linux启动第一个应用程序init && init启动android过程解析 && init.rc与inittab的关系 && android property和linux 环境变量

linux启动第一个应用程序init
linux的运行顺序为uboot传递参数到内核，内核的第一个c编写的函数为start_kernel()，start_kernel来启动内核，最后到到rest_init()函数处完成内核启动过程。
rest_init()中启动第一个应用程序init，init程序的进程号为1，Linux使用了init进程来对组成Linux的服务和应用程序进行初始化。
init进程还负责了：
Linux运行时：init进程会负责收取孤儿进程。如果某个进程创建子进程之后，在子进程终止之前终止，则子进程成为孤儿进程。在Linux中所有的进程必须属于单棵进程树，所以孤立进程必须被收取。一旦进程成为孤儿，它会立即成为init进程的子进程。这是为了保持进程树的完整性。
系统关闭：init负责杀死所有其它的进程，卸载所有的文件系统并停止处理器的工作，以及任何其它被配置成要做的工作。
等等很多使命
完成linux初始化的rest_init()函数中会调用kernel_init，kernel_init函数代码如下：

kernel\init\main.c
static int __ref kernel_init(void *unused)
{
	int ret;
	kernel_init_freeable();
	/* need to finish all async __init code before freeing the memory */
	async_synchronize_full();
	free_initmem();
	mark_readonly();
	system_state = SYSTEM_RUNNING;
	numa_default_policy();
	flush_delayed_fput();
	if (ramdisk_execute_command) {
		ret = run_init_process(ramdisk_execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d)\n",
		       ramdisk_execute_command, ret);
	}
	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are
	 * trying to recover a really broken machine.
	 */
	if (execute_command) {
		ret = run_init_process(execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
			execute_command, ret);
	}
	if (!try_to_run_init_process("/sbin/init") ||
	    !try_to_run_init_process("/etc/init") ||
	    !try_to_run_init_process("/bin/init") ||
	    !try_to_run_init_process("/bin/sh"))
		return 0;

	panic("No working init found.  Try passing init= option to kernel. "
	      "See Linux Documentation/init.txt for guidance.");
}

其中最重要的函数就是try_to_run_init_process，run_init_process切换到用户空间，运行/init 第一个可执行程序
其中“/init”的来源是在kernel_init_freeable函数中赋值到ramdisk_execute_command变量中的。
try_to_run_init_process一旦执行就不会再返回到此函数中了，而是作为linux 1号进程长期存活，直至关机时它关掉其他所有进行最后才会退出。
android系统中linux 1号进程init的源码在system\core\init\init.cpp( 注：linux kernel源码中没有init进程的代码，而是在根文件系统之中，嵌入式中一般使用了busybox中含有的init.c)

system\core\init\init.cpp
int main(int argc, char** argv) {
    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) {
        install_reboot_signal_handlers();
    }
    add_environment("PATH", _PATH_DEFPATH);
    bool is_first_stage = (getenv("INIT_SECOND_STAGE") == nullptr);
    if (is_first_stage) {////////////////////////////////////////////////////////////////////初始化第一阶段
        boot_clock::time_point start_time = boot_clock::now();
        // Clear the umask.
        umask(0);
        // 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");////////////////////////////挂载相关的文件系统
        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...
        InitKernelLogging(argv);
        LOG(INFO) << "init first stage started!";
        if (!DoFirstStageMount()) {
            LOG(ERROR) << "Failed to mount required partitions early ...";
            panic();
        }
        SetInitAvbVersionInRecovery();
        // Set up SELinux, loading the SELinux policy.
        selinux_initialize(true);///////////////////////////////////////////////seLinux初始化
        // 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) {
            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);
        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!";////////////////////////////////////////初始化第二阶段
    // 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);
    // Indicate that booting is in progress to background fw loaders, etc.
    close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));
    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();
    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");/////////////////////////////////读取启动脚本，将命令加入执行队列
        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");////////////////////////////////触发init.rc中的init动作，在init.rc中可以找到 “on init” section 其中的命令就会执行，后文相同
    // 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");///////////////////////////启动模式为charger则启动关机充电，不启动android
    } else if ((strncmp(bootmode.c_str(), "ffbm-00", 7) == 0)
        || (strncmp(bootmode.c_str(), "ffbm-01", 7) == 0)) {
        am.QueueEventTrigger("ffbm");//////////////////////////////启动模式为ffbm则进入ffbm模式
    } else {
        am.QueueEventTrigger("late-init");/////////////////////////其它的启动模式都继续之后的初始化流程，从而启动android
    }
    // Run all property triggers based on current state of the properties.
    am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");///////////////根据当前所有properties的状态执行相应命令

    while (true) {
        // By default, sleep until something happens.
        int epoll_timeout_ms = -1;
        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            am.ExecuteOneCommand();/////////////////////////////////////////执行加入队列中的命令
        }
        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;
}

从上面的注释可以看到在init进程中挂载了相关文件系统；读取并解析了init.rc脚本；根据不同的bootmode来选择进入的模式，如果没有特别指定模式则开启android初始化流程。
详细的函数调用过程见下图：

init.rc文件结构介绍
init.rc文件基本组成单位是section, section分为三种类型，分别由三个关键字(所谓关键字即每一行的第一列)来区分，这三个关键字是 on、service、import。
1、on类型的section表示一系列命令的组合。当on后面的action被触发时执行这个section后面的所有命令。如am.QueueEventTrigger("charger")。on后面也可以是properties的变化再触发执行相关命令，如on property:sys.boot_from_charger_mode=1
2、service类型的section表示一个可执行程序
3、import类型的section表示引入另外一个.rc文件
init.rc实例：

import /init.environ.rc
import /init.usb.rc
import /init.${ro.zygote}.rc
on early-init
    # Set init and its forked children's oom_adj.
    write /proc/1/oom_score_adj -1000
    # Set the security context of /postinstall if present.
    restorecon /postinstall
    start ueventd
on init
    sysclktz 0
    # Mix device-specific information into the entropy pool
    copy /proc/cmdline /dev/urandom
    copy /default.prop /dev/urandom
on property:security.perf_harden=1
    write /proc/sys/kernel/perf_event_paranoid 3
service ueventd /sbin/ueventd
    class core
    critical
    seclabel u:r:ueventd:s0
    shutdown critical

init.rc与inittab的关系
在原生linux中我们发现init进程是读取一个inittab的脚本而不是android中的init.rc。那这个脚本是干啥的呢？
inttab实例

::sysinit:/etc/init.d/rcS
::respawn:-/bin/sh
tty2::askfirst:-/bin/sh
::ctrlaltdel:/bin/umount -a -r

inittab的作用：
inittab为linux初始化文件系统时init初始化程序用到的配置文件。这个文件负责设置init初始化程序初始化脚本?在哪里;每个运行级初始化时运行的命令;?开机、关机、重启对应的命令；各运行级登陆时所运行的命令

android简化了这部分处理直接使用init.rc脚本了。

android property和linux 环境变量
property是android在标准linux基础之上新增的一种系统全局可访问的属性机制。和linux的环境变量机制相似，可看做是android系统的环境变量。
在android上层可以同时使用这两种机制

shell中查看linux中所有环境变量

env

shell中查看android中所有properties

getprop