serviceManager介绍

作者: android小奉先 | 来源:发表于2021-05-31 15:13 被阅读0次

本篇介绍

servicemanager是android中binder服务的管家，一般binder服务会先注册到servicemanager中，然后其他客户端会通过servicemanager来获取目标binder的proxy binder，这时候就可以通过获取的proxy binder进行ipc调用了，本篇介绍下servicemanager的工作流程。

servicemanager的启动

servicemanager是由init拉起的，下面是配置的rc文件。

service servicemanager /system/bin/servicemanager
    class core animation
    user system
    group system readproc
    critical
    onrestart restart apexd
    onrestart restart audioserver
    onrestart restart gatekeeperd
    onrestart class_restart main
    onrestart class_restart hal
    onrestart class_restart early_hal
    writepid /dev/cpuset/system-background/tasks
    shutdown critical

可以看到servicemanager很关键，如果一分钟内crash 4次就会导致设备重启进入bootloader模式。
接下来看下启动流程：

int main(int argc, char** argv) {
    if (argc > 2) {
        LOG(FATAL) << "usage: " << argv[0] << " [binder driver]";
    }

    const char* driver = argc == 2 ? argv[1] : "/dev/binder";

    sp<ProcessState> ps = ProcessState::initWithDriver(driver); // 初始化进程间的单例对象processstate
    ps->setThreadPoolMaxThreadCount(0); // 设置线程池线程数量
    ps->setCallRestriction(ProcessState::CallRestriction::FATAL_IF_NOT_ONEWAY); // 设置binder线程阻塞时候的行为，是打印日志还是退出进程

    sp<ServiceManager> manager = sp<ServiceManager>::make(std::make_unique<Access>()); // 创建servicemanager
    if (!manager->addService("manager", manager, false /*allowIsolated*/, IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT).isOk()) {
        LOG(ERROR) << "Could not self register servicemanager";
    } // 添加servicemanager自己到服务列表中

    IPCThreadState::self()->setTheContextObject(manager);
    ps->becomeContextManager();通知内核，设置binder 管家

    sp<Looper> looper = Looper::prepare(false /*allowNonCallbacks*/);

    BinderCallback::setupTo(looper); // 针对binder的fd设置回调
    ClientCallbackCallback::setupTo(looper, manager);

    while(true) {
        looper->pollAll(-1); // 开始监听binder节点
    }

    // should not be reached
    return EXIT_FAILURE;
}

可以看到servicemanager的启动做的几件事情

初始化ProcessState
创建ServiceManager对象，并设置成binder管家
通过looper设置binder驱动节点回调
接下来挨个介绍下

初始化ProcessState

···
sp<ProcessState> ProcessState::initWithDriver(const char* driver)
{
return init(driver, true /requireDefault/);
}
···

前面文章defaultServiceManager介绍
提到过，android中有三个binder域，分别是/dev/binder, /dev/hwbinder, /dev/vndbinder, 这三个binder是不通的，也就是一个binder服务是无法拿到vndbinder中的服务的，这个是Google在treble项目中做系统解耦的一种方法。

sp<ProcessState> ProcessState::init(const char *driver, bool requireDefault)
{
    [[clang::no_destroy]] static sp<ProcessState> gProcess;
    [[clang::no_destroy]] static std::mutex gProcessMutex;

    if (driver == nullptr) { // driver不能为空
        std::lock_guard<std::mutex> l(gProcessMutex);
        return gProcess;
    }

    [[clang::no_destroy]] static std::once_flag gProcessOnce;
    std::call_once(gProcessOnce, [&](){
        if (access(driver, R_OK) == -1) {
            ALOGE("Binder driver %s is unavailable. Using /dev/binder instead.", driver);
            driver = "/dev/binder";
        }

        std::lock_guard<std::mutex> l(gProcessMutex);
        gProcess = sp<ProcessState>::make(driver); // 构造ProcessState
    });

    if (requireDefault) {
        // Detect if we are trying to initialize with a different driver, and
        // consider that an error. ProcessState will only be initialized once above.
        LOG_ALWAYS_FATAL_IF(gProcess->getDriverName() != driver,
                            "ProcessState was already initialized with %s,"
                            " can't initialize with %s.",
                            gProcess->getDriverName().c_str(), driver);
    }

