sysinit.go

系统初始化模块，对外暴露SysInit方法。

SysInit

func SysInit() {
    //# 需要参数
    if len(os.Args) <= 1 {
        fmt.Println("You should not invoke docker-init manually")
        os.Exit(1)
    }
    //# 从参数中找到用户`username or uid`和网关`gateway address`
    var u = flag.String("u", "", "username or uid")
    var gw = flag.String("g", "", "gateway address")

    flag.Parse()

    //# 设置网络，改变用户，启动守护进程
    //# 如果`gw` != "", 则使用系统命令`/sbin/route add default gw GW`
    //# 设置网关
    setupNetworking(*gw)
    //# 降低权限以便照顾普通用户
    //# 如果·u· != "", 则通过`Setuid`，`Setgid`设置权限。
    changeUser(*u)
    //# 运行程序(Deamon/Debug 模式)
    executeProgram(flag.Arg(0), flag.Args())
}

setupNetworking

// 设置网关
func setupNetworking(gw string) {
    if gw == "" {
        return
    }
    cmd := exec.Command("/sbin/route", "add", "default", "gw", gw)
    if err := cmd.Run(); err != nil {
        log.Fatalf("Unable to set up networking: %v", err)
    }
}

changeUser

// 降低权限以便照顾普通用户
func changeUser(u string) {
    if u == "" {
        return
    }
    userent, err := user.LookupId(u)
    if err != nil {
        userent, err = user.Lookup(u)
    }
    if err != nil {
        log.Fatalf("Unable to find user %v: %v", u, err)
    }

    uid, err := strconv.Atoi(userent.Uid)
    if err != nil {
        log.Fatalf("Invalid uid: %v", userent.Uid)
    }
    gid, err := strconv.Atoi(userent.Gid)
    if err != nil {
        log.Fatalf("Invalid gid: %v", userent.Gid)
    }

    if err := syscall.Setgid(gid); err != nil {
        log.Fatalf("setgid failed: %v", err)
    }
    if err := syscall.Setuid(uid); err != nil {
        log.Fatalf("setuid failed: %v", err)
    }
}

executeProgram

func executeProgram(name string, args []string) {
    path, err := exec.LookPath(name)
    if err != nil {
        log.Printf("Unable to locate %v", name)
        os.Exit(127)
    }

    if err := syscall.Exec(path, args, os.Environ()); err != nil {
        panic(err)
    }
}