Android系统启动之Init流程(下)

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目录

第一篇:Android系统启动之bootloader
第二篇:Android系统启动之Init流程(上)
第三篇:Android系统启动之Init流程(下)
第四篇:Android系统启动之init.rc文件解析过程
第五篇:Android系统启动之zyogte进程
第六篇:Android系统启动之zyogte进程java(上)
第七篇:Android系统启动之zyogte进程java(下)
第八篇:Android系统启动之SystemServer

本节主要是对代码进行解释,十分枯燥.O(∩_∩)O哈哈~

下面开始

启动代码(main)

主要分为七部分:

第一部分

判断启动部分,如果是ueventd,调用ueventd_main主函数,如果是watchdogd,调用watchdogd_main主函数.

第二部分

add_environment导入环境变量,并根据环境变量判断是否是第一次启动.

第三部分

创建一些基本的目录,包括/dev、/porc、/sysfc等。同时把一些文件系统,如tmpfs、devpt、proc、sysfs等mount到项目的目录。

目录 功能
tmpfs 一种基于内存的文件系统,mount后就可以使用。tmpfs文件系统下的文件都存放在内存中,访问速度快,但是关机后所有内容偶读会丢失,因此tmpfs文件系统比较合适存放一些临时性的文件。
devpts 虚拟终端文件系统,它通常mount在目录dev/pts下
proc 一种基于内存的虚拟文件系统,它可以看作是内核内部数据结构的接口,通过它可以获得系统的信息,同时能够在运行时修改特定的内核参数
sysfs proc文件系统类似,它是Linux2.6内核引入的,作用是把系统的设备和总线按层次组织起来,使得它们可以在用户空间存取

然后使用InitKernelLogging开启log,使得init进程可以使用kernel的log系统来输出log.

为什么要使用kernel的log系统?

因为此时Android系统的log还没有启动,所以需要使用kernel的log系统.

第四部分

SELinex的知识参考android之SELinux小记
主要使用函数selinux_initialize启动SELinux.

另外:

close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));

判断/dev/.booting文件是否可读写和创建.

在/dev目录下创建一个空文件".booting"表示初始化正在进行
is_booting()函数会依靠空文件".booting"来判断是否进程处于初始化中,初始化结束后,这个文件会被删除

第五部分

初始化系统属性存储区域:

property_init();

property_init函数在system/core/init/property_service.cpp实现:

  void property_init() {
      if (__system_property_area_init()) {
          LOG(ERROR) << "Failed to initialize property area";
          exit(1);
      }
  }

然后,设定内核处理命令行.
并设定相关系统属性export_kernel_boot_props

  static void export_kernel_boot_props() {
      struct {
          const char *src_prop;
          const char *dst_prop;
          const char *default_value;
      } prop_map[] = {
          { "ro.boot.serialno",   "ro.serialno",   "", },
          { "ro.boot.mode",       "ro.bootmode",   "unknown", },
          { "ro.boot.baseband",   "ro.baseband",   "unknown", },
          { "ro.boot.bootloader", "ro.bootloader", "unknown", },
          { "ro.boot.hardware",   "ro.hardware",   "unknown", },
          { "ro.boot.revision",   "ro.revision",   "0", },
      };
      for (size_t i = 0; i < arraysize(prop_map); i++) {
          std::string value = GetProperty(prop_map[i].src_prop, "");
          property_set(prop_map[i].dst_prop, (!value.empty()) ? value : prop_map[i].defau     lt_value);
      }
  }

export_kernel_boot_props这个函数,它就是设置一些属性,设置ro属性根据之前的ro.boot这类的属性值,如果没有设置成unknown,像之前我们有ro.boot.hardware,那我们就可以设置root.hardware这样的属性。

第六部分

  1. 调用epoll_create1创建epoll句柄,如果创建失败,则退出。
  2. 调用signal_handler_init()函数,装载进程信号处理器。
  3. 调用property_load_boot_defaults()函数解析根目录的default.prop的属性,设置默认属性配置的相关工作。
  4. 调用start_prperty_service()函数,启动属性服务,并接受属性的socket的fd加入到epoll中,定义了处理函数。
  5. 解析rc文件(重要).参考:Android系统启动之init.rc文件解析过程

signal_handler_init函数主要是当子进程被kill之后,会在父进程接受一个信号。

处理这个信号的时候往sockpair一段写数据,而另一端的fd是加入epoll中

init是一个守护进程,为了防止init的子进程称为僵尸进程(zombie process),需要init在子进程结束时获取子进程的结束码,通过结束码将程序表中的子进程移除,防止称为僵尸进程的子进程占用程序表的空间(程序表的空间达到上线时,系统就不能再启动新的进城了,会引起严重的系统问题)。

