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深入研究源码:Android10.0系统启动流程(二)init进程

前言

上篇文章对系统启动流程进行了一个大概的梳理,我们知道了init进程是由内核态的0号进程idle(wrapper)启动起来的,今天我们就来深入挖掘下,init进程到底做了哪些事情

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正文

整体流程概览

具体源码分析

init的源码位于system/core/init包下,我们先从入口类main.cpp来看

int main(int argc, char** argv) {
#if __has_feature(address_sanitizer)
    __asan_set_error_report_callback(AsanReportCallback);
#endif

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

    if (argc > 1) {
        if (!strcmp(argv[1], "subcontext")) {
            android::base::InitLogging(argv, &android::base::KernelLogger);
            const BuiltinFunctionMap function_map;

            return SubcontextMain(argc, argv, &function_map);
        }

        if (!strcmp(argv[1], "selinux_setup")) {
            // This function initializes SELinux then execs init to run in the init SELinux context.
            return SetupSelinux(argv); //对SELinux进行初始化,并通过execs的系统调用开启init进程
        }

        if (!strcmp(argv[1], "second_stage")) {
            return SecondStageMain(argc, argv);  //第二阶段
        }
    }

    return FirstStageMain(argc, argv);   //第一阶段
}
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可以看到main.cpp的函数跟之前版本有了很大的区别

拿Android9.0的源码androidxref.com/9.0.0_r3/xr…来说,Android10中并不只是调用init::main,而是把部分流程性的判断放到的mian.cpp中来做,所以这里如果按照书上或者文章中所说的,直接去找init.cpp中的main函数,其实是找不到入口的

init进程是如何启动的

先来看看init进程是如何启动的

system/core/Selinux.cpp

// This function initializes SELinux then execs init to run in the init SELinux context.
int SetupSelinux(char** argv) {
    InitKernelLogging(argv);

    if (REBOOT_BOOTLOADER_ON_PANIC) {
        InstallRebootSignalHandlers();
    }

    // Set up SELinux, loading the SELinux policy.
    SelinuxSetupKernelLogging();
    SelinuxInitialize();

    // We're in the kernel domain and want to transition to the init domain.  File systems that
    // store SELabels in their xattrs, such as ext4 do not need an explicit restorecon here,
    // but other file systems do.  In particular, this is needed for ramdisks such as the
    // recovery image for A/B devices.
    if (selinux_android_restorecon("/system/bin/init", 0) == -1) {
        PLOG(FATAL) << "restorecon failed of /system/bin/init failed";
    }

    const char* path = "/system/bin/init"; //init二进制文件的目录
    const char* args[] = {path, "second_stage", nullptr};
    execv(path, const_cast<char**>(args));//调用execv开启init进程

    // execv() only returns if an error happened, in which case we
    // panic and never return from this function.
    PLOG(FATAL) << "execv(\"" << path << "\") failed";

    return 1;
}
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上面的SetupSelinux函数主要是通过evecv来开启init进程

FirstStageMain做了哪些工作

我们再来看下第一阶段的函数FirstStageMain做了哪些工作

system/core/first_stage_main.cpp

int FirstStageMain(int argc, char** argv) {
    if (REBOOT_BOOTLOADER_ON_PANIC) {  //是否定义由init.mk决定
        InstallRebootSignalHandlers(); //处理init挂掉的情况,会重启bootloader
    }

    boot_clock::time_point start_time = boot_clock::now();

    std::vector<std::pair<std::string, int>> errors;
#define CHECKCALL(x) 
    if (x != 0) errors.emplace_back(#x " failed", errno);

    // Clear the umask.
    umask(0);
   
    CHECKCALL(clearenv());
    CHECKCALL(setenv("PATH", _PATH_DEFPATH, 1));
    // 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.
    CHECKCALL(mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755"));
    CHECKCALL(mkdir("/dev/pts", 0755));
    CHECKCALL(mkdir("/dev/socket", 0755));
    CHECKCALL(mount("devpts", "/dev/pts", "devpts", 0, NULL));
#define MAKE_STR(x) __STRING(x)
    CHECKCALL(mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC)));
#undef MAKE_STR
    // Don't expose the raw commandline to unprivileged processes.
    CHECKCALL(chmod("/proc/cmdline", 0440));
    gid_t groups[] = {AID_READPROC};
    CHECKCALL(setgroups(arraysize(groups), groups)); //设置用户组
    CHECKCALL(mount("sysfs", "/sys", "sysfs", 0, NULL)); //挂载系统文件
    CHECKCALL(mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL));
    
