custom_kernel_rootfs.md

Custom Kernel / Rootfs

This document explains how to build a custom kernel and use debootstrap to build a rootfs for running crosvm.

For an easier way to get started with prebuilt images, see Example Usage

Build a kernel

The linux kernel in chromiumos comes preconfigured for running in a crosvm guest and is the easiest to build. You can use any mainline kernel though as long as it's configured for para-virtualized (virtio) devices

If you are using the chroot for ChromiumOS development, you already have the kernel source. Otherwise, you can clone it:

git clone --depth 1 -b chromeos-6.6 https://chromium.googlesource.com/chromiumos/third_party/kernel

Either way that you get the kernel, the next steps are to configure and build the bzImage:

cd kernel
CHROMEOS_KERNEL_FAMILY=termina ./chromeos/scripts/prepareconfig container-vm-x86_64
make olddefconfig
make -j$(nproc) bzImage

This kernel does not build any modules, nor does it support loading them, so there is no need to worry about an initramfs, although they are supported in crosvm.

Build a rootfs disk

This stage enjoys the most flexibility. There aren't any special requirements for a rootfs in crosvm, but you will at a minimum need an init binary. This could even be /bin/bash if that is enough for your purposes. To get you started, a Debian rootfs can be created with debootstrap. Make sure to define $CHROOT_PATH.

truncate -s 20G debian.ext4
mkfs.ext4 debian.ext4
mkdir -p "${CHROOT_PATH}"
sudo mount debian.ext4 "${CHROOT_PATH}"
sudo debootstrap stable "${CHROOT_PATH}" http://deb.debian.org/debian/
sudo chroot "${CHROOT_PATH}"
passwd
echo "tmpfs /tmp tmpfs defaults 0 0" >> /etc/fstab
echo "tmpfs /var/log tmpfs defaults 0 0" >> /etc/fstab
echo "tmpfs /root tmpfs defaults 0 0" >> /etc/fstab
echo "sysfs /sys sysfs defaults 0 0" >> /etc/fstab
echo "proc /proc proc defaults 0 0" >> /etc/fstab
exit
sudo umount "${CHROOT_PATH}"

Note: If you run crosvm on a testing device (e.g. Chromebook in Developer mode), another option is to share the host's rootfs with the guest via virtiofs. See the virtiofs usage.

You can simply create a disk image as follows:

fallocate --length 4G disk.img
mkfs.ext4 ./disk.img