第七部分

启动守护进程

源码如下

int main(int argc, char** argv) {
//---------------------第一部分------------------------------------
// 根据传入的参数,运行不同的主函数
//----------------------------------------------------------------------
    //匹配启动程序名
    if (!strcmp(basename(argv[0]), "ueventd")) {
        return ueventd_main(argc, argv);
    }

    if (!strcmp(basename(argv[0]), "watchdogd")) {
        return watchdogd_main(argc, argv);
    }

    if (REBOOT_BOOTLOADER_ON_PANIC) {
        InstallRebootSignalHandlers();
    }
//---------------------第二部分--------------------------------------
// 设定环境变量
//----------------------------------------------------------------------
    //设定环境变量
    add_environment("PATH", _PATH_DEFPATH);

    bool is_first_stage = (getenv("INIT_SECOND_STAGE") == nullptr);

    //判断是否是第一次
    if (is_first_stage) {
        boot_clock::time_point start_time = boot_clock::now();

        // Clear the umask.
        // 清楚权限掩码
        umask(0);
//----------------------第三部分-------------------------------------
// 设定文件目录并挂载对应的设备
//----------------------------------------------------------------------
        // Get the basic filesystem setup we need put together in the initramdisk
        // on / and then we'll let the rc file figure out the rest.
        // 创建文件系统和对应的权限,并挂载
        mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755");
        mkdir("/dev/pts", 0755);
        mkdir("/dev/socket", 0755);
        mount("devpts", "/dev/pts", "devpts", 0, NULL);
        #define MAKE_STR(x) __STRING(x)
        mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC));
        // Don't expose the raw commandline to unprivileged processes.
        chmod("/proc/cmdline", 0440);
        gid_t groups[] = { AID_READPROC };
        setgroups(arraysize(groups), groups);
        mount("sysfs", "/sys", "sysfs", 0, NULL);
        mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL);
        mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11));
        mknod("/dev/random", S_IFCHR | 0666, makedev(1, 8));
        mknod("/dev/urandom", S_IFCHR | 0666, makedev(1, 9));

        // Now that tmpfs is mounted on /dev and we have /dev/kmsg, we can actually
        // talk to the outside world...
        //初始化log
        InitKernelLogging(argv);

        LOG(INFO) << "init first stage started!";

        if (!DoFirstStageMount()) {
            LOG(ERROR) << "Failed to mount required partitions early ...";
            panic();
        }

        SetInitAvbVersionInRecovery();
//-----------------------第四部分-------------------------------------
// 启动SELinux,根据SELinux的配置重新启动init
//----------------------------------------------------------------------
        // Set up SELinux, loading the SELinux policy.
        // 设置SELinux,加载SEPolicy
        selinux_initialize(true);

        // We're in the kernel domain, so re-exec init to transition to the init domain now
        // that the SELinux policy has been loaded.
        // 根据SELinux的要求重新设定init文件属性
        if (restorecon("/init") == -1) {
            PLOG(ERROR) << "restorecon failed";
            security_failure();
        }

        setenv("INIT_SECOND_STAGE", "true", 1);

        static constexpr uint32_t kNanosecondsPerMillisecond = 1e6;
        uint64_t start_ms = start_time.time_since_epoch().count() / kNanosecondsPerMillisecond;
        setenv("INIT_STARTED_AT", StringPrintf("%" PRIu64, start_ms).c_str(), 1);


        //设定参数
        char* path = argv[0];
        char* args[] = { path, nullptr };
        execv(path, args);

        // execv() only returns if an error happened, in which case we
        // panic and never fall through this conditional.
        PLOG(ERROR) << "execv(\"" << path << "\") failed";
        security_failure();
    }

    // At this point we're in the second stage of init.
    InitKernelLogging(argv);
    LOG(INFO) << "init second stage started!";

    // Set up a session keyring that all processes will have access to. It
    // will hold things like FBE encryption keys. No process should override
    // its session keyring.
    keyctl_get_keyring_ID(KEY_SPEC_SESSION_KEYRING, 1);

    // Indicate that booting is in progress to background fw loaders, etc.
    close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));
//-----------------------第五部分-------------------------------------
// 运行属性服务,根据属性值设定内核命令
//----------------------------------------------------------------------
    //设定属性值
    property_init();

    // If arguments are passed both on the command line and in DT,
    // properties set in DT always have priority over the command-line ones.
    process_kernel_dt();
    process_kernel_cmdline();

    // Propagate the kernel variables to internal variables
    // used by init as well as the current required properties.
    export_kernel_boot_props();

    // Make the time that init started available for bootstat to log.
    property_set("ro.boottime.init", getenv("INIT_STARTED_AT"));
    property_set("ro.boottime.init.selinux", getenv("INIT_SELINUX_TOOK"));

