Makefile

# Sources
MUSL_SOURCE := https://git.musl-libc.org/git/musl
BUSYBOX_SOURCE := https://git.busybox.net/busybox
LINUX_SOURCE := https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

# Configuration
BUSYBOX_CONFIG := busybox.config
ROOTFS_OVERLAYDIR := rootfs-overlay
LINUX_CONFIG := linux.config
BOOTFS_LABEL := BOOT
BOOTFS_SIZE := 1440
BOOTFS_CREATE_GPT := 0
QEMU := qemu-system-x86_64
QEMUFLAGS := -bios /usr/share/ovmf/x64/OVMF.fd -smp 2

# Temporary directories
ROOTFS_DIR := rootfs
BOOTFS_MOUNTDIR := floppy

# Recipe targets
MUSL_DIR := musl
MUSL_BUILD_DIR := musl-build
MUSL_GCC := $(MUSL_BUILD_DIR)/bin/musl-gcc
BUSYBOX_DIR := busybox
BUSYBOX := $(BUSYBOX_DIR)/busybox
ROOTFS_CPIO := rootfs.cpio
LINUX_DIR := linux
LINUX_HEADERS_DIR := linux-headers
LINUX_BZIMAGE := $(LINUX_DIR)/arch/x86/boot/bzImage
BOOTFS_IMAGE := floppy.img

.PHONY: all configure source update clean reset
all: $(BOOTFS_IMAGE)
configure: musl-configure busybox-configure linux-configure
source: $(MUSL_DIR) $(BUSYBOX_DIR) $(LINUX_DIR)
update: musl-update busybox-update linux-update
clean: musl-clean busybox-clean rootfs-clean linux-clean bootfs-clean
reset:
	rm -rf '$(MUSL_DIR)' '$(MUSL_BUILD_DIR)' '$(BUSYBOX_DIR)' '$(LINUX_DIR)' \
		'$(LINUX_HEADERS_DIR)' '$(ROOTFS_DIR)' '$(ROOTFS_CPIO)' \
		'$(BOOTFS_MOUNTDIR)' '$(BOOTFS_IMAGE)'

$(MUSL_DIR):
	git clone --depth=1 '$(MUSL_SOURCE)' '$@'

$(MUSL_BUILD_DIR): | $(LINUX_HEADERS_DIR)
	rm -rf '$@'

	mkdir '$@' \
		'$@'/bin \
		'$@'/include \
		'$@'/lib

# These symlinks ensure that programs and headers required for building BusyBox
# under musl are present (besides musl-gcc).
# Refer: https://www.openwall.com/lists/musl/2014/08/08/13
	ln -s "$$(which ar)" '$@'/bin/musl-ar
	ln -s "$$(which strip)" '$@'/bin/musl-strip
	ln -s '../../$(LINUX_HEADERS_DIR)/include/linux' '$@'/include/linux
	ln -s '../../$(LINUX_HEADERS_DIR)/include/mtd' '$@'/include/mtd
	ln -s '../../$(LINUX_HEADERS_DIR)/include/asm' '$@'/include/asm
	ln -s '../../$(LINUX_HEADERS_DIR)/include/asm-generic' '$@'/include/asm-generic

$(MUSL_GCC): | $(MUSL_DIR) $(MUSL_BUILD_DIR)
	$(MAKE) -C '$(MUSL_DIR)' -j "$$(nproc)" all
	$(MAKE) -C '$(MUSL_DIR)' install

.PHONY: musl-update
musl-update: $(MUSL_DIR)
	git -C '$<' pull

.PHONY: musl-configure
musl-configure: $(MUSL_DIR)
	cd '$<' && ./configure --prefix=../'$(MUSL_BUILD_DIR)'

.PHONY: musl-clean
musl-clean:
	[ -d '$(MUSL_DIR)' ] && $(MAKE) -C '$(MUSL_DIR)' distclean
	rm -rf '$(MUSL_BUILD_DIR)'

$(BUSYBOX_DIR):
	git clone --depth=1 '$(BUSYBOX_SOURCE)' '$@'

$(BUSYBOX): $(BUSYBOX_DIR)/.config $(MUSL_GCC) | $(BUSYBOX_DIR)
	$(MAKE) -C '$(<D)' -j "$$(nproc)" CC=../$(MUSL_GCC) all

.PHONY: busybox-update
busybox-update: $(BUSYBOX_DIR)
	git -C '$<' pull

.PHONY: busybox-configure
busybox-configure: $(BUSYBOX_DIR)
	cp '$(BUSYBOX_CONFIG)' '$<'/.config
	$(MAKE) -C '$<' CC=../$(MUSL_GCC) oldconfig

.PHONY: busybox-clean
busybox-clean:
	[ -d '$(BUSYBOX_DIR)' ] && $(MAKE) -C '$(BUSYBOX_DIR)' distclean

