Booting Linux Kernel with a custom init program on QEMU Ubuntu 24

This post documents how I wrote and compiled a minimal init program in C, built a simple initramfs, and successfully booted it using a custom Linux kernel v6.12 on Ubuntu 24.04 via QEMU. Before you begin, make sure you’ve already built your own Linux kernel as shown in this post.

Create a Minimal C Project

mkdir -p my-init/bin
cd my-init
touch init.c

Edit init.c:

// init.c
#include <stdio.h>
#include <unistd.h>

int main() {
    while (1) {
        printf("$ ");
        fflush(stdout);

        char input[256];
        if (fgets(input, sizeof(input), stdin) == NULL) break;

        printf("Sorry, I don't know how to do that.\n");
    }
    return 0;
}

Now compile this as a statically linked binary. This ensures the kernel doesn’t rely on any external shared libraries or runtime:

gcc init.c -static -o bin/init

Build an initramfs Image

Before the kernel can execute the init program, we must place it inside an initial RAM-based file system (initramfs). The kernel will extract this archive into memory at boot and use it as the root filesystem.

cd bin
chmod +x init
cd ..
find . -print0 | cpio --null --create --verbose --format=newc | gzip -9 > ../initrd

This generates an initrd file — a gzipped cpio archive — which contains your custom init binary.

Enable Kernel Features

Make sure your kernel supports initramfs, ELF binaries, and 64-bit execution. Run:

make menuconfig

Then enable these options:

1. General setup —> Initial RAM filesystem and RAM disk (initramfs/initrd) support
2. Executable file formats —> Kernel support for ELF binaries
3. Processor type and features —> 64-bit kernel

Recompile the kernel afterward if needed.

Boot with QEMU

Now boot your kernel and initramfs using QEMU:

qemu-system-x86_64 -kernel arch/x86/boot/bzImage -initrd initrd -nographic -append "console=ttyS0"

Expected output:

$ Sorry, I don't know how to do that.
$ Sorry, I don't know how to do that.
...

Each time you type a line and press Enter, it responds with a fixed message — just like the minimal shell we designed!