android源码学习目录

目录

1. android init进程
2. init进程--属性服务器
3. init进程--init.rc解析与zynote进程启动

介绍

Android init进程是Android系统中用户空间的第一个进程，他有极多工作，例如启动属性服务器和启动Zygote进程，init进程由多个源码组成，位于system/core/init文件夹中。

一:Init进程启动前的准备

Android系统启动过程：介绍了init进程前是linux内核启动，后会寻找init.rc文件进行加载，这时并没有对init.rc进行解析。后启动init进程。

二：Init进程

Iinit进程做了很多工作，主要用来初始化和启动属性服务器，并启动Zygote(孵化器)进程

2.1：init进程的入口函数

/**
* Android 8.0 init进程main函数
**/
int main(int argc, char** argv) {
    if (!strcmp(basename(argv[0]), "ueventd")) {
        return ueventd_main(argc, argv);  //根据参数启动ueventd，关于硬件设备
    }

    if (!strcmp(basename(argv[0]), "watchdogd")) {
        return watchdogd_main(argc, argv); //根据参数启动watchdogd，关于程序监控
    }

    if (REBOOT_BOOTLOADER_ON_PANIC) {
        install_reboot_signal_handlers(); //若是紧急启动则安装对应的消息处理器
    }

    add_environment("PATH", _PATH_DEFPATH); //添加环境变量

    bool is_first_stage = (getenv("INIT_SECOND_STAGE") == nullptr);//获取环境变量是否第一次启动

    if (is_first_stage) { //第一次开启手机
        /*************init的第一个阶段*********/
        /* 用户为root用户
         * 1>创建文件系统目录并挂载相关文件系统
         * 2>重定向输入输出内核log系统
         * 3>挂载一些分区设备
         * 4>完成SELinux相关工作
         * 5>重新启动init进程
         */
        boot_clock::time_point start_time = boot_clock::now(); //用于记录启动时间

        // Clear the umask.
        umask(0);  //清除屏蔽字，保证新建的目录访问权限不受屏蔽影响

        // 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.
        mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755");  //挂载tmpfs文件系统
        mkdir("/dev/pts", 0755);
        mkdir("/dev/socket", 0755);
        mount("devpts", "/dev/pts", "devpts", 0, NULL);  //挂载devpts文件系统
        #define MAKE_STR(x) __STRING(x)
        mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC)); //挂载proc文件系统
        // Don't expose the raw commandline to unprivileged processes.
        chmod("/proc/cmdline", 0440);     //改变收紧cmdline目录权限，8.0新增
        
        gid_t groups[] = { AID_READPROC };
        setgroups(arraysize(groups), groups);  //增加一个用户组，8.0新增
        
        mount("sysfs", "/sys", "sysfs", 0, NULL);    //挂载sysfs文件系统
        mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL);  //挂载selinuxfs,8.0新增
        mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11));  //提前创建kmsg设备节点，用于输出log
        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...
        InitKernelLogging(argv);    //在android内核中重定向输入输出log系统

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

        if (!DoFirstStageMount()) {   //挂载特定的分区设备
            LOG(ERROR) << "Failed to mount required partitions early ...";
            panic();  //挂载不成功，panic尝试reboot
        }

        SetInitAvbVersionInRecovery();

        // Set up SELinux, loading the SELinux policy.
        selinux_initialize(true);  //初始化SELinux，

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

        setenv("INIT_SECOND_STAGE", "true", 1); //第一次启动必要的设置已经完成，

        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); //记录初始化时的时间

        char* path = argv[0];
        char* args[] = { path, nullptr };
        execv(path, args);  //再次执行init的main函数，这次init进程为用户状态，不在需要创建各种系统

        // 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();   //重新执行init的main函数失败，重启reboot
    }

