Kata Containers is a lightweight container runtime that leverages hardware virtualization to provide strong isolation between containers. It is compatible with the Open Container Initiative (OCI) and the Container Runtime Interface (CRI). Kata Containers uses a lightweight VM to run each container, which provides an additional layer of isolation compared to traditional container runtimes like Docker or containerd.

The official documentation is fairly lackluster here and there. For example, see here. There is a lot of prerequisite knowledge assumed. Another tutorial is here, which sheds some light into the building process of a custom kernel image, but leaves out custom kernel modules.

This article aims to provide a step-by-step guide on how to utilize a custom kernel module in a Kata Containers guest. In this example, we will include the igb_uio kernel module, which can be used with DPDK.

This guide assumes you already installed yq.

KATA_VERSION="3.2.0"
KATA_CONFIG_VERSION="114"
wget "https://github.com/kata-containers/kata-containers/archive/refs/tags/$KATA_VERSION.tar.gz"
tar -xf $KATA_VERSION.tar.gz

cd kata-containers-$KATA_VERSION/tools/packaging/kernel
bash build-kernel.sh -v 6.7 -s -f setup # download the kernel source, force create the .config file

mkdir -p kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/igb_uio
cp -r source/folder/to/module/igb_uio/* kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/igb_uio/

# create Kconfig file, this file will make the module visible in make menuconfig
cat > kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/igb_uio/Kconfig << EOF
menuconfig IGB_UIO
  tristate "igb_uio driver"
  depends on UIO
  default y
EOF
# overwrite Makefile to avoid building the module as .ko file
echo "# SPDX-License-Identifier: GPL-2.0" > kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/igb_uio/Makefile
echo "obj-\$(CONFIG_IGB_UIO) += igb_uio.o" >> kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/igb_uio/Makefile

# link into main Kconfig, so that make menuconfig has a new entry; insert into the line after uio entry
sed -i '135i\source "drivers/igb_uio/Kconfig"' kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/Kconfig

# add folder to main Makefile
echo "obj-\$(CONFIG_IGB_UIO) += igb_uio/" >> kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/Makefile

# remove Kbuild file to avoid building the module as .ko file and therefore directly link it into the built kernel
rm kata-linux-6.7-$KATA_CONFIG_VERSION/drivers/igb_uio/Kbuild

# append to .config file
echo "CONFIG_IGB_UIO=y" >> kata-linux-6.7-$KATA_CONFIG_VERSION/.config

# build the kernel with the new module
bash build-kernel.sh -v 6.7 build

Why Kata 3.2.0, an ancient version, you might ask? Unfortunately, we were unable to get newer version to work with SEV-SNP.