Heads up Please see my updated version of this tutorial: How to Build a Custom Linux Kernel for Qemu (2015 Edition). I'm leaving this post here for historical purposes only.

In this howto, we’re going to build a Linux system from the ground up using kernel sources from kernel.org and a busybox-based initramfs, which we will then run under qemu.

Initial Ramdisk

Linux needs something called the initial ramdisk in order to complete its boot process. The initial ramdisk is a filesystem image that is mounted temporarily at / while the Linux boots. The purpose of the initial ramdisk is to provide kernel modules and necessary utilities that might be needed in order to bring up the real root filesystem.

As outlined in the initrd Wikipedia article, there are currently two approaches that fulfill the role of the initial ramdisk: initrd and initramfs. We’ll be using the initramfs approach.

The gentoo-wiki has a great article about initramfs. We’ll be following that article closely here.

We’ll be creating everything under ~/linux_qemu:

$ mkdir ~/linux_qemu
$ cd ~/linux_qemu

First, create some directory structure for our initramfs:

$ mkdir initramfs
$ cd initramfs/
$ mkdir -pv bin lib dev etc mnt/root proc root sbin sys
$ sudo cp -va /dev/{null,console,tty} dev/

Now we need some stuff to put in our initial ramdisk. We’ll be using busybox since it provides a multitude of useful utilities all in a single binary.

Building BusyBox

Now we’ll compile busybox:

$ cd ~/linux_qemu
$ wget http://busybox.net/downloads/busybox-1.19.4.tar.bz2
$ tar xf busybox-1.19.4.tar.bz2
$ cd busybox-1.19.4/

For some reason I had to fix up some includes to get things working here. Newer versions of busybox might not have this issue, I haven’t really pursued the issue… If you have issues compiling you might want to apply my patch:

$ patch -p1 < <(wget https://raw.github.com/gist/3863017/f6d96af7b7cab9346adaf21aa7f05e0cfb722bef/struct_rlimit.diff -q -O-)

Now configure busybox:

$ make menuconfig

The options I changed were:

• CONFIG_DESKTOP=n
• CONFIG_EXTRA_CFLAGS=-m32 -march=i386 (might not need this if compiling on a 32-bit host)
• CONFIG_MKFS_EXT2=n

Compile:

$ make
$ make install

Copy the freshly-built busybox system to our initramfs staging area:

$ sudo cp -avR _install/* ../initramfs/

Now let’s have a look at what shared libraries we’ll need to include in our system:

$ cd ~/linux_qemu/initramfs
$ ldd bin/busybox
        linux-gate.so.1 =>  (0xf76f7000)
        libm.so.6 => /usr/lib/libm.so.6 (0xf76a1000)
        libc.so.6 => /usr/lib/libc.so.6 (0xf74fe000)
        /lib/ld-linux.so.2 (0xf76f8000)

At this point I did something very hacky and copied over some libraries from the host machine. This is almost definitely not the right way to do it, but it’s working for now so oh well…

$ mkdir -pv usr/lib
$ cp -av /usr/lib/lib[mc].so.6 usr/lib/
$ cp -av /usr/lib/lib[mc]-2.16.so usr/lib/
$ cp -av /usr/lib/ld-2.16.so usr/lib/
$ cp -av /lib/ld-linux.so.2 lib/
$ cp -av /lib/ld-2.16.so lib/

I believe the correct way to do it would be cross-compile glibc in a bootstrapped environment similar to how it’s done in the Linux From Scratch book. It’s really kind of tricky to get around the host-library dependency stuff…

It’s also worth mentioning another tool at this point: uclibc. uclibc is a small C Library targeting embedded systems. It also comes with a very slick build system called buildroot that makes it dead simple to build a full embedded system complete with a cross-compiled toolchain, root filesystem, kernel image and bootloader. It basically automates everything we’re doing in this tutorial (and uses a different C Libary). Anyways, it’s a very cool tool, so maybe I’ll cover building a qemu system with uclibc in another howto.

Init

Now we just need to create /init which will be called by Linux at the last stage of the boot process.

$ emacs ~/linux_qemu/initramfs/init

Here’s the contents of my /init:

#!/bin/sh

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

cat <<'EOF'
                       _             _ _                  
 _ __ ___   __ _  __ _| | __ _ ___  | (_)_ __  _   ___  __
| '_ ` _ \ / _` |/ _` | |/ _` / __| | | | '_ \| | | \ \/ /
| | | | | | (_| | (_| | | (_| \__ \ | | | | | | |_| |>  < 
|_| |_| |_|\__, |\__,_|_|\__, |___/ |_|_|_| |_|\__,_/_/\_\
           |___/         |___/                            

EOF
echo 'Enjoy your new system!'

/bin/sh

Make /init executable:

$ chmod 755 ~/linux_qemu/initramfs/init

We should now have everything necessary for our initramfs. We will cpio it up:

$ cd ~/linux_qemu/initramfs
$ find . -print0 | cpio --null -ov --format=newc > ../my-initramfs.cpio

We avoid gzip‘ing it here because the emulator takes forever to unpack it if we do…

If you want to see how to build a tiny Linux system from scratch using the initrd method, you can refer to this awesome presentation.

Kernel Configuration

$ cd ~/linux_qemu
$ wget http://www.kernel.org/pub/linux/kernel/v3.x/linux-3.3.tar.xz
$ tar xf linux-3.3.tar.xz
$ cd linux-3.3/

We’ll start with a minimal configuration, then add only what we need:

$ make allnoconfig
$ make menuconfig

I added:

• CONFIG_BLK_DEV_INITRD=y
• CONFIG_INITRAMFS_SOURCE=/home/mgalgs/linux_qemu/initramfs
• CONFIG_PCI=y
• CONFIG_BINFMT_ELF=y
• CONFIG_SERIAL_8250
• CONFIG_EXT2_FS=y
• CONFIG_IA32_EMULATION=y # might not need if on 32-bit host
• CONFIG_NET=y
• CONFIG_PACKET=y
• CONFIG_UNIX=y
• CONFIG_INET=y
• CONFIG_WIRELESS=n
• CONFIG_ATA=y
• CONFIG_NETDEVICES=y
• CONFIG_NET_VENDOR_REALTEK=y
• CONFIG_8139TOO=y (unchecked all other Ethernet drivers)

• CONFIG_WLAN=n

• CONFIG_DEVTMPFS=y

You can see my entire kernel .config here.

Now we’re ready to build the kernel:

$ make

Our kernel image should now be available at linux-3.3/arch/x86/boot/.

Final Preparations

Now we’ll just create a little hard disk to play around with:

$ cd ~/linux_qemu
$ qemu-img create disk.img 512M
$ mkfs.ext2 -F disk.img

Boot

We can now run our kernel in qemu:

$ qemu-system-i386 -hda disk.img -kernel ../linux-3.3/arch/x86/boot/bzImage -initrd my-initramfs.cpio

Success!