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Android系统进程间通信(IPC)机制Binder中的Client获得Server远程接口过程源代码分析

 
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在上一篇文章中,我们分析了Android系统进程间通信机制Binder中的Server在启动过程使用Service Manager的addService接口把自己添加到Service Manager守护过程中接受管理。在这一篇文章中,我们将深入到Binder驱动程序源代码去分析Client是如何通过Service Manager的getService接口中来获得Server远程接口的。Client只有获得了Server的远程接口之后,才能进一步调用Server提供的服务。

这里,我们仍然是通过Android系统中自带的多媒体播放器为例子来说明Client是如何通过IServiceManager::getService接口来获得MediaPlayerService这个Server的远程接口的。假设计读者已经阅读过前面三篇文章浅谈Service Manager成为Android进程间通信(IPC)机制Binder守护进程之路浅谈Android系统进程间通信(IPC)机制Binder中的Server和Client获得Service Manager接口之路Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析,即假设Service Manager和MediaPlayerService已经启动完毕,Service Manager现在等待Client的请求。

这里,我们要举例子说明的Client便是MediaPlayer了,它声明和实现在frameworks/base/include/media/mediaplayer.h和frameworks/base/media/libmedia/mediaplayer.cpp文件中。MediaPlayer继承于IMediaDeathNotifier类,这个类声明和实现在frameworks/base/include/media/IMediaDeathNotifier.h和frameworks/base/media/libmedia//IMediaDeathNotifier.cpp文件中,里面有一个静态成员函数getMeidaPlayerService,它通过IServiceManager::getService接口来获得MediaPlayerService的远程接口。

在介绍IMediaDeathNotifier::getMeidaPlayerService函数之前,我们先了解一下这个函数的目标。看来前面浅谈Android系统进程间通信(IPC)机制Binder中的Server和Client获得Service Manager接口之路这篇文章的读者知道,我们在获取Service Manager远程接口时,最终是获得了一个BpServiceManager对象的IServiceManager接口。类似地,我们要获得MediaPlayerService的远程接口,实际上就是要获得一个称为BpMediaPlayerService对象的IMediaPlayerService接口。现在,我们就先来看一下BpMediaPlayerService的类图:


从这个类图可以看到,BpMediaPlayerService继承于BpInterface<IMediaPlayerService>类,即BpMediaPlayerService继承了IMediaPlayerService类和BpRefBase类,这两个类又分别继续了RefBase类。BpRefBase类有一个成员变量mRemote,它的类型为IBinder,实际是一个BpBinder对象。BpBinder类使用了IPCThreadState类来与Binder驱动程序进行交互,而IPCThreadState类有一个成员变量mProcess,它的类型为ProcessState,IPCThreadState类借助ProcessState类来打开Binder设备文件/dev/binder,因此,它可以和Binder驱动程序进行交互。

BpMediaPlayerService的构造函数有一个参数impl,它的类型为const sp<IBinder>&,从上面的描述中,这个实际上就是一个BpBinder对象。这样,要创建一个BpMediaPlayerService对象,首先就要有一个BpBinder对象。再来看BpBinder类的构造函数,它有一个参数handle,类型为int32_t,这个参数的意义就是请求MediaPlayerService这个远程接口的进程对MediaPlayerService这个Binder实体的引用了。因此,获取MediaPlayerService这个远程接口的本质问题就变为从Service Manager中获得MediaPlayerService的一个句柄了。

现在,我们就来看一下IMediaDeathNotifier::getMeidaPlayerService的实现:

// establish binder interface to MediaPlayerService
/*static*/const sp<IMediaPlayerService>&
IMediaDeathNotifier::getMediaPlayerService()
{
    LOGV("getMediaPlayerService");
    Mutex::Autolock _l(sServiceLock);
    if (sMediaPlayerService.get() == 0) {
        sp<IServiceManager> sm = defaultServiceManager();
        sp<IBinder> binder;
        do {
            binder = sm->getService(String16("media.player"));
            if (binder != 0) {
                break;
             }
             LOGW("Media player service not published, waiting...");
             usleep(500000); // 0.5 s
        } while(true);

        if (sDeathNotifier == NULL) {
        sDeathNotifier = new DeathNotifier();
    }
    binder->linkToDeath(sDeathNotifier);
    sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
    }
    LOGE_IF(sMediaPlayerService == 0, "no media player service!?");
    return sMediaPlayerService;
}
函数首先通过defaultServiceManager函数来获得Service Manager的远程接口,实际上就是获得BpServiceManager的IServiceManager接口,具体可以参考浅谈Android系统进程间通信(IPC)机制Binder中的Server和Client获得Service Manager接口之路一文。总的来说,这里的语句:

sp<IServiceManager> sm = defaultServiceManager();
相当于是:

sp<IServiceManager> sm = new BpServiceManager(new BpBinder(0)); 
这里的0表示Service Manager的远程接口的句柄值是0。