    return gProcess;
}

这儿就是用提供的driver构造一个ProcessState，默认是/dev/binder。接下来再看下ProcessState的构造方法：

ProcessState::ProcessState(const char *driver)
    : mDriverName(String8(driver))
    , mDriverFD(open_driver(driver)) // 通过binder节点，创建对应的binder_proc对象
    , mVMStart(MAP_FAILED)
    , mThreadCountLock(PTHREAD_MUTEX_INITIALIZER)
    , mThreadCountDecrement(PTHREAD_COND_INITIALIZER)
    , mExecutingThreadsCount(0)
    , mWaitingForThreads(0)
    , mMaxThreads(DEFAULT_MAX_BINDER_THREADS)
    , mStarvationStartTimeMs(0)
    , mThreadPoolStarted(false)
    , mThreadPoolSeq(1)
    , mCallRestriction(CallRestriction::NONE)
{

// TODO(b/166468760): enforce in build system
#if defined(__ANDROID_APEX__)
    LOG_ALWAYS_FATAL("Cannot use libbinder in APEX (only system.img libbinder) since it is not stable.");
#endif
// apex 里面不能用libbinder，apex是android 10引入的一个机制。目的还是提升系统的升级速度，以前的treble项目也是这个目的
//treble是google希望可以自己独立升级自己的android代码，不用关芯片厂商的硬件代码。
//apex就更近一步，毕竟升级android镜像还是比较麻烦，apex干脆让系统分为一个个模块，每个模块就是一个apex，
//可以直接从play store上下载升级，这样就更加简单了。
    if (mDriverFD >= 0) {
        // mmap the binder, providing a chunk of virtual address space to receive transactions.
        mVMStart = mmap(nullptr, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);
        if (mVMStart == MAP_FAILED) {
            // *sigh*
            ALOGE("Using %s failed: unable to mmap transaction memory.\n", mDriverName.c_str());
            close(mDriverFD);
            mDriverFD = -1;
            mDriverName.clear();
        }
    }

#ifdef __ANDROID__
    LOG_ALWAYS_FATAL_IF(mDriverFD < 0, "Binder driver '%s' could not be opened.  Terminating.", driver);
#endif
}

这时候就完成了ProcessState的构造。这里面提到了一个话题外的概念就是apex，也就是模块化升级，这块后面再专门介绍下。

创建ServiceManager对象，并设置成binder管家

创建servicemanager后，调用addService将自己添加到服务列表中。
看下addService的内容

Status ServiceManager::addService(const std::string& name, const sp<IBinder>& binder, bool allowIsolated, int32_t dumpPriority) {
    auto ctx = mAccess->getCallingContext();

    // apps cannot add services
    if (multiuser_get_app_id(ctx.uid) >= AID_APP) { 
        return Status::fromExceptionCode(Status::EX_SECURITY); 
    }

    if (!mAccess->canAdd(ctx, name)) { // selinux检查
        return Status::fromExceptionCode(Status::EX_SECURITY);
    }

    if (binder == nullptr) {
        return Status::fromExceptionCode(Status::EX_ILLEGAL_ARGUMENT);
    }

    if (!isValidServiceName(name)) {
        LOG(ERROR) << "Invalid service name: " << name;
        return Status::fromExceptionCode(Status::EX_ILLEGAL_ARGUMENT);
    }

#ifndef VENDORSERVICEMANAGER
    if (!meetsDeclarationRequirements(binder, name)) {
        // already logged
        return Status::fromExceptionCode(Status::EX_ILLEGAL_ARGUMENT);
    }
#endif  // !VENDORSERVICEMANAGER

    // implicitly unlinked when the binder is removed
    if (binder->remoteBinder() != nullptr &&
        binder->linkToDeath(sp<ServiceManager>::fromExisting(this)) != OK) { // 注册死亡通知
        LOG(ERROR) << "Could not linkToDeath when adding " << name;
        return Status::fromExceptionCode(Status::EX_ILLEGAL_STATE);
    }

