Binder系列5—注册服务(addService)

Posted by Gityuan on November 14, 2015

基于Android 6.0的源码剖析, 本文讲解如何向ServiceManager注册Native层的服务的过程。

framework/native/libs/binder/
  - Binder.cpp
  - BpBinder.cpp
  - IPCThreadState.cpp
  - ProcessState.cpp
  - IServiceManager.cpp
  - IInterface.cpp
  - Parcel.cpp

frameworks/native/include/binder/
  - IInterface.h (包括BnInterface, BpInterface)

一.概述

1.1 media服务注册

media入口函数是main_mediaserver.cpp中的main()方法,代码如下:

int main(int argc __unused, char** argv)
{
    ...
    InitializeIcuOrDie();
    //获得ProcessState实例对象【见小节2.1】
    sp<ProcessState> proc(ProcessState::self());
    //获取BpServiceManager对象
    sp<IServiceManager> sm = defaultServiceManager();
    AudioFlinger::instantiate();
    //注册多媒体服务  【见小节3.1】
    MediaPlayerService::instantiate();
    ResourceManagerService::instantiate();
    CameraService::instantiate();
    AudioPolicyService::instantiate();
    SoundTriggerHwService::instantiate();
    RadioService::instantiate();
    registerExtensions();
    //启动Binder线程池
    ProcessState::self()->startThreadPool();
    //当前线程加入到线程池
    IPCThreadState::self()->joinThreadPool();
 }

过程说明:

  1. 获取ServiceManager: 讲解了defaultServiceManager()返回的是BpServiceManager对象, 用于跟servicemanager进程通信;
  2. 理解Binder线程池的管理, 讲解了startThreadPool和joinThreadPool过程.

本文的重点就是讲解Native层服务注册的过程.

1.2 类图

在Native层的服务以media服务为例,来说一说服务注册过程,先来看看media的整个的类关系图。 点击查看大图

add_media_player_service

图解:

  • 蓝色代表的是注册MediaPlayerService服务所涉及的类
  • 绿色代表的是Binder架构中与Binder驱动通信过程中的最为核心的两个类;
  • 紫色代表的是注册服务和获取服务的公共接口/父类;

1.3 时序图

先通过一幅图来说说,media服务启动过程是如何向servicemanager注册服务的。

点击查看大图

addService

二. ProcessState

2.1 ProcessState::self

[-> ProcessState.cpp]

sp<ProcessState> ProcessState::self()
{
    Mutex::Autolock _l(gProcessMutex);
    if (gProcess != NULL) {
        return gProcess;
    }

    //实例化ProcessState 【见小节2.2】
    gProcess = new ProcessState;
    return gProcess;
}

获得ProcessState对象: 这也是单例模式,从而保证每一个进程只有一个ProcessState对象。其中gProcessgProcessMutex是保存在Static.cpp类的全局变量。

2.2 ProcessState初始化

[-> ProcessState.cpp]

ProcessState::ProcessState()
    : mDriverFD(open_driver()) // 打开Binder驱动【见小节2.3】
    , mVMStart(MAP_FAILED)
    , mThreadCountLock(PTHREAD_MUTEX_INITIALIZER)
    , mThreadCountDecrement(PTHREAD_COND_INITIALIZER)
    , mExecutingThreadsCount(0)
    , mMaxThreads(DEFAULT_MAX_BINDER_THREADS)
    , mManagesContexts(false)
    , mBinderContextCheckFunc(NULL)
    , mBinderContextUserData(NULL)
    , mThreadPoolStarted(false)
    , mThreadPoolSeq(1)
{
    if (mDriverFD >= 0) {
        //采用内存映射函数mmap,给binder分配一块虚拟地址空间【见小节2.4】
        mVMStart = mmap(0, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);
        if (mVMStart == MAP_FAILED) {
            close(mDriverFD); //没有足够空间分配给/dev/binder,则关闭驱动
            mDriverFD = -1;
        }
    }
}
  • ProcessState的单例模式的惟一性,因此一个进程只打开binder设备一次,其中ProcessState的成员变量mDriverFD记录binder驱动的fd,用于访问binder设备。
  • BINDER_VM_SIZE = (1*1024*1024) - (4096 *2), binder分配的默认内存大小为1M-8k。
  • DEFAULT_MAX_BINDER_THREADS = 15,binder默认的最大可并发访问的线程数为16。

