, it extracts kernel/x86/microcode/AuthenticAMD.bin and stops.

Inspecting initrd file contents I can see a "TRAILER" section showing that there are multiple archives in the file. As suggested in other threads about cpio archives, I tried to extract the compound archive as such: cat initrd | while cpio -id --no-absolute-filenames; do :; done. In this case, the first entry extracts but then it has a bunch of "0 blocks" and doesn't extract anything else.

Does anyone know how this archive was created or how to extract it?


Same problem here -- the solution was to not unpack with Ubuntu version 20.04 using unmkinitramfs.

I created a bootable USB and booted my machine into Ubuntu 20 -- unmkinitramfs was then able to unpack the initrd. I'm still finding my way around building a new bootable USB that doesn't autologin (and works correctly with casper-rw).

If you want to see more about what unmkinitramfs does, it's a bash script. In a comparison between Bionic and Focal, Bionic does not supprt lz4 compression.

  • So is the point here that the version of unmkinitramfs shipped with versions previous to 20.04 not compatible with the format used? – virullius May 15 '20 at 16:08
  • That is indeed the issue. I needed the version shipped with 20.04. – virullius May 15 '20 at 19:34
  • when tried I got unmkinitramfs command not found message even though I have initram tools – incompetent May 3 at 19:13

/usr/sbin/update-initramfs calls /usr/sbin/mkinitramfs, which calls

cpio --quiet -R 0:0 --reproducible -o -H newc | lz4 -9 -l

where the lz4 is coming from /etc/initramfs-tools/initramfs.conf

reversing that into

cat /boot/initrd.img-5.4.0-26-generic | unlz4 -9 -l | cpio -i -H newc

fails though.

cat /boot/initrd.img-5.4.0-26-generic | file -
    /dev/stdin: ASCII cpio archive (SVR4 with no CRC)

keeps persisting it's a cpio archive, but

cat /boot/initrd.img-5.4.0-26-generic | cpio -i -H newc

still ends up in the same AuthenticAMD.bin. Maybe somebody else can build on this?

Edit: nope. According to https://unix.stackexchange.com/a/511224/44864 The answer is

 unmkinitramfs -v initrd.img-5.4.0-26-generic .
  • unmkinitramfs also extracted GenuineIntel.bin and a junk directory named .enuineIntel.align.0123456789abc, seemingly misunderstanding some metadata. It then errors with premature end of archive. – virullius Apr 27 '20 at 14:48
  • I suggest you look into the checksum of your downloaded copy of ubuntu-20.04-live-server-amd64.iso as I just downloaded it, and successfully unpacked the initrd.img from it with unmkinitramfs. – JdeHaan Apr 27 '20 at 14:59
  • is the file you're looking at called "initrd.img"? I am looking at /casper/initrd. – virullius Apr 28 '20 at 17:36
  • Yes: pastebin.com/MqDVq6P8 – JdeHaan Apr 29 '20 at 6:38
  • I suggest you download nl.archive.ubuntu.com/ubuntu/pool/main/i/initramfs-tools/… and extract it with mkdir initramfs-tools-core cd initramfs-tools-core dpkg -x ../initramfs-tools/initramfs-tools-core_0.136ubuntu6_all.deb . dpkg -e ../initramfs-tools/initramfs-tools-core_0.136ubuntu6_all.deb ls -al ./usr/bin/unmkinitramfs -rwxr-xr-x 1 me me 3595 Feb 18 02:19 ./usr/bin/unmkinitramfs – JdeHaan Apr 29 '20 at 6:45

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