Binder系列6—获取服务(getService)

Posted by Gityuan on November 15, 2015

基于Android 6.0的源码剖析, 本文Client如何向Server获取服务的过程。

一、 获取服务

在Native层的服务注册,我们选择以media为例来展开讲解,先来看看media的类关系图。

1.1 类图

点击查看大图

get_media_player_service

图解:

  • 蓝色: 代表获取MediaPlayerService服务相关的类;
  • 绿色: 代表Binder架构中与Binder驱动通信过程中的最为核心的两个类;
  • 紫色: 代表注册服务和获取服务的公共接口/父类;

二. 获取Media服务

2.1 getMediaPlayerService

[-> framework/av/media/libmedia/IMediaDeathNotifier.cpp]

sp<IMediaPlayerService>&
IMediaDeathNotifier::getMediaPlayerService()
{
    Mutex::Autolock _l(sServiceLock);
    if (sMediaPlayerService == 0) {
        sp<IServiceManager> sm = defaultServiceManager(); //获取ServiceManager
        sp<IBinder> binder;
        do {
            //获取名为"media.player"的服务 【见2.2】
            binder = sm->getService(String16("media.player"));
            if (binder != 0) {
                break;
            }
            usleep(500000); // 0.5s
        } while (true);

        if (sDeathNotifier == NULL) {
            sDeathNotifier = new DeathNotifier(); //创建死亡通知对象
        }

        //将死亡通知连接到binder 【见流程14】
        binder->linkToDeath(sDeathNotifier);
        sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
    }
    return sMediaPlayerService;
}

其中defaultServiceManager()过程在上一篇文章获取ServiceManager已讲过,返回BpServiceManager。

在请求获取名为”media.player”的服务过程中,采用不断循环获取的方法。由于MediaPlayerService服务可能还没向ServiceManager注册完成或者尚未启动完成等情况,故则binder返回为NULL,休眠0.5s后继续请求,直到获取服务为止。

2.2 BpSM.getService

[-> IServiceManager.cpp ::BpServiceManager]

virtual sp<IBinder> getService(const String16& name) const
    {
        unsigned n;
        for (n = 0; n < 5; n++){
            sp<IBinder> svc = checkService(name); //【见2.3】
            if (svc != NULL) return svc;
            sleep(1);
        }
        return NULL;
    }

通过BpServiceManager来获取MediaPlayer服务:检索服务是否存在,当服务存在则返回相应的服务,当服务不存在则休眠1s再继续检索服务。该循环进行5次。为什么是循环5次呢,这估计跟Android的ANR时间为5s相关。如果每次都无法获取服务,循环5次,每次循环休眠1s,忽略checkService()的时间,差不多就是5s的时间

2.3 BpSM.checkService

[-> IServiceManager.cpp ::BpServiceManager]

virtual sp<IBinder> checkService( const String16& name) const
{
    Parcel data, reply;
    //写入RPC头
    data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
    //写入服务名
    data.writeString16(name);
    remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply); //【见2.4】
    return reply.readStrongBinder(); //【见小节2.9】
}

检索指定服务是否存在, 其中remote()为BpBinder。

2.4 BpBinder::transact

[-> BpBinder.cpp]

status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    if (mAlive) {
        //【见流程2.5】
        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,

2.4.1 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2.4.2】
    }

    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。

2.4.2 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。

2.5 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 【见流程2.6】
        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
    }

    if (err != NO_ERROR) {
        if (reply) reply->setError(err);
        return (mLastError = err);
    }

    if ((flags & TF_ONE_WAY) == 0) { //flgs=0进入该分支
        if (reply) {
            //等待响应  【见流程2.7】
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }

    } else {
        //不需要响应消息的binder则进入该分支
        err = waitForResponse(NULL, NULL);
    }
    return err;
}

2.6 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 = CHECK_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

2.7 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; // 【见流程2.8】
        err = mIn.errorCheck();
        if (err < NO_ERROR) break;
        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:
            {
              binder_transaction_data tr;
              err = mIn.read(&tr, sizeof(tr));
              if (reply) {
                  if ((tr.flags & TF_STATUS_CODE) == 0) {
                      reply->ipcSetDataReference(
                          reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                          tr.data_size,
                          reinterpret_cast<const binder_size_t*>(tr.data.ptr.offsets),
                          tr.offsets_size/sizeof(binder_size_t),
                          freeBuffer, this);
                  } else {
                      ...
                  }
              }
            }
            goto finish;

            default:
                err = executeCommand(cmd);
                if (err != NO_ERROR) goto finish;
                break;
        }
    }
    ...
    return err;
}

