How to make linux kernel with Nasm, Go binary, mini linux

Github link for project: https://github.com/creotiv/howto-bootable-golang-binary

Pre-install

Some things needed for building & running stuff

apt-get install bison flex nasm qemu-system-x86_64 libelf-dev bc

Nasm simple example

Standard boot sector is 512 bytes that should end with 0xaa55 bytes. I’ve created simple Nasm script that print “Hello Wolrd!” on boot.

[org 0x7c00]
mov ah, 0x0e
mov bx, hello 

print:
    mov al, [bx]
    cmp al, 0
    je end
    int 0x10
    inc bx
    jmp print
end:

jmp $

hello:     
    db 'Hello world!',0

times 510-($-$$) db 0  
dw 0xaa55       ; this bytes should end boot sector

Compile Boot binary:

nasm -f bin boot.nasm -o boot.bin

Run it with emulator

qemu-system-x86_64 boot.bin

Run go binary on Linux Kernel

First we need to download ltest stable linux Kernel

curl https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.17.4.tar.xz | tar Jx

Then we need to configure it and select all things that we need. That’s kind of hard, because there are tons of settings. I’ve made some default config in root/.config that u can copy to linux-5.17.4 folder to use.

If you want to build your own config use: make menuconfig

Compile kernel (it will take a while)

cd linux-5.17.4 && make -j4

Now lets run image with our kernel to see if it works

qemu-system-x86_64 -serial stdio -kernel linux-5.17.4/arch/x86/boot/bzImage

We got error saying that we didn’t mount a disk — it ok, cause we really didn’t do it.

Now we need mount disk and add our binary, libs, etc on it. We will use initramfs for that, basically put everything into the RAM.

For that we need to build our go binary with static linking and create gzip archive with our ram mount(basically only our binary)

Very simple go script

package main

import (
	"fmt"
)

func main() {
    var data string
    fmt.Println("Hello from your Go image!")
    for {
	fmt.Println("Enter something:")
    	fmt.Scanln(&data)
	fmt.Printf("Echo: %s\n", data)
    }
}

./build-ramfs.sh

#!/bin/bash
SPATH=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )

cd $SPATH/gobinary
go build -ldflags="-extldflags=-static" -o init
cd ../
mkdir -p goram
mv $SPATH/gobinary/init $SPATH/goram/init
cd $SPATH/goram
find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../goram.cpio.gz

Now we can run it as before just add our ram mount

qemu-system-x86_64 -serial stdio -kernel linux-5.17.4/arch/x86/boot/bzImage -initrd go-ram.cpio.gz

Creating some mini-linux image.

Of course we can create our custom linux. For example we can use Buxybox image for that

curl https://busybox.net/downloads/busybox-1.34.1.tar.bz2 | tar Jz
cd busybox-1.33.2
make menuconfig 
make -j4
make install
cd ..

Now let’s fill our image

mkdir -p bbram/{bin,sbin,etc,proc,sys,usr/bin,usr/sbin}
cp -a busybox-1.34.1/_install/* bbram/

Let’s create welcome init script

#!/bin/sh

/bin/mount -t devtmpfs devtmpfs /dev
/bin/mount -t proc none /proc
/bin/mount -t sysfs none /sys

dmesg -n 1
 
cat << EOF

 Boot took $(cut -d' ' -f1 /proc/uptime) seconds

  __  __ _       _   _      _                  
 |  \/  (_)     (_) | |    (_)                 
 | \  / |_ _ __  _  | |     _ _ __  _   ___  __
 | |\/| | | '_ \| | | |    | | '_ \| | | \ \/ /
 | |  | | | | | | | | |____| | | | | |_| |>  < 
 |_|  |_|_|_| |_|_| |______|_|_| |_|\__,_/_/\_\
                                               
                                               

EOF
exec setsid cttyhack sh #https://busybox.net/FAQ.html#job_control
exec /bin/sh

Save it

cp init bbram/bin/init
chmod +x bbram/bin/init

Create ram archive

cd bbram
find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../bbram.cpio.gz
cd ../

Run it

qemu-system-x86_64 -kernel linux-5.17.4/arch/x86/boot/bzImage -initrd bbram.cpio.gz -m1024