2.3 open_driver

[-> ProcessState.cpp]

static int open_driver()
{
    // 打开/dev/binder设备,建立与内核的Binder驱动的交互通道
    int fd = open("/dev/binder", O_RDWR);
    if (fd >= 0) {
        fcntl(fd, F_SETFD, FD_CLOEXEC);
        int vers = 0;
        status_t result = ioctl(fd, BINDER_VERSION, &vers);
        if (result == -1) {
            close(fd);
            fd = -1;
        }
        if (result != 0 || vers != BINDER_CURRENT_PROTOCOL_VERSION) {
            close(fd);
            fd = -1;
        }
        size_t maxThreads = DEFAULT_MAX_BINDER_THREADS;

        // 通过ioctl设置binder驱动,能支持的最大线程数
        result = ioctl(fd, BINDER_SET_MAX_THREADS, &maxThreads);
        if (result == -1) {
            ALOGE("Binder ioctl to set max threads failed: %s", strerror(errno));
        }
    } else {
        ALOGW("Opening '/dev/binder' failed: %s\n", strerror(errno));
    }
    return fd;
}

open_driver作用是打开/dev/binder设备,设定binder支持的最大线程数。关于binder驱动的相应方法,见文章Binder Driver初探

ProcessState采用单例模式,保证每一个进程都只打开一次Binder Driver。

2.4 mmap

//原型
void* mmap(void* addr, size_t size, int prot, int flags, int fd, off_t offset)
//此处
mmap(0, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);

参数说明:

  • addr: 代表映射到进程地址空间的起始地址,当值等于0则由内核选择合适地址,此处为0;
  • size: 代表需要映射的内存地址空间的大小,此处为1M-8K;
  • prot: 代表内存映射区的读写等属性值,此处为PROT_READ(可读取);
  • flags: 标志位,此处为MAP_PRIVATE(私有映射,多进程间不共享内容的改变)和 MAP_NORESERVE(不保留交换空间)
  • fd: 代表mmap所关联的文件描述符,此处为mDriverFD;
  • offset:偏移量,此处为0。

mmap()经过系统调用,执行http://gityuan.com/2015/11/01/binder-driver/过程。

三. 服务注册

3.1 instantiate

[-> MediaPlayerService.cpp]

void MediaPlayerService::instantiate() {
    //注册服务【见小节3.2】
    defaultServiceManager()->addService(String16("media.player"), new MediaPlayerService());
}

注册服务MediaPlayerService:由defaultServiceManager()返回的是BpServiceManager,同时会创建ProcessState对象和BpBinder对象。 故此处等价于调用BpServiceManager->addService。其中MediaPlayerService位于libmediaplayerservice库.

3.2 BpSM.addService

[-> IServiceManager.cpp ::BpServiceManager]

virtual status_t addService(const String16& name, const sp<IBinder>& service,
        bool allowIsolated)
{
    Parcel data, reply; //Parcel是数据通信包
    //写入头信息"android.os.IServiceManager"
    data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());   
    data.writeString16(name);        // name为 "media.player"
    data.writeStrongBinder(service); // MediaPlayerService对象【见小节3.2.1】
    data.writeInt32(allowIsolated ? 1 : 0); // allowIsolated= false
    //remote()指向的是BpBinder对象【见小节3.3】
    status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
    return err == NO_ERROR ? reply.readExceptionCode() : err;
}

服务注册过程:向ServiceManager注册服务MediaPlayerService,服务名为”media.player”;

3.2.1 writeStrongBinder

[-> parcel.cpp]

status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
{
    return flatten_binder(ProcessState::self(), val, this);
}

3.2.2 flatten_binder

[-> parcel.cpp]

status_t flatten_binder(const sp<ProcessState>& /*proc*/,
    const sp<IBinder>& binder, Parcel* out)
{
    flat_binder_object obj;

    obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
    if (binder != NULL) {
        IBinder *local = binder->localBinder(); //本地Binder不为空
        if (!local) {
            BpBinder *proxy = binder->remoteBinder();
            const int32_t handle = proxy ? proxy->handle() : 0;
            obj.type = BINDER_TYPE_HANDLE; 
            obj.binder = 0; 
            obj.handle = handle;
            obj.cookie = 0;
        } else { //进入该分支
            obj.type = BINDER_TYPE_BINDER; 
            obj.binder = reinterpret_cast<uintptr_t>(local->getWeakRefs());
            obj.cookie = reinterpret_cast<uintptr_t>(local);
        }
    } else {
        ...
    }
    //【见小节3.2.3】
    return finish_flatten_binder(binder, obj, out);
}