    CHECKCALL(mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11)));
    
    if constexpr (WORLD_WRITABLE_KMSG) {
        CHECKCALL(mknod("/dev/kmsg_debug", S_IFCHR | 0622, makedev(1, 11)));
    }

    CHECKCALL(mknod("/dev/random", S_IFCHR | 0666, makedev(1, 8)));
    CHECKCALL(mknod("/dev/urandom", S_IFCHR | 0666, makedev(1, 9)));

    // This is needed for log wrapper, which gets called before ueventd runs.
    CHECKCALL(mknod("/dev/ptmx", S_IFCHR | 0666, makedev(5, 2)));
    CHECKCALL(mknod("/dev/null", S_IFCHR | 0666, makedev(1, 3)));

    // These below mounts are done in first stage init so that first stage mount can mount
    // subdirectories of /mnt/{vendor,product}/.  Other mounts, not required by first stage mount,
    // should be done in rc files.
    // Mount staging areas for devices managed by vold
    // See storage config details at http://source.android.com/devices/storage/
    CHECKCALL(mount("tmpfs", "/mnt", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                    "mode=0755,uid=0,gid=1000"));
    // /mnt/vendor is used to mount vendor-specific partitions that can not be
    // part of the vendor partition, e.g. because they are mounted read-write.
    CHECKCALL(mkdir("/mnt/vendor", 0755));
    // /mnt/product is used to mount product-specific partitions that can not be
    // part of the product partition, e.g. because they are mounted read-write.
    CHECKCALL(mkdir("/mnt/product", 0755));

    // /apex is used to mount APEXes
    CHECKCALL(mount("tmpfs", "/apex", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                    "mode=0755,uid=0,gid=0"));

    // /debug_ramdisk is used to preserve additional files from the debug ramdisk
    CHECKCALL(mount("tmpfs", "/debug_ramdisk", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV,
                    "mode=0755,uid=0,gid=0"));
#undef CHECKCALL

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

    if (!errors.empty()) {
        for (const auto& [error_string, error_errno] : errors) {
            LOG(ERROR) << error_string << " " << strerror(error_errno);
        }
        LOG(FATAL) << "Init encountered errors starting first stage, aborting";
    }

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

    auto old_root_dir = std::unique_ptr<DIR, decltype(&closedir)>{opendir("/"), closedir};
    if (!old_root_dir) {
        PLOG(ERROR) << "Could not opendir(\"/\"), not freeing ramdisk";
    }

    struct stat old_root_info;
    if (stat("/", &old_root_info) != 0) {
        PLOG(ERROR) << "Could not stat(\"/\"), not freeing ramdisk";
        old_root_dir.reset();
    }

    if (ForceNormalBoot()) {
        mkdir("/first_stage_ramdisk", 0755);
        // SwitchRoot() must be called with a mount point as the target, so we bind mount the
        // target directory to itself here.
        if (mount("/first_stage_ramdisk", "/first_stage_ramdisk", nullptr, MS_BIND, nullptr) != 0) {
            LOG(FATAL) << "Could not bind mount /first_stage_ramdisk to itself";
        }
        SwitchRoot("/first_stage_ramdisk");
    }

    // If this file is present, the second-stage init will use a userdebug sepolicy
    // and load adb_debug.prop to allow adb root, if the device is unlocked.
    if (access("/force_debuggable", F_OK) == 0) { //如果该文件存在且已经解锁bootbloade r,则允许调用adb root指令(userdebug sepolicy)
        std::error_code ec;  // to invoke the overloaded copy_file() that won't throw.
        if (!fs::copy_file("/adb_debug.prop", kDebugRamdiskProp, ec) ||
            !fs::copy_file("/userdebug_plat_sepolicy.cil", kDebugRamdiskSEPolicy, ec)) {
            LOG(ERROR) << "Failed to setup debug ramdisk";
        } else {
            // setenv for second-stage init to read above kDebugRamdisk* files.
            setenv("INIT_FORCE_DEBUGGABLE", "true", 1);
        }
    }

