Buildroot, QEMU and Linux kernel for x86_64

How to build a minimum Linux kernel with customized rootfs and run it with QEMU?

Download Buildroot

Please go here to download the latest Buildroot package and decompress it.

Add new external packages (user-space program and kernel module) (optional)

To write an external user space program or install an external kernel module to validate your idea, e.g., to reproducing a bug, I refer to this and this.

I have created a repo with support of the external user space program and kernel module. Please git clone https://github.com/cyruscyliu/buildroot-external-packages.git.

Compile Buildroot

The directory layout is like below.

.
├── buildroot-2022.02.4
└── buildroot-external-packages

Go to buildroot
If no external packages, just make qemu_x86_64_defconfig, otherwise, make BR2_EXTERNAL="$(pwd)/../buildroot-external-packages" qemu_x86_64_defconfig. If there is an old config, make oldconfig or make BR2_EXTERNAL="$(pwd)/../buildroot-external-packages" oldconfig.
Before going on, we should update the C library and the TTY target by make menuconfig.
- Toolchain -> C library -> glibc
- System Configuration -> Run a getty (login prompt) after boot -> TTY port -> ttyS0
We can optionally enable lspci -v to show more information on PCI devices.
- Target Packages -> Hardware Handling -> pciutils
We can optionally enable the external packages.
- External options -> check what you want
make with -jN to accelerate your compilation.

Launch all of them

qemu-system-x86_64 \
    -M pc \
    -kernel ./output/images/bzImage \
    -drive file=./output/images/rootfs.ext2,if=virtio,format=raw \
    -append "root=/dev/vda console=ttyS0" \
    -net user,hostfwd=tcp:127.0.0.1:3333-:22 \
    -net nic,model=virtio \
    -nographic

If the external packages exist, please do as follows.

$ modprobe hello
hello: loading out-of-tree module taints kernel.
hello init
$ desg
...
hello: loading out-of-tree module taints kernel.
hello init
$ userspace_program 
Hello World!