将Binder对象扁平化,转换成flat_binder_object对象。

  • 对于Binder实体,则cookie记录Binder实体的指针;
  • 对于Binder代理,则用handle记录Binder代理的句柄;

关于localBinder,代码见Binder.cpp。

BBinder* BBinder::localBinder()
{
    return this;
}

BBinder* IBinder::localBinder()
{
    return NULL;
}

3.2.3 finish_flatten_binder

inline static status_t finish_flatten_binder(
    const sp<IBinder>& , const flat_binder_object& flat, Parcel* out)
{
    return out->writeObject(flat, false);
}

将flat_binder_object写入out。

3.3 BpBinder::transact

[-> BpBinder.cpp]

status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    if (mAlive) {
        // code=ADD_SERVICE_TRANSACTION【见小节3.4】
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }
    return DEAD_OBJECT;
}

Binder代理类调用transact()方法,真正工作还是交给IPCThreadState来进行transact工作。先来 看看IPCThreadState::self的过程。

3.3.1 IPCThreadState::self

[-> IPCThreadState.cpp]

IPCThreadState* IPCThreadState::self()
{
    if (gHaveTLS) {
restart:
        const pthread_key_t k = gTLS;
        IPCThreadState* st = (IPCThreadState*)pthread_getspecific(k);
        if (st) return st;
        return new IPCThreadState;  //初始IPCThreadState 【见小节3.3.2】
    }

    if (gShutdown) return NULL;

    pthread_mutex_lock(&gTLSMutex);
    if (!gHaveTLS) { //首次进入gHaveTLS为false
        if (pthread_key_create(&gTLS, threadDestructor) != 0) { //创建线程的TLS
            pthread_mutex_unlock(&gTLSMutex);
            return NULL;
        }
        gHaveTLS = true;
    }
    pthread_mutex_unlock(&gTLSMutex);
    goto restart;
}

TLS是指Thread local storage(线程本地储存空间),每个线程都拥有自己的TLS,并且是私有空间,线程之间不会共享。通过pthread_getspecific/pthread_setspecific函数可以获取/设置这些空间中的内容。从线程本地存储空间中获得保存在其中的IPCThreadState对象。

3.3.2 IPCThreadState初始化

[-> IPCThreadState.cpp]

IPCThreadState::IPCThreadState()
    : mProcess(ProcessState::self()),
      mMyThreadId(gettid()),
      mStrictModePolicy(0),
      mLastTransactionBinderFlags(0)
{
    pthread_setspecific(gTLS, this);
    clearCaller();
    mIn.setDataCapacity(256);
    mOut.setDataCapacity(256);
}

每个线程都有一个IPCThreadState,每个IPCThreadState中都有一个mIn、一个mOut。成员变量mProcess保存了ProcessState变量(每个进程只有一个)。

  • mIn 用来接收来自Binder设备的数据,默认大小为256字节;
  • mOut用来存储发往Binder设备的数据,默认大小为256字节。

3.4 IPC::transact

[-> IPCThreadState.cpp]

status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
    status_t err = data.errorCheck(); //数据错误检查
    flags |= TF_ACCEPT_FDS;
    ....
    if (err == NO_ERROR) { // 传输数据 【见小节3.5】
        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
    }
    ...

    if ((flags & TF_ONE_WAY) == 0) {
        if (reply) {
            //等待响应 【见小节3.6】
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }

    } else {
        //oneway,则不需要等待reply的场景
        err = waitForResponse(NULL, NULL);
    }
    return err;
}

IPCThreadState进行transact事务处理分3部分:

  • errorCheck() //数据错误检查
  • writeTransactionData() // 传输数据
  • waitForResponse() //f等待响应

3.5 IPC.writeTransactionData

[-> IPCThreadState.cpp]

status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags,
    int32_t handle, uint32_t code, const Parcel& data, status_t* statusBuffer)
{
    binder_transaction_data tr;
    tr.target.ptr = 0;
    tr.target.handle = handle; // handle = 0
    tr.code = code;            // code = ADD_SERVICE_TRANSACTION
    tr.flags = binderFlags;    // binderFlags = 0
    tr.cookie = 0;
    tr.sender_pid = 0;
    tr.sender_euid = 0;