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

    struct stat new_root_info;
    if (stat("/", &new_root_info) != 0) {
        PLOG(ERROR) << "Could not stat(\"/\"), not freeing ramdisk";
        old_root_dir.reset();
    }

    if (old_root_dir && old_root_info.st_dev != new_root_info.st_dev) {
        FreeRamdisk(old_root_dir.get(), old_root_info.st_dev);
    }

    SetInitAvbVersionInRecovery();

    static constexpr uint32_t kNanosecondsPerMillisecond = 1e6;
    uint64_t start_ms = start_time.time_since_epoch().count() / kNanosecondsPerMillisecond;
    setenv("INIT_STARTED_AT", std::to_string(start_ms).c_str(), 1);

    const char* path = "/system/bin/init"; //找到init的二进制文件目录
    const char* args[] = {path, "selinux_setup", nullptr};
    execv(path, const_cast<char**>(args)); //通过execv来启动init进程

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

    return 1;
}
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主要代码处已经做处了注释,现在来总结下FirstStageMain的工作

  1. 处理init进程挂掉的情况
  2. 设置用户组,挂载相关系统文件
  3. 根据/force_debuggable文件来判断是否允许adb root指令
  4. 找到init的二进制文件目录,通过execv来启动init进程

至此init进程就被启动起来了,我们在回来看看SecondStageMain函数接下来做了哪些工作

SecondStageMain做了哪些工作

int SecondStageMain(int argc, char** argv) {
    if (REBOOT_BOOTLOADER_ON_PANIC) {
        InstallRebootSignalHandlers();
    }

    SetStdioToDevNull(argv);
    InitKernelLogging(argv);
    LOG(INFO) << "init second stage started!";

    // Set init and its forked children's oom_adj.
    if (auto result = WriteFile("/proc/1/oom_score_adj", "-1000"); !result) {
        LOG(ERROR) << "Unable to write -1000 to /proc/1/oom_score_adj: " << result.error();
    }

    // Enable seccomp if global boot option was passed (otherwise it is enabled in zygote).
    GlobalSeccomp();

    // 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));

    property_init(); //初始化系统属性,使用mmap共享内存,"/dev/__properties__/property_info"

    // 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);

    // See if need to load debug props to allow adb root, when the device is unlocked.
    const char* force_debuggable_env = getenv("INIT_FORCE_DEBUGGABLE");
    if (force_debuggable_env && AvbHandle::IsDeviceUnlocked()) {
        load_debug_prop = "true"s == force_debuggable_env;
    }

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

    // Now set up SELinux for second stage.
    SelinuxSetupKernelLogging();
    SelabelInitialize();
    SelinuxRestoreContext();

    Epoll epoll; //使用IO复用机制,epoll,即 event poll,是poll机制的升级版
    if (auto result = epoll.Open(); !result) {
        PLOG(FATAL) << result.error();
    }

    InstallSignalFdHandler(&epoll);  //使用epoll对init子进程的信号进行监听

    property_load_boot_defaults(load_debug_prop);
    UmountDebugRamdisk();
    fs_mgr_vendor_overlay_mount_all();
    export_oem_lock_status();
    StartPropertyService(&epoll); //开启属性服务,并注册到epoll中
    MountHandler mount_handler(&epoll);
    set_usb_controller();

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

    if (!SetupMountNamespaces()) {
        PLOG(FATAL) << "SetupMountNamespaces failed";
    }

    subcontexts = InitializeSubcontexts();

    ActionManager& am = ActionManager::GetInstance();
    ServiceList& sm = ServiceList::GetInstance();

    LoadBootScripts(am, sm); //加载系统启动脚本"/init.rc"

    // 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) DumpState();

    // Make the GSI status available before scripts start running.
    if (android::gsi::IsGsiRunning()) {
        property_set("ro.gsid.image_running", "1");
    } else {
        property_set("ro.gsid.image_running", "0");
    }

    am.QueueBuiltinAction(SetupCgroupsAction, "SetupCgroups");

    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(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng");
    am.QueueBuiltinAction(SetMmapRndBitsAction, "SetMmapRndBits");
    am.QueueBuiltinAction(SetKptrRestrictAction, "SetKptrRestrict");
    Keychords keychords;
    am.QueueBuiltinAction(
        [&epoll, &keychords](const BuiltinArguments& args) -> Result<Success> {
            for (const auto& svc : ServiceList::GetInstance()) {
                keychords.Register(svc->keycodes());
            }
            keychords.Start(&epoll, HandleKeychord);
            return Success();
        },
        "KeychordInit");
    am.QueueBuiltinAction(console_init_action, "console_init");