# https://www.kernel.org/doc/Documentation/admin-guide/devices.txt
define _ROOTFS_BUILD_CMDS :=
set -e
rm -rf '$(ROOTFS_DIR)'
mkdir '$(ROOTFS_DIR)' '$(ROOTFS_DIR)'/root '$(ROOTFS_DIR)'/bin '$(ROOTFS_DIR)'/sbin '$(ROOTFS_DIR)'/dev '$(ROOTFS_DIR)'/proc '$(ROOTFS_DIR)'/etc '$(ROOTFS_DIR)'/sys '$(ROOTFS_DIR)'/tmp
$(MAKE) -C '$(BUSYBOX_DIR)' CC=../'$(MUSL_GCC)' CONFIG_PREFIX=../'$(ROOTFS_DIR)' install
mknod '$(ROOTFS_DIR)'/dev/null c 1 3
mknod '$(ROOTFS_DIR)'/dev/zero c 1 5
mknod '$(ROOTFS_DIR)'/dev/random c 1 8
mknod '$(ROOTFS_DIR)'/dev/urandom c 1 9
mknod '$(ROOTFS_DIR)'/dev/tty0 c 4 0
mknod '$(ROOTFS_DIR)'/dev/tty1 c 4 1
mknod '$(ROOTFS_DIR)'/dev/tty2 c 4 2
mknod '$(ROOTFS_DIR)'/dev/tty3 c 4 3
mknod '$(ROOTFS_DIR)'/dev/tty c 5 0
ln -s '/proc/self/fd' '$(ROOTFS_DIR)'/dev/fd
ln -s '/proc/self/fd/0' '$(ROOTFS_DIR)'/dev/stdin
ln -s '/proc/self/fd/1' '$(ROOTFS_DIR)'/dev/stdout
ln -s '/proc/self/fd/2' '$(ROOTFS_DIR)'/dev/stderr
cp -rfv '$(ROOTFS_OVERLAYDIR)'/* '$(ROOTFS_DIR)'
cd '$(ROOTFS_DIR)' && find . | cpio -H newc -o > ../'$(ROOTFS_CPIO)'
rm -rf '$(ROOTFS_DIR)'
endef
export ROOTFS_BUILD_CMDS := $(value _ROOTFS_BUILD_CMDS)
$(ROOTFS_CPIO): $(BUSYBOX) $(ROOTFS_OVERLAYDIR)
# We don't need actual root privileges for this operation.
	echo "$${ROOTFS_BUILD_CMDS}" | fakeroot

.PHONY: rootfs-clean
rootfs-clean:
	rm -rf '$(ROOTFS_DIR)' '$(ROOTFS_CPIO)'

$(LINUX_DIR):
	git clone --depth=1 '$(LINUX_SOURCE)' '$@'

$(LINUX_HEADERS_DIR): | $(LINUX_DIR)
	$(MAKE) -C '$(LINUX_DIR)' \
		headers_install INSTALL_HDR_PATH=../'$@'

$(LINUX_BZIMAGE): $(LINUX_DIR)/.config $(ROOTFS_CPIO) | $(LINUX_DIR)
	$(MAKE) -C '$(<D)' -j "$$(nproc)" bzImage

.PHONY: linux-update
linux-update: $(LINUX_DIR)
	git -C '$<' pull

.PHONY: linux-configure
linux-configure: $(LINUX_DIR)
	cp '$(LINUX_CONFIG)' '$<'/.config
	$(MAKE) -C '$<' oldconfig

.PHONY: linux-clean
linux-clean:
	[ -d '$(LINUX_DIR)' ] && $(MAKE) -C '$(LINUX_DIR)' distclean
	rm -rf '$(LINUX_HEADERS_DIR)'

define _BOOTFS_BUILD_CMDS :=
set -e
losetup -j '$(BOOTFS_IMAGE)' | cut -d ':' -f 1 | xargs -r losetup -d
if [ '$(BOOTFS_CREATE_GPT)' = 1 ]; then
  sgdisk -a 2 -N -t 1:EF00
  loopdev="$$(losetup -P -f '$(BOOTFS_IMAGE)' --show)"p1
else
  loopdev="$$(losetup -f '$(BOOTFS_IMAGE)' --show)"
fi
mkfs.fat -F 12 -n '$(BOOTFS_LABEL)' "$${loopdev}"
mountpoint -q '$(BOOTFS_MOUNTDIR)' && umount '$(BOOTFS_MOUNTDIR)'
mount -m "$${loopdev}" '$(BOOTFS_MOUNTDIR)'
mkdir -p '$(BOOTFS_MOUNTDIR)'/EFI/BOOT
cleanup() {
  umount '$(BOOTFS_MOUNTDIR)'
  rmdir '$(BOOTFS_MOUNTDIR)'
  losetup -d "$${loopdev%p1}"
  exit "$$1"
}
cp '$(LINUX_BZIMAGE)' '$(BOOTFS_MOUNTDIR)'/EFI/BOOT/BOOTX64.EFI || cleanup 1
cleanup 0
endef
export BOOTFS_BUILD_CMDS := $(value _BOOTFS_BUILD_CMDS)
$(BOOTFS_IMAGE): $(LINUX_BZIMAGE)
	dd if=/dev/zero of='$@' \
		bs=1024 count='$(BOOTFS_SIZE)' conv=fsync
# losetup, mount and mkfs.fat likely require actual root privileges.
	echo "$${BOOTFS_BUILD_CMDS}" | sudo -s

.PHONY: bootfs-clean
bootfs-clean:
	rm -f '$(BOOTFS_IMAGE)'

.PHONY: runqemu-bzImage
runqemu-bzImage:
	$(QEMU) $(QEMUFLAGS) -kernel '$(LINUX_BZIMAGE)'

.PHONY: runqemu-floppy.img
runqemu-$(BOOTFS_IMAGE):
# QEMU's UEFI implementation doesn't seem to support booting from an internal
# FDD, so we simulate an external USB-connected drive instead.
	$(QEMU) $(QEMUFLAGS) \
		-drive id=usbstick,if=none,file='$(BOOTFS_IMAGE)',format=raw \
		-usb \
		-device usb-ehci,id=ehci \
		-device usb-storage,bus=ehci.0,drive=usbstick,bootindex=1