build.sh

#!/usr/bin/env bash

# Build a custom WSL2 kernel with zswap

set -e
set -o pipefail

sudo apt update
sudo apt install build-essential flex bison libssl-dev libelf-dev libncurses-dev autoconf libudev-dev libtool dwarves cpio qemu-utils

WSL2_KERNEL_VERSION="$(uname -r | grep -o '^[0-9\.]\+')"
KERNEL_MAJOR_VERSION="$(echo "${WSL2_KERNEL_VERSION}" | cut -d. -f1)"

# Validate kernel version was extracted correctly
if [[ -z "${KERNEL_MAJOR_VERSION}" ]] || ! [[ "${KERNEL_MAJOR_VERSION}" =~ ^[0-9]+$ ]]; then
  echo "Error: Could not determine kernel major version from: $(uname -r)"
  echo "Expected format: X.Y.Z.W (e.g., 5.15.153.1 or 6.6.36.3)"
  exit 1
fi

echo "Detected WSL2 kernel version: ${WSL2_KERNEL_VERSION} (major: ${KERNEL_MAJOR_VERSION})"

wget -c https://github.com/microsoft/WSL2-Linux-Kernel/archive/refs/tags/linux-msft-wsl-${WSL2_KERNEL_VERSION}.tar.gz
tar xvf linux-msft-wsl-${WSL2_KERNEL_VERSION}.tar.gz

cd "WSL2-Linux-Kernel-linux-msft-wsl-${WSL2_KERNEL_VERSION}"

cp Microsoft/config-wsl .config           # Use WSL default kernel config as the base

# Add zswap configuration
# Common configuration for all kernel versions
cat << EOF >> .config

CONFIG_CRYPTO_ZSTD=y
CONFIG_ZSTD_COMMON=y
CONFIG_ZSTD_COMPRESS=y

EOF

# Add CONFIG_FRONTSWAP for kernel 5.x only (removed in 6.x)
if [[ "${KERNEL_MAJOR_VERSION}" -lt 6 ]]; then
  cat << EOF >> .config
CONFIG_FRONTSWAP=y
EOF
fi

# Add remaining zswap configuration (common to all versions)
cat << EOF >> .config
CONFIG_ZSWAP=y
CONFIG_ZSWAP_COMPRESSOR_DEFAULT_ZSTD=y
CONFIG_ZSWAP_COMPRESSOR_DEFAULT="zstd"
CONFIG_ZSWAP_ZPOOL_DEFAULT_ZBUD=y
CONFIG_ZSWAP_ZPOOL_DEFAULT="zbud"
CONFIG_ZSWAP_DEFAULT_ON=y
CONFIG_ZPOOL=y
CONFIG_ZBUD=y
EOF

make olddefconfig

make -j $(nproc)

# For kernel 6.x, also build and package modules
if [[ "${KERNEL_MAJOR_VERSION}" -ge 6 ]]; then
  echo "Building and packaging kernel modules for WSL2 kernel 6.x..."

  # Save current directory
  BUILD_DIR=$(pwd)

  # Set up cleanup trap to ensure modules directory is removed on error
  cleanup_modules() {
    if [[ -d "${BUILD_DIR}/modules" ]]; then
      rm -rf "${BUILD_DIR}/modules"
    fi
  }
  trap cleanup_modules EXIT

  # Install modules to a modules directory
  if ! make modules_install INSTALL_MOD_PATH="${BUILD_DIR}/modules"; then
    echo "Error: Failed to install kernel modules"
    exit 1
  fi

  # Get kernel release version
  KERNEL_RELEASE=$(make -s kernelrelease)
  
  # Validate kernel release was extracted correctly
  if [[ -z "${KERNEL_RELEASE}" ]]; then
    echo "Error: Could not determine kernel release version"
    exit 1
  fi

  # Use Microsoft's gen_modules_vhdx.sh script
  echo "Creating modules VHDX using Microsoft's gen_modules_vhdx.sh script..."
  if ! sudo ./Microsoft/scripts/gen_modules_vhdx.sh "${BUILD_DIR}/modules" "${KERNEL_RELEASE}" "${BUILD_DIR}/modules.vhdx"; then
    echo "Error: Failed to create modules VHDX"
    exit 1
  fi

  # Cleanup modules directory and remove trap
  trap - EXIT
  cleanup_modules

  cat << EOF

Kernel build complete for WSL2 kernel ${WSL2_KERNEL_VERSION}!

Next steps:
1. Copy "arch/x86/boot/bzImage" to "/mnt/c/bzImage"
2. Copy "modules.vhdx" to "/mnt/c/modules.vhdx"
3. Add the following to your ".wslconfig" file in your Windows user directory:

[wsl2]
kernel=C:\\\\bzImage
kernelModules=C:\\\\modules.vhdx

4. Restart your WSL2 instance:
   wsl --shutdown
   
Then reopen your WSL2 terminal. The new kernel with zswap support will be active.
EOF
else
  cat << EOF

Kernel build complete for WSL2 kernel ${WSL2_KERNEL_VERSION}!

Next steps:
1. Copy "arch/x86/boot/bzImage" to "/mnt/c/bzImage"
2. Add the following to your ".wslconfig" file in your Windows user directory:

[wsl2]
kernel=C:\\\\bzImage

3. Restart your WSL2 instance:
   wsl --shutdown

Then reopen your WSL2 terminal. The new kernel with zswap support will be active.
EOF
fi