    // data为记录Media服务信息的Parcel对象
    const status_t err = data.errorCheck();
    if (err == NO_ERROR) {
        tr.data_size = data.ipcDataSize();  // mDataSize
        tr.data.ptr.buffer = data.ipcData(); //mData
        tr.offsets_size = data.ipcObjectsCount()*sizeof(binder_size_t); //mObjectsSize
        tr.data.ptr.offsets = data.ipcObjects(); //mObjects
    } else if (statusBuffer) {
        ...
    } else {
        return (mLastError = err);
    }

    mOut.writeInt32(cmd);         //cmd = BC_TRANSACTION
    mOut.write(&tr, sizeof(tr));  //写入binder_transaction_data数据
    return NO_ERROR;
}

其中handle的值用来标识目的端,注册服务过程的目的端为service manager,此处handle=0所对应的是binder_context_mgr_node对象,正是service manager所对应的binder实体对象。binder_transaction_data结构体是binder驱动通信的数据结构,该过程最终是把Binder请求码BC_TRANSACTION和binder_transaction_data结构体写入到mOut

transact过程,先写完binder_transaction_data数据,其中Parcel data的重要成员变量:

  • mDataSize:保存再data_size,binder_transaction的数据大小;
  • mData: 保存在ptr.buffer, binder_transaction的数据的起始地址;
  • mObjectsSize:保存在ptr.offsets_size,记录着flat_binder_object结构体的个数;
  • mObjects: 保存在offsets, 记录着flat_binder_object结构体在数据偏移量;

接下来执行waitForResponse()方法。

3.6 IPC.waitForResponse

[-> IPCThreadState.cpp]

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    int32_t cmd;
    int32_t err;
    while (1) {
        if ((err=talkWithDriver()) < NO_ERROR) break; // 【见小节3.7】
        ...
        if (mIn.dataAvail() == 0) continue;

        cmd = mIn.readInt32();
        switch (cmd) {
            case BR_TRANSACTION_COMPLETE: ...
            case BR_DEAD_REPLY: ...
            case BR_FAILED_REPLY: ...
            case BR_ACQUIRE_RESULT: ...
            case BR_REPLY: ...
                goto finish;

            default:
                err = executeCommand(cmd);  //【见小节3.x】
                if (err != NO_ERROR) goto finish;
                break;
        }
    }
    ...
    return err;
}

在waitForResponse过程, 首先执行BR_TRANSACTION_COMPLETE;另外,目标进程收到事务后,处理BR_TRANSACTION事务。 然后发送给当前进程,再执行BR_REPLY命令。

3.7 IPC.talkWithDriver

[-> IPCThreadState.cpp]

status_t IPCThreadState::talkWithDriver(bool doReceive)
{
    ...
    binder_write_read bwr;
    const bool needRead = mIn.dataPosition() >= mIn.dataSize();
    const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;

    bwr.write_size = outAvail;
    bwr.write_buffer = (uintptr_t)mOut.data();

    if (doReceive && needRead) {
        //接收数据缓冲区信息的填充。如果以后收到数据,就直接填在mIn中了。
        bwr.read_size = mIn.dataCapacity();
        bwr.read_buffer = (uintptr_t)mIn.data();
    } else {
        bwr.read_size = 0;
        bwr.read_buffer = 0;
    }
    //当读缓冲和写缓冲都为空,则直接返回
    if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;

    bwr.write_consumed = 0;
    bwr.read_consumed = 0;
    status_t err;
    do {
        //通过ioctl不停的读写操作,跟Binder Driver进行通信
        if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
            err = NO_ERROR;
        ...
    } while (err == -EINTR); //当被中断,则继续执行
    ...
    return err;
}

binder_write_read结构体用来与Binder设备交换数据的结构, 通过ioctl与mDriverFD通信,是真正与Binder驱动进行数据读写交互的过程。 主要是操作mOut和mIn变量。

ioctl()经过系统调用后进入Binder Driver.

四. Binder Driver

ioctl -> binder_ioctl -> binder_ioctl_write_read

4.1 binder_ioctl_write_read

[-> binder.c]

static int binder_ioctl_write_read(struct file *filp,
                unsigned int cmd, unsigned long arg,
                struct binder_thread *thread)
{
    struct binder_proc *proc = filp->private_data;
    void __user *ubuf = (void __user *)arg;
    struct binder_write_read bwr;

    //将用户空间bwr结构体拷贝到内核空间
    copy_from_user(&bwr, ubuf, sizeof(bwr));
    ...