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

    // Starting the BoringSSL self test, for NIAP certification compliance.
    am.QueueBuiltinAction(StartBoringSslSelfTest, "StartBoringSslSelfTest");

    // 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(MixHwrngIntoLinuxRngAction, "MixHwrngIntoLinuxRng");

    // Initialize binder before bringing up other system services
    am.QueueBuiltinAction(InitBinder, "InitBinder");

    // 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");
	//解析启动脚本
    while (true) {
        // By default, sleep until something happens.
        auto epoll_timeout = std::optional<std::chrono::milliseconds>{};

        if (do_shutdown && !shutting_down) {
            do_shutdown = false;
            if (HandlePowerctlMessage(shutdown_command)) {
                shutting_down = true;
            }
        }

        if (!(waiting_for_prop || Service::is_exec_service_running())) {
            am.ExecuteOneCommand();
        }
        if (!(waiting_for_prop || Service::is_exec_service_running())) {
            if (!shutting_down) {
                auto next_process_action_time = HandleProcessActions();

                // If there's a process that needs restarting, wake up in time for that.
                if (next_process_action_time) {
                    epoll_timeout = std::chrono::ceil<std::chrono::milliseconds>(
                            *next_process_action_time - boot_clock::now());
                    if (*epoll_timeout < 0ms) epoll_timeout = 0ms;
                }
            }

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

        if (auto result = epoll.Wait(epoll_timeout); !result) {
            LOG(ERROR) << result.error();
        }
    }

    return 0;
}

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SecondStageMain的主要工作总结

我们发现,该函数是其实是system/core/init/init.cpp的入口函数,在Android9.0的版本中,该类的入口函数为main,代码中的关键部分我已经做了注释,现在来我们来总结一下SecondStageMain的主要工作

  1. 使用epoll对init子进程的信号进行监听
  2. 初始化系统属性,使用mmap共享内存,"/dev/properties/property_info" (重要)
  3. 开启属性服务,并注册到epoll中(重要)
  4. 加载系统启动脚本"/init.rc"
  5. 解析启动脚本,启动相关服务

深入挖掘系统属性

我们再来进一步去深入挖掘下,系统属性是如何初始化的

system/core/init/property_service.cpp

void property_init() {
    mkdir("/dev/__properties__", S_IRWXU | S_IXGRP | S_IXOTH);
    CreateSerializedPropertyInfo();
    if (__system_property_area_init()) { //初始化system_property内存区域
        LOG(FATAL) << "Failed to initialize property area";
    }
    if (!property_info_area.LoadDefaultPath()) {
        LOG(FATAL) << "Failed to load serialized property info file";
    }
}
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再来看下__system_property_area_init()这个函数,这里已经到了bionic包中 /Volumes/Fwk_Jack/WORKING_DIRECTORY/bionic/libc/bionic/system_property_api.cpp

__BIONIC_WEAK_FOR_NATIVE_BRIDGE
int __system_property_area_init() {
  bool fsetxattr_failed = false;
  return system_properties.AreaInit(PROP_FILENAME, &fsetxattr_failed) && !fsetxattr_failed ? 0 : -1;
}
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这个函数又调用了system_properties.AreaInit(),我们接着跟下去 /bionic/libc/system_properties/system_properties.cpp

bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed) {
  if (strlen(filename) >= PROP_FILENAME_MAX) {
    return false;
  }
  strcpy(property_filename_, filename);

  contexts_ = new (contexts_data_) ContextsSerialized(); 
  if (!contexts_->Initialize(true, property_filename_, fsetxattr_failed)) {
    return false;
  }
  initialized_ = true;
  return true;
}
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这里调用了contexts_->Initialize()函数

