Include binary in initramfs

include_bin_initrd.md

Include a binary in initial ram disk image

Below are the steps to include a binary to the kernel, which is not there by default. The binary used in this example is the openssl one.

Overview of steps to be done:

1. backup kernel image
2. create hook script with initramfs-tools
3. rebuild kernel

Backup current initramfs

Locate the latest initrd image inside the /boot/ folder and back it up via:

sudo cp -aRf /boot/initrd.img-5.10.0-23-amd64 /boot/initrd.img-5.10.0-23-amd64.bak

If something goes wrong in the steps later, this backup initramfs image can be used anytime. To specify this backup image to be loaded instead of any other, press e on the GRUB menu before boot and go to line starting with initrd and change the name of the image to the name of backup image and press Ctrl+x to boot.

Create hook script

Initramfs-tools allows creating custom hook scripts, which can execute custom scripts during build of the kernel image, more info here.

Create hook script: sudo nano /etc/initramfs-tools/hooks/openssl

With the following content:

#!/bin/sh
PREREQ=""
prereqs()
{
     echo "$PREREQ"
}

case $1 in
prereqs)
     prereqs
     exit 0
     ;;
esac

# openssl
. /usr/share/initramfs-tools/hook-functions    #provides copy_exec
rm -f ${DESTDIR}/bin/openssl                   #copy_exec won't overwrite an existing file
copy_exec /usr/bin/openssl /bin/openssl        #Takes location in filesystem and location in initramfs as arguments

Add execution rights: sudo chmod +x /etc/initramfs-tools/hooks/openssl

Rebuild kernel

Via: sudo update-initramfs -u -k all