    if (bwr.write_size > 0) {
        //将数据放入目标进程【见小节4.2】
        ret = binder_thread_write(proc, thread,
                      bwr.write_buffer,
                      bwr.write_size,
                      &bwr.write_consumed);
        ...
    }
    if (bwr.read_size > 0) {
        //读取自己队列的数据 【见小节】
        ret = binder_thread_read(proc, thread, bwr.read_buffer,
             bwr.read_size,
             &bwr.read_consumed,
             filp->f_flags & O_NONBLOCK);
        if (!list_empty(&proc->todo))
            wake_up_interruptible(&proc->wait);
        ...
    }

    //将内核空间bwr结构体拷贝到用户空间
    copy_to_user(ubuf, &bwr, sizeof(bwr));
    ...
}   

4.2 binder_thread_write

static int binder_thread_write(struct binder_proc *proc,
            struct binder_thread *thread,
            binder_uintptr_t binder_buffer, size_t size,
            binder_size_t *consumed)
{
    uint32_t cmd;
    void __user *buffer = (void __user *)(uintptr_t)binder_buffer;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;
    while (ptr < end && thread->return_error == BR_OK) {
        //拷贝用户空间的cmd命令,此时为BC_TRANSACTION
        if (get_user(cmd, (uint32_t __user *)ptr)) -EFAULT;
        ptr += sizeof(uint32_t);
        switch (cmd) {
        case BC_TRANSACTION:
        case BC_REPLY: {
            struct binder_transaction_data tr;
            //拷贝用户空间的binder_transaction_data
            if (copy_from_user(&tr, ptr, sizeof(tr)))   return -EFAULT;
            ptr += sizeof(tr);
            // 见小节4.3】
            binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
            break;
        }
        ...
    }
    *consumed = ptr - buffer;
  }
  return 0;
}

4.3 binder_transaction

static void binder_transaction(struct binder_proc *proc,
               struct binder_thread *thread,
               struct binder_transaction_data *tr, int reply){
    struct binder_transaction *t;
   	struct binder_work *tcomplete;
    ...
    
    if (reply) {
        ...
    }else {
        if (tr->target.handle) {
            ...
        } else {
            // handle=0则找到servicemanager实体
            target_node = binder_context_mgr_node;
        }
        //target_proc为servicemanager进程
        target_proc = target_node->proc;
    }

    if (target_thread) {
        ...
    } else {
        //找到servicemanager进程的todo队列
        target_list = &target_proc->todo;
        target_wait = &target_proc->wait;
    }

    t = kzalloc(sizeof(*t), GFP_KERNEL);
    tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);

    //非oneway的通信方式,把当前thread保存到transaction的from字段
    if (!reply && !(tr->flags & TF_ONE_WAY))
        t->from = thread;
    else
        t->from = NULL;

    t->sender_euid = task_euid(proc->tsk);
    t->to_proc = target_proc; //此次通信目标进程为servicemanager进程
    t->to_thread = target_thread;
    t->code = tr->code;  //此次通信code = ADD_SERVICE_TRANSACTION
    t->flags = tr->flags;  // 此次通信flags = 0
    t->priority = task_nice(current);

    //从servicemanager进程中分配buffer
    t->buffer = binder_alloc_buf(target_proc, tr->data_size,
        tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));

    t->buffer->allow_user_free = 0;
    t->buffer->transaction = t;
    t->buffer->target_node = target_node;

    if (target_node)
        binder_inc_node(target_node, 1, 0, NULL); //引用计数加1
    offp = (binder_size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

    //分别拷贝用户空间的binder_transaction_data中ptr.buffer和ptr.offsets到内核
    copy_from_user(t->buffer->data,
        (const void __user *)(uintptr_t)tr->data.ptr.buffer, tr->data_size);
    copy_from_user(offp,
        (const void __user *)(uintptr_t)tr->data.ptr.offsets, tr->offsets_size);

    off_end = (void *)offp + tr->offsets_size;

    for (; offp < off_end; offp++) {
        struct flat_binder_object *fp;
        fp = (struct flat_binder_object *)(t->buffer->data + *offp);
        off_min = *offp + sizeof(struct flat_binder_object);
        switch (fp->type) {
            case BINDER_TYPE_BINDER:
            case BINDER_TYPE_WEAK_BINDER: {
              struct binder_ref *ref;
              //【见4.3.1】
              struct binder_node *node = binder_get_node(proc, fp->binder);
              if (node == NULL) { 
                //服务所在进程 创建binder_node实体【见4.3.2】
                node = binder_new_node(proc, fp->binder, fp->cookie);
                ...
              }
              //servicemanager进程binder_ref【见4.3.3】
              ref = binder_get_ref_for_node(target_proc, node);
              ...
              //调整type为HANDLE类型
              if (fp->type == BINDER_TYPE_BINDER)
                fp->type = BINDER_TYPE_HANDLE;
              else
                fp->type = BINDER_TYPE_WEAK_HANDLE;
              fp->binder = 0;
              fp->handle = ref->desc; //设置handle值
              fp->cookie = 0;
              binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE,
                       &thread->todo);
            } break;
            case :...
    }

