How do I get an initrd made out of a cpio archive loaded as the root device by grub2 in a distro-agnostic way?

Question

I'm building an LFS (Linux From Scratch) system in a VM and so far I've managed to get a workable, desktop system, booting from a known device, /dev/sda1 in my case. I'm now trying to make a live system that boots from an ISO image. Instead of using /dev/sr0 as the root, which I've already established is possible (and, since it's more likely to be used from a USB stick than an actual CD-ROM, is too inflexible) I've set my mind on booting it into an initrd root. The idea is to use that as the system's root instead of using it as a temporal root to load the "real" root, and since it's already in memory, it saves me the trouble of setting up a tmpfs root, copy all the files, and switch to it.

I had been previously been experimenting with a squashfs image as I had seen that Ubuntu seemed to use that and has what I needed: a small sized root, being faster to load, using less memory, and is fast (xz is SSLLOOWW to extract and gzip is slow to load). At first I was having trouble booting it, so i switched to the cpio based initrd. After some initial trouble due to missing files on the archive I did manage to boot it.

I left that aside for the time being (around a month ago) to do other tasks on the system. I lost the original GRUB2 settings and kernel config so went about doing it again but Ive been running into a brick wall. I'm hoping someone here might know what I'm missing.

When I boot up I never see any message about the loading of the initrd file, it goes straight into the loading, uncompressing and booting up of the kernel. And this ends up in a kernel panic with the message

VFS: cannot open root device "(null)" or unknown-block (1,0): error -6
Please append the correct "root=" boot option; here are the available partitions:

No partitions are shown and following that is the "kernel panic" message, just repeating the first line. If I use the "rootfstype=ramfs" I get:

VFS: mounted root (ramfs filesystem) readonly on device 0:15.
devtmpfs: error mounting -2

Essentially, it's mounting an EMPTY ramfs file system as root, so mounting the devtmpfs fails because the /dev entry doesn't exist. But certain I used that boot option before.

Here's my GRUB 2 config:

menuentry = "LFS (inird test)" {
        linux   /boot/kernel/initrd/linux ro rdinit=/etc/init
        initrd  /boot/kernel/initrd/root.cpio.gz
}

Yes, /boot/kernel/initrd/ directory entry exists, linux is the kernel (the bzimage file produced by compiling the kernel), and root.cpio.gz is my compressed initrd root cpio archive.

Here's my kernel's .config file (sorry can't paste it here).

If any more info is needed, don't hesitate to ask. That you.

Answer 1

OK, I managed to solve the problem! Apparently, it wasn't the kernel's configuration, GRUB2, or even the bootup sequence. It was the initrd archive itself. Deep in the bowels of the Linux kernel's configuration lied the answer: the archive must be built using cpio's --newc option. The one I built manually lacked this option, so the kernel was ignoring the archive and just proceeding with the normal boot procedure.

This came about because I managed to stumble across an older script I used to build them and saw all the options in it for cpio. I checked the much more recent script I hastily put together and double-checked the kernel documentation (as well as the init/do_mounts.c and init/initramfs.c files) and realized what was going on. I tried it with the corrections and the system now happily boots into the initrd with no problem! :D