How to extract modern Ubuntu initramfs

5 Comments

1. Pingback: Running Systems » Blog Archive » Extracting multi-layered initramfs

2. I am working on Lubuntu 22.04, and either the lubuntu team creates the ramdisk differently, or the command is now out of date.

   This works for me:
   (cpio -id; cpio -id; zstdcat | cpio -id) < /path/to/initrd.img The ramdisk I am working with now has 2x uncompressed CPIO archives, prepending a "Z Standard" compressed main ramdisk.

   Reply

   1. They can modify it, however – how? Can you run the command ‘file’ on the initrd file? Or ‘lsinitrd’? It will give you a lot of details and insights.
      In any case, on Ubuntu 22.0.4 my command works, and there is no reason to assume that this mechanism has changed. Just to be clear – your command failed on my test with the following output:
      cpio: Malformed number
      and
      cpio: premature end of archive

      Reply

      1. Hi etzion, while I was initially confused by your response, I think I understand where the difference comes from. With my initrd I have 3 CPIO archives, the first 2 being microcode for AMD and Intel CPUs respectively – they each have their own CPIO archive. Then the third archive is the main initial ramdisk, encrypted Z standard encryption.

         My ramdisk comes from the Lubuntu ISO. It is the ramdisk used to load the live environment. I assume the ramdisk you are using is for Ubuntu, installed on your computer? It would make sense to me that they would drop one of the first two microcode ramdisks, depending on what is applicable to the machine’s processor.

         Thanks for your response, by the way you have some great info on your website. Thanks for what you do!

         Reply

         1. I understand. You were using the special LiveCD initrd. On systems which are on-disk, the initrd is created per the hardware configuration and layout of the system. Your note is good and important – and it exposes a structure including both microcodes as the same time, for both types of CPUs. When the system is installed on disk, it will integrate only the CPIO image for the relevant microcode. Makes a lot of sense.

            I really appreciate the feedback. I have been collecting and sharing (and using it as an extended memory) for a long while. Some of it is still relevant even today 🙂
            Thanks!

            Reply

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