    if (reply) {
        ..
    } else if (!(t->flags & TF_ONE_WAY)) {
        //BC_TRANSACTION 且 非oneway,则设置事务栈信息
        t->need_reply = 1;
        t->from_parent = thread->transaction_stack;
        thread->transaction_stack = t;
    } else {
        ...
    }

    //将BINDER_WORK_TRANSACTION添加到目标队列,本次通信的目标队列为target_proc->todo
    t->work.type = BINDER_WORK_TRANSACTION;
    list_add_tail(&t->work.entry, target_list);

    //将BINDER_WORK_TRANSACTION_COMPLETE添加到当前线程的todo队列
    tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
    list_add_tail(&tcomplete->entry, &thread->todo);

    //唤醒等待队列,本次通信的目标队列为target_proc->wait
    if (target_wait)
        wake_up_interruptible(target_wait);
    return;
}

注册服务的过程,传递的是BBinder对象,故[小节3.2.1]的writeStrongBinder()过程中localBinder不为空, 从而flat_binder_object.type等于BINDER_TYPE_BINDER。

服务注册过程是在服务所在进程创建binder_node,在servicemanager进程创建binder_ref。 对于同一个binder_node,每个进程只会创建一个binder_ref对象。

向servicemanager的binder_proc->todo添加BINDER_WORK_TRANSACTION事务,接下来进入ServiceManager进程。

4.3.1 binder_get_node

static struct binder_node *binder_get_node(struct binder_proc *proc,
             binder_uintptr_t ptr)
{
  struct rb_node *n = proc->nodes.rb_node;
  struct binder_node *node;

  while (n) {
    node = rb_entry(n, struct binder_node, rb_node);

    if (ptr < node->ptr)
      n = n->rb_left;
    else if (ptr > node->ptr)
      n = n->rb_right;
    else
      return node;
  }
  return NULL;
}

从binder_proc来根据binder指针ptr值,查询相应的binder_node。

4.3.2 binder_new_node

static struct binder_node *binder_new_node(struct binder_proc *proc,
                       binder_uintptr_t ptr,
                       binder_uintptr_t cookie)
{
    struct rb_node **p = &proc->nodes.rb_node;
    struct rb_node *parent = NULL;
    struct binder_node *node;
    ... //红黑树位置查找

    //给新创建的binder_node 分配内核空间
    node = kzalloc(sizeof(*node), GFP_KERNEL);
    
    // 将新创建的node添加到proc红黑树;
    rb_link_node(&node->rb_node, parent, p);
    rb_insert_color(&node->rb_node, &proc->nodes);
    node->debug_id = ++binder_last_id;
    node->proc = proc;
    node->ptr = ptr;
    node->cookie = cookie;
    node->work.type = BINDER_WORK_NODE; //设置binder_work的type
    INIT_LIST_HEAD(&node->work.entry);
    INIT_LIST_HEAD(&node->async_todo);
    return node;
}

4.3.3 binder_get_ref_for_node

static struct binder_ref *binder_get_ref_for_node(struct binder_proc *proc,
              struct binder_node *node)
{
  struct rb_node *n;
  struct rb_node **p = &proc->refs_by_node.rb_node;
  struct rb_node *parent = NULL;
  struct binder_ref *ref, *new_ref;
  //从refs_by_node红黑树,找到binder_ref则直接返回。
  while (*p) {
    parent = *p;
    ref = rb_entry(parent, struct binder_ref, rb_node_node);

    if (node < ref->node)
      p = &(*p)->rb_left;
    else if (node > ref->node)
      p = &(*p)->rb_right;
    else
      return ref;
  }
  
  //创建binder_ref
  new_ref = kzalloc_preempt_disabled(sizeof(*ref));
  
  new_ref->debug_id = ++binder_last_id;
  new_ref->proc = proc; //记录进程信息
  new_ref->node = node; //记录binder节点
  rb_link_node(&new_ref->rb_node_node, parent, p);
  rb_insert_color(&new_ref->rb_node_node, &proc->refs_by_node);