    // Overwrite the old service if it exists
    mNameToService[name] = Service { // 将服务添加到服务列表
        .binder = binder,
        .allowIsolated = allowIsolated,
        .dumpPriority = dumpPriority,
        .debugPid = ctx.debugPid,
    };

    auto it = mNameToRegistrationCallback.find(name); // 查看是否有client关心该service的注册事件，如果有，则通知对应client
    if (it != mNameToRegistrationCallback.end()) {
        for (const sp<IServiceCallback>& cb : it->second) {
            mNameToService[name].guaranteeClient = true;
            // permission checked in registerForNotifications
            cb->onRegistration(name, binder);
        }
    }

    return Status::ok();
}

从上面代码可以看出来，addservice其实就是将servie的名字和对应的binder保存起来即可，在一些场景中，可能需要获得指定名字的binder，可是又不知道什么时候可以获取到，那么就可以有2种方法来实现了。一种是直接在轮询中使用getService，一种就是注册添加服务通知。
这时候会有一个疑问，那就是如果重复名字的binder服务注册会怎样？从上面的实现看，后者会替换掉前者，并不会因为已经注册了而注册失败。
接着看下如何成为binder管家

bool ProcessState::becomeContextManager()
{
    AutoMutex _l(mLock);

    flat_binder_object obj {
        .flags = FLAT_BINDER_FLAG_TXN_SECURITY_CTX,
    };

    int result = ioctl(mDriverFD, BINDER_SET_CONTEXT_MGR_EXT, &obj);

    // fallback to original method
    if (result != 0) {
        android_errorWriteLog(0x534e4554, "121035042");

        int unused = 0;
        result = ioctl(mDriverFD, BINDER_SET_CONTEXT_MGR, &unused);
    }

    if (result == -1) {
        ALOGE("Binder ioctl to become context manager failed: %s\n", strerror(errno));
    }

    return result == 0;
}

直接ioctl进入内核，发送命令BINDER_SET_CONTEXT_MGR_EXT。

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    ...
        case BINDER_SET_CONTEXT_MGR_EXT: {
        struct flat_binder_object fbo;

        if (copy_from_user(&fbo, ubuf, sizeof(fbo))) {
            ret = -EINVAL;
            goto err;
        }
        ret = binder_ioctl_set_ctx_mgr(filp, &fbo); // 设置binder管家
        if (ret)
            goto err;
        break;
    }
    ...
}

再看下binder_ioctl_set_ctx_mgr

static int binder_ioctl_set_ctx_mgr(struct file *filp,
                    struct flat_binder_object *fbo)
{
    int ret = 0;
    struct binder_proc *proc = filp->private_data;
    struct binder_context *context = proc->context;
    struct binder_node *new_node;
    kuid_t curr_euid = current_euid();

    mutex_lock(&context->context_mgr_node_lock);
    if (context->binder_context_mgr_node) { // 如果已经有管家了，就返回
        pr_err("BINDER_SET_CONTEXT_MGR already set\n");
        ret = -EBUSY;
        goto out;
    }
    ret = security_binder_set_context_mgr(proc->tsk); // 设置binder管家
    if (ret < 0)
        goto out;
    if (uid_valid(context->binder_context_mgr_uid)) {
        if (!uid_eq(context->binder_context_mgr_uid, curr_euid)) {
            pr_err("BINDER_SET_CONTEXT_MGR bad uid %d != %d\n",
                   from_kuid(&init_user_ns, curr_euid),
                   from_kuid(&init_user_ns,
                     context->binder_context_mgr_uid));
            ret = -EPERM;
            goto out;
        }
    } else {
        context->binder_context_mgr_uid = curr_euid;
    }
    new_node = binder_new_node(proc, fbo); // 创建binder管家实体
    if (!new_node) {
        ret = -ENOMEM;
        goto out;
    }
    binder_node_lock(new_node);
    new_node->local_weak_refs++;
    new_node->local_strong_refs++;
    new_node->has_strong_ref = 1;
    new_node->has_weak_ref = 1;
    context->binder_context_mgr_node = new_node;
    binder_node_unlock(new_node);
    binder_put_node(new_node);
out:
    mutex_unlock(&context->context_mgr_node_lock);
    return ret;
}

