How to build a kernel

build_kernel.sh

#!/bin/bash
set -ex


KVER="$(make -s -C "${BUILD_DIR}" kernelversion 2>/dev/null)"
BUILD_DIR="/usr/src/build-${KVER}"
SRC_DIR=/usr/src/linux

function copy_config() {
        mkdir -vp "${BUILD_DIR}"
        if [[ -e /root/config ]]; then
                cp -v /root/config "${BUILD_DIR}/.config"
        else
                zcat /proc/config.gz > "${BUILD_DIR}/.config"
        fi
        make -C "${SRC_DIR}" O="${BUILD_DIR}" olddefconfig
}

if [[ ! -d /boot/loader ]]; then
        mount /boot
fi

copy_config
make -C "${BUILD_DIR}" "-j$(nproc)" olddefconfig modules_prepare scripts

export KBUILD_OUTPUT="${BUILD_DIR}"
env EXTRA_ECONF="--with-spl=${SRC_DIR} --enable-linux-builtin --with-spl-obj=${BUILD_DIR}" \
        ebuild "$(portageq get_repo_path / gentoo)/sys-fs/zfs-kmod/zfs-kmod-9999.ebuild" clean configure
(cd /var/tmp/portage/sys-fs/zfs-kmod-9999/work/zfs-kmod-9999/ && ./copy-builtin "${SRC_DIR}")
unset KBUILD_OUTPUT

emerge --usepkg=n sys-fs/zfs

copy_config
make -C "${BUILD_DIR}" "-j$(nproc)"
make -C "${BUILD_DIR}" modules_install
depmod -a

# Create the initramfs
dracut -f "/root/initramfs.img" "${KVER}"

# Bundle boot artifacts together, the offsets are arbitrary and are only used by linuxx64.efi.stub.
objcopy \
        --add-section .osrel=/etc/os-release --change-section-vma .osrel=0x20000 \
        --add-section .cmdline=/etc/cmdline.txt --change-section-vma .cmdline=0x30000 \
        --add-section .linux="${BUILD_DIR}/arch/x86_64/boot/bzImage" --change-section-vma .linux=0x2000000 \
        --add-section .initrd=/root/initramfs.img --change-section-vma .initrd=0x3000000 \
        /usr/lib/systemd/boot/efi/linuxx64.efi.stub /root/linux.efi

# Sign boot artifact
sbsign \
        --key /etc/efikeys/db.key \
        --cert /etc/efikeys/db.crt \
        --output "/boot/${KVER}".efi \
        "/root/linux.efi"

# Reconfigure the bootloader
sed "s/KVER/${KVER}/" /boot/loader/entries/gentoo-template > "/boot/loader/entries/${KVER}.conf"