2.8 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【2.8.1】
        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。 先向service manager进程发送查询服务的请求(BR_TRANSACTION),见Binder系列3—启动ServiceManager。当service manager进程收到该命令后,会执行do_find_service() 查询服务所对应的handle,然后再binder_send_reply()应答 发起者,发送BC_REPLY协议,然后调用binder_transaction(),再向服务请求者的Todo队列 插入事务。

接下来,再看看binder_transaction过程。

2.8.1 binder_transaction

static void binder_transaction(struct binder_proc *proc,
               struct binder_thread *thread,
               struct binder_transaction_data *tr, int reply){
    //根据各种判定,获取以下信息:
    struct binder_thread *target_thread; //目标线程
    struct binder_proc *target_proc;    //目标进程
    struct binder_node *target_node;    //目标binder节点
    struct list_head *target_list;      //目标TODO队列
    wait_queue_head_t *target_wait;     //目标等待队列
    ...

    //分配两个结构体内存
    struct binder_transaction *t = kzalloc(sizeof(*t), GFP_KERNEL);
    struct binder_work *tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
    //从target_proc分配一块buffer
    t->buffer = binder_alloc_buf(target_proc, tr->data_size,

    for (; offp < off_end; offp++) {
        switch (fp->type) {
        case BINDER_TYPE_BINDER: ...
        case BINDER_TYPE_WEAK_BINDER: ...

        case BINDER_TYPE_HANDLE:
        case BINDER_TYPE_WEAK_HANDLE: {
          struct binder_ref *ref = binder_get_ref(proc, fp->handle,
                fp->type == BINDER_TYPE_HANDLE);
          ...
          //此时运行在servicemanager进程,故ref->node是指向服务所在进程的binder实体,
          //而target_proc为请求服务所在的进程,此时并不相等。
          if (ref->node->proc == target_proc) {
            if (fp->type == BINDER_TYPE_HANDLE)
              fp->type = BINDER_TYPE_BINDER;
            else
              fp->type = BINDER_TYPE_WEAK_BINDER;
            fp->binder = ref->node->ptr;
            fp->cookie = ref->node->cookie; //BBinder服务的地址
            binder_inc_node(ref->node, fp->type == BINDER_TYPE_BINDER, 0, NULL);

          } else {
            struct binder_ref *new_ref;
            //请求服务所在进程并非服务所在进程,则为请求服务所在进程创建binder_ref
            new_ref = binder_get_ref_for_node(target_proc, ref->node);
            fp->binder = 0;
            fp->handle = new_ref->desc; //重新赋予handle值
            fp->cookie = 0;
            binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
          }
        } break;

        case BINDER_TYPE_FD: ...
        }
    }
    //分别target_list和当前线程TODO队列插入事务
    t->work.type = BINDER_WORK_TRANSACTION;
    list_add_tail(&t->work.entry, target_list);
    tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
    list_add_tail(&tcomplete->entry, &thread->todo);
    if (target_wait)
        wake_up_interruptible(target_wait);
    return;
}

这个过程非常重要,分两种情况来说:

  1. 当请求服务的进程与服务属于不同进程,则为请求服务所在进程创建binder_ref对象,指向服务进程中的binder_node;
  2. 当请求服务的进程与服务属于同一进程,则不再创建新对象,只是引用计数加1,并且修改type为BINDER_TYPE_BINDER或BINDER_TYPE_WEAK_BINDER。

2.8.2 binder_thread_read

binder_thread_read(...){
    ...
    //当线程todo队列有数据则执行往下执行;当线程todo队列没有数据,则进入休眠等待状态
    ret = wait_event_freezable(thread->wait, binder_has_thread_work(thread));
    ...
    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;
        //先从线程todo队列获取事务数据
        if (!list_empty(&thread->todo)) {
            w = list_first_entry(&thread->todo, struct binder_work, entry);
        // 线程todo队列没有数据, 则从进程todo对获取事务数据
        } else if (!list_empty(&proc->todo) && wait_for_proc_work) {
            ...
        }
        switch (w->type) {
            case BINDER_WORK_TRANSACTION:
                //获取transaction数据
                t = container_of(w, struct binder_transaction, work);
                break;

            case : ...  
        }

        //只有BINDER_WORK_TRANSACTION命令才能继续往下执行
        if (!t) continue;

        if (t->buffer->target_node) {
            ...
        } else {
            tr.target.ptr = NULL;
            tr.cookie = NULL;
            cmd = BR_REPLY; //设置命令为BR_REPLY
        }
        tr.code = t->code;
        tr.flags = t->flags;
        tr.sender_euid = t->sender_euid;

        if (t->from) {
            struct task_struct *sender = t->from->proc->tsk;
            //当非oneway的情况下,将调用者进程的pid保存到sender_pid
            tr.sender_pid = task_tgid_nr_ns(sender, current->nsproxy->pid_ns);
        } else {
            ...
        }

        tr.data_size = t->buffer->data_size;
        tr.offsets_size = t->buffer->offsets_size;
        tr.data.ptr.buffer = (void *)t->buffer->data +
                    proc->user_buffer_offset;
        tr.data.ptr.offsets = tr.data.ptr.buffer +
                    ALIGN(t->buffer->data_size,
                        sizeof(void *));