再看下servicemanager中提供的接口

接口名称	含义
getService	获取binder服务，如果binder服务没有在运行，则会尝试拉起该服务
checkService	获取binder服务，如果没找到，也不会执行拉起动作
addService	添加binder服务
listServices	获取binder服务列表
un/registerForNotifications	注销/添加监听服务注册回调
getDeclaredInstances	获取制定接口的binder服务
registerClientCallback	注册感知client变化回调，clients变化时会调用回调
getServiceDebugInfo	注册的binder服务名字和pid列表

设置驱动回调

首先创建了一个Looper，然后设置两个回调，一个用于处理binder请求，一个用于定时通知client变化，先看下第一个。
BinderCallback::setupTo(looper);

class BinderCallback : public LooperCallback {
public:
    static sp<BinderCallback> setupTo(const sp<Looper>& looper) {
        sp<BinderCallback> cb = sp<BinderCallback>::make();

        int binder_fd = -1;
        IPCThreadState::self()->setupPolling(&binder_fd); // 获取binder的fd
        LOG_ALWAYS_FATAL_IF(binder_fd < 0, "Failed to setupPolling: %d", binder_fd);

        int ret = looper->addFd(binder_fd, // 监听binder 的事件
                                Looper::POLL_CALLBACK,
                                Looper::EVENT_INPUT,
                                cb,
                                nullptr /*data*/);
        LOG_ALWAYS_FATAL_IF(ret != 1, "Failed to add binder FD to Looper");

        return cb;
    }

    int handleEvent(int /* fd */, int /* events */, void* /* data */) override {
        IPCThreadState::self()->handlePolledCommands(); // 处理binder请求，读取命令，并执行
        return 1;  // Continue receiving callbacks.
    }
};

再看下第二个回调：

class ClientCallbackCallback : public LooperCallback {
public:
    static sp<ClientCallbackCallback> setupTo(const sp<Looper>& looper, const sp<ServiceManager>& manager) {
        sp<ClientCallbackCallback> cb = sp<ClientCallbackCallback>::make(manager);

        int fdTimer = timerfd_create(CLOCK_MONOTONIC, 0 /*flags*/);  // 创建定时timerfd
        LOG_ALWAYS_FATAL_IF(fdTimer < 0, "Failed to timerfd_create: fd: %d err: %d", fdTimer, errno);

        itimerspec timespec {
            .it_interval = {
                .tv_sec = 5,
                .tv_nsec = 0,
            },
            .it_value = {
                .tv_sec = 5,
                .tv_nsec = 0,
            },
        };

        int timeRes = timerfd_settime(fdTimer, 0 /*flags*/, &timespec, nullptr);
        LOG_ALWAYS_FATAL_IF(timeRes < 0, "Failed to timerfd_settime: res: %d err: %d", timeRes, errno);

        int addRes = looper->addFd(fdTimer, // 监听定时通知
                                   Looper::POLL_CALLBACK,
                                   Looper::EVENT_INPUT,
                                   cb,
                                   nullptr);
        LOG_ALWAYS_FATAL_IF(addRes != 1, "Failed to add client callback FD to Looper");

        return cb;
    }

    int handleEvent(int fd, int /*events*/, void* /*data*/) override {
        uint64_t expirations;
        int ret = read(fd, &expirations, sizeof(expirations));
        if (ret != sizeof(expirations)) {
            ALOGE("Read failed to callback FD: ret: %d err: %d", ret, errno);
        }

        mManager->handleClientCallbacks(); // 固定周期通知client变化
        return 1;  // Continue receiving callbacks.
    }
private:
    friend sp<ClientCallbackCallback>;
    ClientCallbackCallback(const sp<ServiceManager>& manager) : mManager(manager) {}
    sp<ServiceManager> mManager;
};

最后servicemanager就只要在这个looper上poll就可以了。

本篇总结

本篇介绍了下servicemanager的内容，包含servicemanager的创建，启动，运行等，可以看到servicemanager相比以前添加了service和client的互相监听机制，使用上更加方便。

serviceManager介绍

本篇介绍

servicemanager的启动

初始化ProcessState

创建ServiceManager对象，并设置成binder管家

设置驱动回调

本篇总结

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