    /***************init第二阶段，重启后用户为user********************/
    /* 1>重定向输入输出kernel log系统给
     * 2>初始化属性服务器， 重要
     * 3>清除用过的环境变量
     * 4>完成SELinux相关工作
     * 5>创建epoll句柄
     * 6>装载子进程信号处理器
     * 7>启动属性服务器
     * 8>匹配命令和函数之间的对应关系。
     */
    // At this point we're in the second stage of init.
    InitKernelLogging(argv);   //屏蔽标准的输入输出，并重定向输入输出kernel log系统
    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(KEYCTL_GET_KEYRING_ID, KEY_SPEC_SESSION_KEYRING, 1); //设置安全相关的值。

    // Indicate that booting is in progress to background fw loaders, etc.
    close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));

    property_init();  //1 初始化属性服务器

    // 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();  //处理内核命令

    // 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.  //清空环境变量
    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();    //再次完成SELinux的相关工作

    epoll_fd = epoll_create1(EPOLL_CLOEXEC); //创建epoll句柄
    if (epoll_fd == -1) {
        PLOG(ERROR) << "epoll_create1 failed";
        exit(1);
    }

    signal_handler_init();  //装在子进程信号处理器

    property_load_boot_defaults(); //进行默认的属性配置相关工作
    export_oem_lock_status();       //最终决定ro.boot.fllash.locked的值
    start_property_service();       //2  启动属性服务器
    set_usb_controller();

    //匹配命令和函数之间的对应关系
    const BuiltinFunctionMap function_map; //system/core/init/builtins.cpp，定义Action中的function_map为BuiltinFuntionMap
    Action::set_function_map(&function_map); //在Action中保存的function_map队形，记录了命令和函数之间的关系

    /****************init第三阶段************************/
    /* 1>>构建解析init.rc等文件的解析器对象
     * 2>解析init.rc文件
     */
    
    Parser& parser = Parser::GetInstance();  //构建解析init.rc等文件的解析器对象
    //为解析器增加解析能力，增加一个ServiceParser对应Serrvice,on对应ActionParser,import--importParse
    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", ""); //判断bootScript是否存在
    if (bootscript.empty()) {
        parser.ParseConfig("/init.rc");  //解析init.rc文件
        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); //如果存在bootscript则解析
        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();

    /******************init进程第四阶段***********************/
    /* 1>通过am对命令执行顺序进行控制
     * 2>向am中添加执行action
     * 3>执行命令
     */
    
    ActionManager& am = ActionManager::GetInstance();

    am.QueueEventTrigger("early-init"); //添加触发器early-init,执行on early-init内容

    // 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");   //添加触发器init,执行on init内容

    // 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");   //充电模式下运行 on charger
    } else {
        am.QueueEventTrigger("late-init"); //非充电模式下运行 on late-init
    }

    // Run all property triggers based on current state of the properties.
    am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");

    while (true) {
        // By default, sleep until something happens.
        int epoll_timeout_ms = -1;

        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            am.ExecuteOneCommand();   //依次执行action中的command
        }
        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            restart_processes(); // 重启一些挂掉的进程

            // 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; // 有action待处理，不等待
            }

            // 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) {
            //有事件到来，执行对应处理函数
            //根据上文知道，epoll句柄（即epoll_fd）主要监听子进程结束，及其它进程设置系统属性的请求
            ((void (*)()) ev.data.ptr)();
        }
    }

    return 0;
}

init main函数做了很多工作比较复杂，在开始时进行了文件系统的创建和挂载，对于一个系统我们不需要了解那么详细，我们只需要关注几点就可以了。

• init进程--属性服务器
• init进程--init.rc解析与zynote进程启动
• signal_handler_init()函数用于设置子进程，定义在system/core/init/signale_handler.cpp中，主要用于防止init进程的子进程成为僵尸进程， 当子进程暂停或者终止会发出SIGCHLD信号，signal_handler_init函数会接受这个信号.
  eg：如果init进程的子进程zynote进程终止了，signal_handler_init函数会收到终止信号，并找到zynote进程，移除所有zynote进程的所有消息，在根据配置(是否需要重启)重启zynote进程。