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!

Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.

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Eugen Mihailescu

Founder/programmer/one-man-show at Cubique Software

Always looking to learn more about *nix world, about the fundamental concepts of math, physics, electronics. I am also passionate about programming, database and systems administration. 16+ yrs experience in software development, designing enterprise systems, IT support and troubleshooting.

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