  //计算binder引用的handle值,该值返回给target_proc进程
  new_ref->desc = (node == binder_context_mgr_node) ? 0 : 1;
  //从红黑树最最左边的handle对比,依次递增,直到红黑树遍历结束或者找到更大的handle则结束。
  for (n = rb_first(&proc->refs_by_desc); n != NULL; n = rb_next(n)) {
    //根据binder_ref的成员变量rb_node_desc的地址指针n,来获取binder_ref的首地址
    ref = rb_entry(n, struct binder_ref, rb_node_desc);
    if (ref->desc > new_ref->desc)
      break;
    new_ref->desc = ref->desc + 1;
  }

  // 将新创建的new_ref 插入proc->refs_by_desc红黑树
  p = &proc->refs_by_desc.rb_node;
  while (*p) {
    parent = *p;
    ref = rb_entry(parent, struct binder_ref, rb_node_desc);

    if (new_ref->desc < ref->desc)
      p = &(*p)->rb_left;
    else if (new_ref->desc > ref->desc)
      p = &(*p)->rb_right;
    else
      BUG();
  }
  rb_link_node(&new_ref->rb_node_desc, parent, p);
  rb_insert_color(&new_ref->rb_node_desc, &proc->refs_by_desc);
  if (node) {
    hlist_add_head(&new_ref->node_entry, &node->refs);
  } 
  return new_ref;
}

handle值计算方法规律:

  • 每个进程binder_proc所记录的binder_ref的handle值是从1开始递增的;
  • 所有进程binder_proc所记录的handle=0的binder_ref都指向service manager;
  • 同一个服务的binder_node在不同进程的binder_ref的handle值可以不同;

五. ServiceManager

Binder系列3—启动ServiceManager已介绍其原理,循环在binder_loop()过程, 会调用binder_parse()方法。

5.1 binder_parse

[-> servicemanager/binder.c]

int binder_parse(struct binder_state *bs, struct binder_io *bio,
                 uintptr_t ptr, size_t size, binder_handler func)
{
    int r = 1;
    uintptr_t end = ptr + (uintptr_t) size;

    while (ptr < end) {
        uint32_t cmd = *(uint32_t *) ptr;
        ptr += sizeof(uint32_t);
        switch(cmd) {
        case BR_TRANSACTION: {
            struct binder_transaction_data *txn = (struct binder_transaction_data *) ptr;
            ...
            binder_dump_txn(txn);
            if (func) {
                unsigned rdata[256/4];
                struct binder_io msg; 
                struct binder_io reply;
                int res;

                bio_init(&reply, rdata, sizeof(rdata), 4);
                bio_init_from_txn(&msg, txn); //从txn解析出binder_io信息
                 // 收到Binder事务 【见小节5.2】
                res = func(bs, txn, &msg, &reply);
                // 发送reply事件【见小节5.4】
                binder_send_reply(bs, &reply, txn->data.ptr.buffer, res);
            }
            ptr += sizeof(*txn);
            break;
        }
        case : ...
    }
    return r;
}

5.2 svcmgr_handler

[-> service_manager.c]

int svcmgr_handler(struct binder_state *bs,
                   struct binder_transaction_data *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    size_t len;
    uint32_t handle;
    uint32_t strict_policy;
    int allow_isolated;
    ...
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    ...

    switch(txn->code) {
      case SVC_MGR_ADD_SERVICE: 
          s = bio_get_string16(msg, &len);
          ...
          handle = bio_get_ref(msg); //获取handle
          allow_isolated = bio_get_uint32(msg) ? 1 : 0;
           //注册指定服务 【见小节5.3】
          if (do_add_service(bs, s, len, handle, txn->sender_euid,
              allow_isolated, txn->sender_pid))
              return -1;
          break;
       case :...
    }

    bio_put_uint32(reply, 0);
    return 0;
}

5.3 do_add_service

[-> service_manager.c]

int do_add_service(struct binder_state *bs,
                   const uint16_t *s, size_t len,
                   uint32_t handle, uid_t uid, int allow_isolated,
                   pid_t spid)
{
    struct svcinfo *si;

    if (!handle || (len == 0) || (len > 127))
        return -1;

