initrd/main.go

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strings"
	"syscall"
	"time"

	yaml "gopkg.in/yaml.v2"
	"novit.nc/direktil/pkg/sysfs"
)

const (
	// VERSION is the current version of init
	VERSION = "Direktil init v0.1"

	rootMountFlags  = 0
	bootMountFlags  = syscall.MS_NOEXEC | syscall.MS_NODEV | syscall.MS_NOSUID | syscall.MS_RDONLY
	layerMountFlags = syscall.MS_RDONLY
)

var (
	bootVersion string
)

type config struct {
	Layers []string `yaml:"layers"`
}

func main() {
	runtime.LockOSThread()

	log.Print("Welcome to ", VERSION)

	// essential mounts
	mount("none", "/proc", "proc", 0, "")
	mount("none", "/sys", "sysfs", 0, "")
	mount("none", "/dev", "devtmpfs", 0, "")

	// get the "boot version"
	bootVersion = param("version", "current")
	log.Printf("booting system %q", bootVersion)

	// find and mount /boot
	bootMatch := param("boot", "")
	if bootMatch != "" {
		bootFS := param("boot.fs", "vfat")
		for i := 0; ; i++ {
			devNames := sysfs.DeviceByProperty("block", bootMatch)

			if len(devNames) == 0 {
				if i > 30 {
					fatal("boot partition not found after 30s")
				}
				log.Print("boot partition not found, retrying")
				time.Sleep(1 * time.Second)
				continue
			}

			devFile := filepath.Join("/dev", devNames[0])

			log.Print("boot partition found: ", devFile)

			mount(devFile, "/boot", bootFS, bootMountFlags, "")
			break
		}
	} else {
		log.Print("Assuming /boot is already populated.")
	}

	// load config
	cfgPath := param("config", "/boot/config.yaml")

	cfgBytes, err := ioutil.ReadFile(cfgPath)
	if err != nil {
		fatalf("failed to read %s: %v", cfgPath, err)
	}

	cfg := &config{}
	if err := yaml.Unmarshal(cfgBytes, cfg); err != nil {
		fatal("failed to load config: ", err)
	}

	// mount layers
	if len(cfg.Layers) == 0 {
		fatal("no layers configured!")
	}

	log.Printf("wanted layers: %q", cfg.Layers)

	lowers := make([]string, len(cfg.Layers))
	for i, layer := range cfg.Layers {
		path := layerPath(layer)

		info, err := os.Stat(path)
		if err != nil {
			fatal(err)
		}

		log.Printf("layer %s found (%d bytes)", layer, info.Size())

		dir := "/layers/" + layer

		lowers[i] = dir

		loopDev := fmt.Sprintf("/dev/loop%d", i)
		losetup(loopDev, path)

		mount(loopDev, dir, "squashfs", layerMountFlags, "")
	}

	// prepare system root
	mount("mem", "/changes", "tmpfs", 0, "")

	mkdir("/changes/workdir", 0755)
	mkdir("/changes/upperdir", 0755)

	mount("overlay", "/system", "overlay", rootMountFlags,
		"lowerdir="+strings.Join(lowers, ":")+",upperdir=/changes/upperdir,workdir=/changes/workdir")
	mount("/boot", "/system/boot", "", syscall.MS_BIND, "")

	// - write configuration
	log.Print("writing /config.yaml")
	if err := ioutil.WriteFile("/system/config.yaml", cfgBytes, 0600); err != nil {
		fatal("failed: ", err)
	}

	// clean zombies
	cleanZombies()

	// switch root
	log.Print("switching root")
	err = syscall.Exec("/sbin/switch_root", []string{"switch_root",
		"-c", "/dev/console", "/system", "/sbin/init"}, os.Environ())
	fatal("switch_root failed: ", err)
}

func layerPath(name string) string {
	return fmt.Sprintf("/boot/%s/layers/%s.fs", bootVersion, name)
}

func fatal(v ...interface{}) {
	log.Print("*** FATAL ***")
	log.Print(v...)
	select {}
}

func fatalf(pattern string, v ...interface{}) {
	log.Print("*** FATAL ***")
	log.Printf(pattern, v...)
	select {}
}

func losetup(dev, file string) {
	run("/sbin/losetup", dev, file)
}

func run(cmd string, args ...string) {
	if output, err := exec.Command(cmd, args...).CombinedOutput(); err != nil {
		fatalf("command %s %q failed: %v\n%s", cmd, args, err, string(output))
	}
}

func mkdir(dir string, mode os.FileMode) {
	if err := os.MkdirAll(dir, mode); err != nil {
		fatalf("mkdir %q failed: %v", dir, err)
	}
}

func mount(source, target, fstype string, flags uintptr, data string) {
	if _, err := os.Stat(target); os.IsNotExist(err) {
		mkdir(target, 0755)
	}

	if err := syscall.Mount(source, target, fstype, flags, data); err != nil {
		fatalf("mount %q %q -t %q -o %q failed: %v", source, target, fstype, data, err)
	}
	log.Printf("mounted %q", target)
}