/bionic/libc/system_properties/contexts_serialized.cpp

bool ContextsSerialized::Initialize(bool writable, const char* filename, bool* fsetxattr_failed) {
  filename_ = filename;
  if (!InitializeProperties()) { //初始化系统属性
    return false;
  }

  if (writable) {
    mkdir(filename_, S_IRWXU | S_IXGRP | S_IXOTH);
    bool open_failed = false;
    if (fsetxattr_failed) {
      *fsetxattr_failed = false;
    }

    for (size_t i = 0; i < num_context_nodes_; ++i) {
      if (!context_nodes_[i].Open(true, fsetxattr_failed)) {
        open_failed = true;
      }
    }
    if (open_failed || !MapSerialPropertyArea(true, fsetxattr_failed)) {
      FreeAndUnmap();
      return false;
    }
  } else {
    if (!MapSerialPropertyArea(false, nullptr)) {
      FreeAndUnmap();
      return false;
    }
  }
  return true;
}
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调用InitializeProperties()初始化系统属性

bool ContextsSerialized::InitializeProperties() {
  if (!property_info_area_file_.LoadDefaultPath()) { //加载默认系统属性路径
    return false;
  }

  if (!InitializeContextNodes()) {
    FreeAndUnmap();
    return false;
  }

  return true;
}
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/system/core/property_service/libpropertyinfoparser/property_info_parser.cpp

bool PropertyInfoAreaFile::LoadDefaultPath() {
  return LoadPath("/dev/__properties__/property_info");//把文件加载到内存
}
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bool PropertyInfoAreaFile::LoadPath(const char* filename) {
  int fd = open(filename, O_CLOEXEC | O_NOFOLLOW | O_RDONLY);

  struct stat fd_stat;
  if (fstat(fd, &fd_stat) < 0) {
    close(fd);
    return false;
  }

  if ((fd_stat.st_uid != 0) || (fd_stat.st_gid != 0) ||
      ((fd_stat.st_mode & (S_IWGRP | S_IWOTH)) != 0) ||
      (fd_stat.st_size < static_cast<off_t>(sizeof(PropertyInfoArea)))) {
    close(fd);
    return false;
  }

  auto mmap_size = fd_stat.st_size;
  //千呼万唤始出来,终于到了最后一个,调用mmap函数创建共享内存,供其他进程获取系统属性
  void* map_result = mmap(nullptr, mmap_size, PROT_READ, MAP_SHARED, fd, 0);
  if (map_result == MAP_FAILED) {
    close(fd);
    return false;
  }

  auto property_info_area = reinterpret_cast<PropertyInfoArea*>(map_result);
  if (property_info_area->minimum_supported_version() > 1 ||
      property_info_area->size() != mmap_size) {
    munmap(map_result, mmap_size);
    close(fd);
    return false;
  }

  close(fd);
  mmap_base_ = map_result;
  mmap_size_ = mmap_size;
  return true;
}
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分析到这里,系统属性的初始化终于分析完了,现在来总结下,系统属性初始化是使用了mmap的内存共享机制,来让其他进程来获取系统属性的

那既然可以通过共享内存来访问,为什么还需要开启一个属性服务呢?直接通过共享内存来设置系统属性,不就好了么?

这里其实就涉及到安全性的问题了,如果所有进程都可以自由的修改系统属性,那系统属性,还能被成为系统属性吗?所以Android设计为,其他进程只能通过共享内存来获取系统属性,而“修改”的权限则统一收拢到init进程中,开启一个属性服务,如下图所示

深入挖掘系统服务

下面再来进一步分析下,系统服务是如何开启的

/system/core/init/property_service.cpp

void StartPropertyService(Epoll* epoll) {
    selinux_callback cb;
    cb.func_audit = SelinuxAuditCallback;
    selinux_set_callback(SELINUX_CB_AUDIT, cb);

    property_set("ro.property_service.version", "2"); //设置系统属性

    property_set_fd = CreateSocket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
                                   false, 0666, 0, 0, nullptr); //创建socket服务端
    if (property_set_fd == -1) {
        PLOG(FATAL) << "start_property_service socket creation failed";
    }

    listen(property_set_fd, 8);  //监听sokcet服务,最大并发数是8
    //注册到epoll的handler中进行IO优化处理
    if (auto result = epoll->RegisterHandler(property_set_fd, handle_property_set_fd); !result) {
        PLOG(FATAL) << result.error();
    }
}
复制代码

开启属性服务的重要步骤已经注释说明了,现在再来总结下

系统服务开启流程

  1. 创建socket服务端
  2. 监听sokcet服务,最大并发数是8
  3. 注册到epoll的handler中进行IO优化处理

至此,init进程的主要工作流程和重要原理已分析完成

接下来的这篇文章,我会以视频加讲义的方式,带着大家去读源码,让大家真正了解应该如何把源码读起来

写在最后

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本文原作者为释然,版权©️归Android研习社所有,侵权必究