feat: use Linux kernel as a bootloader

.github/workflows/rust.yml

@@ -5,8 +5,12 @@ permissions: read-all
 on:
   push:
     branches: ["main"]
+    paths-ignore:
+      - 'bootloader/**'
   pull_request:
     branches: ["main"]
+    paths-ignore:
+      - 'bootloader/**'
 
 env:
   CARGO_TERM_COLOR: always

bootloader/build.sh

@@ -0,0 +1,101 @@
+#!/bin/bash
+
+set -eu
+
+readonly ARCH=${ARCH:=$(uname -m)}
+readonly TARGET_DIR=target/bootloader-${ARCH}
+readonly LINUX_SRC=https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.18.5.tar.xz
+
+function fetch_source() {
+    if [[ -d ${TARGET_DIR}/linux ]]; then
+        return 0
+    fi
+
+    pushd ${TARGET_DIR}
+
+    echo "Fetching Linux source..."
+    wget ${LINUX_SRC} -O linux.tar.xz
+    tar xf linux.tar.xz
+    mv linux-* linux
+
+    popd
+}
+
+function build_linux() {
+    case ${ARCH} in
+        x86|x86_64|amd64)
+            local arch=x86
+            local image=arch/x86/boot/bzImage
+            ;;
+        arm|arm64|aarch64)
+            local arch=arm64
+            local image=arch/arm64/boot/Image
+            ;;
+    esac
+
+    local kconfig="${TARGET_DIR}/linux/.config"
+
+    cp bootloader/config ${kconfig}
+    cat bootloader/config-${arch} >> ${kconfig}
+    echo 'CONFIG_INITRAMFS_SOURCE="../initramfs/initramfs.cpio"' \
+        >> ${kconfig}
+
+    make -C ${TARGET_DIR}/linux olddefconfig LLVM=1 ARCH=${arch}
+
+    # Validate the configuration
+    sort -b -o ${kconfig}.old.sorted ${kconfig}.old
+    local missing=$(comm -23 ${kconfig}.old.sorted <(sort -b ${kconfig}))
+    if [[ -n ${missing} ]]; then
+        echo "Missing configuration options:"
+        echo "${missing}"
+        exit 1
+    fi
+
+    echo "Building Linux kernel..."
+    make -C ${TARGET_DIR}/linux -j$((2 * $(nproc))) LLVM=1 ARCH=${arch}
+
+    echo "Image: ${PWD}/${TARGET_DIR}/linux/${image}"
+}
+
+
+function build_initramfs() {
+    mkdir -p ${TARGET_DIR}/initramfs
+    pushd ${TARGET_DIR}/initramfs
+
+    go install github.com/u-root/u-root@latest
+    if [[ ! -f go.mod ]]; then
+        go mod init initramfs
+
+        go get github.com/u-root/u-root/cmds/boot/boot
+        go get github.com/u-root/u-root/cmds/core/init
+    fi
+
+    case ${ARCH} in
+        x86|x86_64|amd64)
+            local goarch=amd64
+            local cmdline="console=ttyS0 reboot=acpi"
+            ;;
+        arm|aarch64|arm64)
+            local goarch=arm64
+            local cmdline=console=ttyAMA0
+            ;;
+    esac
+
+    echo "Building initramfs..."
+
+    GOARCH=${goarch} u-root -o initramfs.cpio \
+        -defaultsh '' -uinitcmd "boot -append '${cmdline}'" \
+        github.com/u-root/u-root/cmds/boot/boot \
+        github.com/u-root/u-root/cmds/core/init
+
+    popd
+}
+
+function main() {
+    build_initramfs
+
+    fetch_source
+    build_linux
+}
+
+main

bootloader/config

@@ -0,0 +1,51 @@
+CONFIG_WERROR=y
+# CONFIG_MODULES is not set
+
+CONFIG_SMP=y
+
+CONFIG_DEFAULT_HOSTNAME="bootloader"
+CONFIG_BLK_DEV_INITRD=y
+
+# CONFIG_NET is not set
+
+CONFIG_TMPFS=y
+CONFIG_DEVTMPFS=y
+CONFIG_EXT4_FS=y
+
+CONFIG_RTC_CLASS=y
+CONFIG_HIGH_RES_TIMERS=y
+CONFIG_NO_HZ=y
+
+CONFIG_COMMON_CLK=y
+CONFIG_PCI=y
+CONFIG_PCI_MSI=y
+# CONFIG_USB_SUPPORT is not set
+# CONFIG_HID_SUPPORT is not set
+# CONFIG_VFIO is not set
+
+CONFIG_SERIAL_CORE=y
+CONFIG_SERIAL_CORE_CONSOLE=y
+CONFIG_SERIAL_EARLYCON=y
+
+CONFIG_TTY=y
+# CONFIG_INPUT_KEYBOARD is not set
+# CONFIG_INPUT_MOUSE is not set
+
+CONFIG_PARAVIRT=y
+CONFIG_PARAVIRT_TIME_ACCOUNTING=y
+CONFIG_VIRT_DRIVERS=y
+# CONFIG_VIRTIO_MMIO is not set
+
+CONFIG_VIRTIO_PCI=y
+CONFIG_HW_RANDOM_VIRTIO=y
+CONFIG_VIRTIO_BLK=y
+CONFIG_VIRTIO_CONSOLE=y
+CONFIG_PVPANIC=y
+CONFIG_PVPANIC_PCI=y
+
+CONFIG_PRINTK_TIME=y
+
+CONFIG_KEXEC_CORE=y
+CONFIG_KEXEC_FILE=y
+
+CONFIG_INITRAMFS_COMPRESSION_ZSTD=y

bootloader/config-arm64

@@ -0,0 +1,10 @@
+CONFIG_OF=y
+
+CONFIG_PCI_HOST_COMMON=y
+CONFIG_PCI_HOST_GENERIC=y
+
+CONFIG_SERIAL_AMBA_PL011=y
+CONFIG_SERIAL_AMBA_PL011_CONSOLE=y
+CONFIG_RTC_DRV_PL031=y
+
+CONFIG_CMDLINE="quiet console=ttyAMA0"

bootloader/config-x86

@@ -0,0 +1,16 @@
+CONFIG_KERNEL_ZSTD=y
+
+CONFIG_ACPI=y
+
+# CONFIG_X86_MCE is not set
+CONFIG_ACPI_MADT_WAKEUP=y
+
+CONFIG_HYPERVISOR_GUEST=y
+CONFIG_KVM_GUEST=y
+CONFIG_PARAVIRT_SPINLOCKS=y
+
+CONFIG_SERIAL_8250_CONSOLE=y
+CONFIG_SERIAL_8250=y
+
+CONFIG_CMDLINE_BOOL=y
+CONFIG_CMDLINE="quiet console=ttyS0"