Android init进程

android源码学习目录

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());    parser.AddSectionParser("on", std::make_unique());    parser.AddSectionParser("import", std::make_unique());    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。

android源码学习目录

目录

介绍

一:Init进程启动前的准备

二：Init进程

2.1：init进程的入口函数

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