        //将cmd和数据写回用户空间
        put_user(cmd, (uint32_t __user *)ptr);
        ptr += sizeof(uint32_t);
        copy_to_user(ptr, &tr, sizeof(tr));
        ptr += sizeof(tr);

        list_del(&t->work.entry);
        t->buffer->allow_user_free = 1;
        if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
            ...
        } else {
            t->buffer->transaction = NULL;
            kfree(t); //通信完成则运行释放
        }
        break;
    }
done:
    *consumed = ptr - buffer;
    if (proc->requested_threads + proc->ready_threads == 0 &&
        proc->requested_threads_started < proc->max_threads &&
        (thread->looper & (BINDER_LOOPER_STATE_REGISTERED |
         BINDER_LOOPER_STATE_ENTERED))) {
        proc->requested_threads++;
        // 生成BR_SPAWN_LOOPER命令,用于创建新的线程
        put_user(BR_SPAWN_LOOPER, (uint32_t __user *)buffer);
    }
    return 0;
}

2.9 readStrongBinder

[-> Parcel.cpp]

sp<IBinder> Parcel::readStrongBinder() const
{
    sp<IBinder> val;
    //【见小节2.9.1】
    unflatten_binder(ProcessState::self(), *this, &val);
    return val;
}

2.9.1 unflatten_binder

[-> Parcel.cpp]

status_t unflatten_binder(const sp<ProcessState>& proc,
    const Parcel& in, sp<IBinder>* out)
{
    const flat_binder_object* flat = in.readObject(false);
    if (flat) {
        switch (flat->type) {
            case BINDER_TYPE_BINDER:
                // 当请求服务的进程与服务属于同一进程
                *out = reinterpret_cast<IBinder*>(flat->cookie);
                return finish_unflatten_binder(NULL, *flat, in);
            case BINDER_TYPE_HANDLE:
                //请求服务的进程与服务属于不同进程【见2.9.2】
                *out = proc->getStrongProxyForHandle(flat->handle);
                //创建BpBinder对象
                return finish_unflatten_binder(
                    static_cast<BpBinder*>(out->get()), *flat, in);
        }
    }
    return BAD_TYPE;
}

2.9.2 getStrongProxyForHandle

[-> ProcessState.cpp]

sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
    sp<IBinder> result;

    AutoMutex _l(mLock);
    //查找handle对应的资源项[2.9.3]
    handle_entry* e = lookupHandleLocked(handle);

    if (e != NULL) {
        IBinder* b = e->binder;
        if (b == NULL || !e->refs->attemptIncWeak(this)) {
            ...
            //当handle值所对应的IBinder不存在或弱引用无效时,则创建BpBinder对象
            b = new BpBinder(handle);
            e->binder = b;
            if (b) e->refs = b->getWeakRefs();
            result = b;
        } else {
            result.force_set(b);
            e->refs->decWeak(this);
        }
    }
    return result;
}

readStrongBinder的功能是flat_binder_object解析并创建BpBinder对象.

2.9.3 lookupHandleLocked

ProcessState::handle_entry* ProcessState::lookupHandleLocked(int32_t handle)
{
    const size_t N=mHandleToObject.size();
    //当handle大于mHandleToObject的长度时,进入该分支
    if (N <= (size_t)handle) {
        handle_entry e;
        e.binder = NULL;
        e.refs = NULL;
        //从mHandleToObject的第N个位置开始,插入(handle+1-N)个e到队列中
        status_t err = mHandleToObject.insertAt(e, N, handle+1-N);
        if (err < NO_ERROR) return NULL;
    }
    return &mHandleToObject.editItemAt(handle);
}

根据handle值来查找对应的handle_entry.

二. 总结

请求服务(getService)过程,就是向servicemanager进程查询指定服务,当执行binder_transaction()时,会区分请求服务所属进程情况。

  1. 当请求服务的进程与服务属于不同进程,则为请求服务所在进程创建binder_ref对象,指向服务进程中的binder_node;
    • 最终readStrongBinder(),返回的是BpBinder对象;
  2. 当请求服务的进程与服务属于同一进程,则不再创建新对象,只是引用计数加1,并且修改type为BINDER_TYPE_BINDER或BINDER_TYPE_WEAK_BINDER。
    • 最终readStrongBinder(),返回的是BBinder对象的真实子类;

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