I have attempted to create a Initramfs file system for my Gentoo kernel in order to use it as a rescue shell whenever I am getting boot problems. Gentoo have a comprehensive wiki about this but I was unable to make it work by the book. I have tried several times, I have goggled until I forgot what I was looking for, but no result.

Gentoo Initramfs - howto

Finally I succeeded to create a Gentoo Initramfs and I will share the minimal required steps bellow:

1. Make sure you have busybox compiled as static with the following flags:

   USE="static" emerge --root=/usr/src/initramfs/ -av busybox

2. create a directory structure as following:

   mkdir /usr/src/initramfs cd /usr/src/initramfs mkdir -p bin lib dev etc mnt/root proc root sbin sys

3. copy the minimal device nodes and busybox:

   cp -a /dev/{null,console,tty,sd??} /usr/src/initramfs/dev/ cp -a /bin/busybox /usr/src/initramfs/bin/busybox

4. Create the /usr/src/initramfs/init executable file as following:

   #!/bin/busybox sh echo "Starting init!" rescue_shell() { echo "Something went wrong while mounting rootfs. Dropping you to a shell with network support..." /bin/busybox --install -s # configure the eth0 network interface ifconfig eth0 10.7.1.12 route add default gw 10.7.1.1 # make sure we enable CTRL-C feature setsid cttyhack sh exec /bin/sh } # Mount the /proc and /sys filesystems. mount -t proc none /proc mount -t sysfs none /sys mount -t devtmpfs none /dev # Do your stuff here. echo "Mounting /dev/sda3 root file system..." # Mount the root filesystem. mount -t ext4 -o ro /dev/sda3 /mnt/root || rescue_shell # Clean up. umount /proc umount /sys umount /dev # Boot the real thing. exec switch_root /mnt/root /sbin/init

5. Make sure you configure and compile your kernel with the following option: General setup ---> [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support

6. Create your initramfs (make sure that /dev/sda1 is mounted as /boot):

   cd /usr/src/initramfs find . -print0 | cpio --null -ov --format=newc | gzip -9 > /boot/initramfs.cpio.gz

7. Edit your /boot/grub/grub.conf layout as following :

   ... title Gentoo Linux root (hd0,0) kernel /boot/bzImage initrd /boot/initramfs.cpio.gz ...

8. Reboot your machine. If something goes wrong when mounting the root partition (eg: /dev/sda3) then you will be dropped directly to the busybox shell. You can test this by intentionally changing the /usr/src/initramfs/init line:

   mount -o ro /dev/sda3 /mnt/root || rescue_shell

   with

   mount -o ro /dev/sda9 /mnt/root || rescue_shell.

   When init will try to mount the sda9 invalid partition then the rescue_shell function will be called and then busybox shell will take the control.

Note: anytime you change something in the /usr/src/initramfs/ make sure you re-run the step 6. That's all folks!