    // Set libavb version for Framework-only OTA match in Treble build.
    const char* avb_version = getenv("INIT_AVB_VERSION");
    if (avb_version) property_set("ro.boot.avb_version", avb_version);

    // Clean up our environment.
    unsetenv("INIT_SECOND_STAGE");
    unsetenv("INIT_STARTED_AT");
    unsetenv("INIT_SELINUX_TOOK");
    unsetenv("INIT_AVB_VERSION");

    // Now set up SELinux for second stage.
    selinux_initialize(false);
    selinux_restore_context();
//-----------------------第六部分------------------------------------
// 启动服务,并解析rc文件,根据rc文件启动进程
//----------------------------------------------------------------------
    epoll_fd = epoll_create1(EPOLL_CLOEXEC);
    if (epoll_fd == -1) {
        PLOG(ERROR) << "epoll_create1 failed";
        exit(1);
    }

    signal_handler_init();

    property_load_boot_defaults();
    export_oem_lock_status();
    start_property_service();
    set_usb_controller();

    const BuiltinFunctionMap function_map;
    Action::set_function_map(&function_map);

    Parser& parser = Parser::GetInstance();
    parser.AddSectionParser("service",std::make_unique<ServiceParser>());
    parser.AddSectionParser("on", std::make_unique<ActionParser>());
    parser.AddSectionParser("import", std::make_unique<ImportParser>());
    std::string bootscript = GetProperty("ro.boot.init_rc", "");
    if (bootscript.empty()) {
        parser.ParseConfig("/init.rc");
        parser.set_is_system_etc_init_loaded(
                parser.ParseConfig("/system/etc/init"));
        parser.set_is_vendor_etc_init_loaded(
                parser.ParseConfig("/vendor/etc/init"));
        parser.set_is_odm_etc_init_loaded(parser.ParseConfig("/odm/etc/init"));
    } else {
        parser.ParseConfig(bootscript);
        parser.set_is_system_etc_init_loaded(true);
        parser.set_is_vendor_etc_init_loaded(true);
        parser.set_is_odm_etc_init_loaded(true);
    }

    // Turning this on and letting the INFO logging be discarded adds 0.2s to
    // Nexus 9 boot time, so it's disabled by default.
    if (false) parser.DumpState();

    ActionManager& am = ActionManager::GetInstance();

    am.QueueEventTrigger("early-init");

    // Queue an action that waits for coldboot done so we know ueventd has set up all of /dev...
    am.QueueBuiltinAction(wait_for_coldboot_done_action, "wait_for_coldboot_done");
    // ... so that we can start queuing up actions that require stuff from /dev.
    am.QueueBuiltinAction(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
    am.QueueBuiltinAction(set_mmap_rnd_bits_action, "set_mmap_rnd_bits");
    am.QueueBuiltinAction(set_kptr_restrict_action, "set_kptr_restrict");
    am.QueueBuiltinAction(keychord_init_action, "keychord_init");
    am.QueueBuiltinAction(console_init_action, "console_init");

    // Trigger all the boot actions to get us started.
    am.QueueEventTrigger("init");

    // Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random
    // wasn't ready immediately after wait_for_coldboot_done
    am.QueueBuiltinAction(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");

    // Don't mount filesystems or start core system services in charger mode.
    std::string bootmode = GetProperty("ro.bootmode", "");
    if (bootmode == "charger") {
        am.QueueEventTrigger("charger");
    } else {
        am.QueueEventTrigger("late-init");
    }

    // Run all property triggers based on current state of the properties.
    am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");
//---------------------第七部分--------------------------------------
// 启动结束,开始守护服务(守护进程)
//----------------------------------------------------------------------
    while (true) {
        // By default, sleep until something happens.
        int epoll_timeout_ms = -1;

        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            am.ExecuteOneCommand();
        }
        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            restart_processes();

            // If there's a process that needs restarting, wake up in time for that.
            if (process_needs_restart_at != 0) {
                epoll_timeout_ms = (process_needs_restart_at - time(nullptr)) * 1000;
                if (epoll_timeout_ms < 0) epoll_timeout_ms = 0;
            }

            // If there's more work to do, wake up again immediately.
            if (am.HasMoreCommands()) epoll_timeout_ms = 0;
        }

        epoll_event ev;
        int nr = TEMP_FAILURE_RETRY(epoll_wait(epoll_fd, &ev, 1, epoll_timeout_ms));
        if (nr == -1) {
            PLOG(ERROR) << "epoll_wait failed";
        } else if (nr == 1) {
            ((void (*)()) ev.data.ptr)();
        }
    }

    return 0;
}

参考

Android系统启动——2init进程

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