    //权限检查
    if (!svc_can_register(s, len, spid)) {
        return -1;
    }

    //服务检索
    si = find_svc(s, len);
    if (si) {
        if (si->handle) {
            svcinfo_death(bs, si); //服务已注册时,释放相应的服务
        }
        si->handle = handle;
    } else {
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        if (!si) {  //内存不足,无法分配足够内存
            return -1;
        }
        si->handle = handle;
        si->len = len;
        memcpy(si->name, s, (len + 1) * sizeof(uint16_t)); //内存拷贝服务信息
        si->name[len] = '\0';
        si->death.func = (void*) svcinfo_death;
        si->death.ptr = si;
        si->allow_isolated = allow_isolated;
        si->next = svclist; // svclist保存所有已注册的服务
        svclist = si;
    }

    //以BC_ACQUIRE命令,handle为目标的信息,通过ioctl发送给binder驱动
    binder_acquire(bs, handle);
    //以BC_REQUEST_DEATH_NOTIFICATION命令的信息,通过ioctl发送给binder驱动,主要用于清理内存等收尾工作。
    binder_link_to_death(bs, handle, &si->death);
    return 0;
}

svcinfo记录着服务名和handle信息,保存到svclist列表。

5.4 binder_send_reply

[-> servicemanager/binder.c]

void binder_send_reply(struct binder_state *bs,
                       struct binder_io *reply,
                       binder_uintptr_t buffer_to_free,
                       int status)
{
    struct {
        uint32_t cmd_free;
        binder_uintptr_t buffer;
        uint32_t cmd_reply;
        struct binder_transaction_data txn;
    } __attribute__((packed)) data;

    data.cmd_free = BC_FREE_BUFFER; //free buffer命令
    data.buffer = buffer_to_free;
    data.cmd_reply = BC_REPLY; // reply命令
    data.txn.target.ptr = 0;
    data.txn.cookie = 0;
    data.txn.code = 0;
    if (status) {
        ...
    } else {
        data.txn.flags = 0;
        data.txn.data_size = reply->data - reply->data0;
        data.txn.offsets_size = ((char*) reply->offs) - ((char*) reply->offs0);
        data.txn.data.ptr.buffer = (uintptr_t)reply->data0;
        data.txn.data.ptr.offsets = (uintptr_t)reply->offs0;
    }
    //向Binder驱动通信
    binder_write(bs, &data, sizeof(data));
}

binder_write进入binder驱动后,将BC_FREE_BUFFER和BC_REPLY命令协议发送给Binder驱动, 向client端发送reply.

六. 总结

服务注册过程(addService)核心功能:在服务所在进程创建binder_node,在servicemanager进程创建binder_ref。 其中binder_ref的desc再同一个进程内是唯一的:

  • 每个进程binder_proc所记录的binder_ref的handle值是从1开始递增的;
  • 所有进程binder_proc所记录的handle=0的binder_ref都指向service manager;
  • 同一个服务的binder_node在不同进程的binder_ref的handle值可以不同;

Media服务注册的过程涉及到MediaPlayerService(作为Client进程)和Service Manager(作为Service进程),通信流程图如下所示:

media_player_service_ipc

过程分析:

  1. MediaPlayerService进程调用ioctl()向Binder驱动发送IPC数据,该过程可以理解成一个事务binder_transaction(记为T1),执行当前操作的线程binder_thread(记为thread1),则T1->from_parent=NULL,T1->from = thread1,thread1->transaction_stack=T1。其中IPC数据内容包含:
    • Binder协议为BC_TRANSACTION;
    • Handle等于0;
    • RPC代码为ADD_SERVICE;
    • RPC数据为”media.player”。
  2. Binder驱动收到该Binder请求,生成BR_TRANSACTION命令,选择目标处理该请求的线程,即ServiceManager的binder线程(记为thread2),则 T1->to_parent = NULL,T1->to_thread = thread2。并将整个binder_transaction数据(记为T2)插入到目标线程的todo队列;

  3. Service Manager的线程thread2收到T2后,调用服务注册函数将服务”media.player”注册到服务目录中。当服务注册完成后,生成IPC应答数据(BC_REPLY),T2->form_parent = T1,T2->from = thread2, thread2->transaction_stack = T2。

  4. Binder驱动收到该Binder应答请求,生成BR_REPLY命令,T2->to_parent = T1,T2->to_thread = thread1, thread1->transaction_stack = T2。 在MediaPlayerService收到该命令后,知道服务注册完成便可以正常使用。

整个过程中,BC_TRANSACTION和BR_TRANSACTION过程是一个完整的事务过程;BC_REPLY和BR_REPLY是一个完整的事务过程。 到此,其他进行便可以获取该服务,使用服务提供的方法,下一篇文章将会讲述如何获取服务


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