接下去的while循环是通过sm->getService接口来不断尝试获得名称为“media.player”的Service,即MediaPlayerService。为什么要通过这无穷循环来得MediaPlayerService呢?因为这时候MediaPlayerService可能还没有启动起来,所以这里如果发现取回来的binder接口为NULL,就睡眠0.5秒,然后再尝试获取,这是获取Service接口的标准做法。
我们来看一下BpServiceManager::getService的实现:

class BpServiceManager : public BpInterface<IServiceManager>
{
    ......

	virtual sp<IBinder> getService(const String16& name) const
	{
		unsigned n;
		for (n = 0; n < 5; n++){
			sp<IBinder> svc = checkService(name);
			if (svc != NULL) return svc;
			LOGI("Waiting for service %s...\n", String8(name).string());
			sleep(1);
		}
		return NULL;
	}

	virtual sp<IBinder> checkService( const String16& name) const
	{
		Parcel data, reply;
		data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
		data.writeString16(name);
		remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
		return reply.readStrongBinder();
	}

	......
};
BpServiceManager::getService通过BpServiceManager::checkService执行操作。

在BpServiceManager::checkService中,首先是通过Parcel::writeInterfaceToken往data写入一个RPC头,这个我们在Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析一文已经介绍过了,就是写往data里面写入了一个整数和一个字符串“android.os.IServiceManager”, Service Manager来处理CHECK_SERVICE_TRANSACTION请求之前,会先验证一下这个RPC头,看看是否正确。接着再往data写入一个字符串name,这里就是“media.player”了。回忆一下Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析这篇文章,那里已经往Service Manager中注册了一个名字为“media.player”的MediaPlayerService。

这里的remote()返回的是一个BpBinder,具体可以参考浅谈Android系统进程间通信(IPC)机制Binder中的Server和Client获得Service Manager接口之路一文,于是,就进行到BpBinder::transact函数了:

status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // Once a binder has died, it will never come back to life.
    if (mAlive) {
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }

    return DEAD_OBJECT;
}

这里的mHandle = 0,code =CHECK_SERVICE_TRANSACTION,flags = 0。

这里再进入到IPCThread::transact函数中:

status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
    status_t err = data.errorCheck();

    flags |= TF_ACCEPT_FDS;

    IF_LOG_TRANSACTIONS() {
        TextOutput::Bundle _b(alog);
        alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "
            << handle << " / code " << TypeCode(code) << ": "
            << indent << data << dedent << endl;
    }
    
    if (err == NO_ERROR) {
        LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),
            (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");
        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) {
        #if 0
        if (code == 4) { // relayout
            LOGI(">>>>>> CALLING transaction 4");
        } else {
            LOGI(">>>>>> CALLING transaction %d", code);
        }
        #endif
        if (reply) {
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
        #if 0
        if (code == 4) { // relayout
            LOGI("<<<<<< RETURNING transaction 4");
        } else {
            LOGI("<<<<<< RETURNING transaction %d", code);
        }
        #endif
        
        IF_LOG_TRANSACTIONS() {
            TextOutput::Bundle _b(alog);
            alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "
                << handle << ": ";
            if (reply) alog << indent << *reply << dedent << endl;
            else alog << "(none requested)" << endl;
        }
    } else {
        err = waitForResponse(NULL, NULL);
    }
    
    return err;
}
首先是调用函数writeTransactionData写入将要传输的数据到IPCThreadState的成员变量mOut中去:

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.handle = handle;
    tr.code = code;
    tr.flags = binderFlags;
    
    const status_t err = data.errorCheck();
    if (err == NO_ERROR) {
        tr.data_size = data.ipcDataSize();
        tr.data.ptr.buffer = data.ipcData();
        tr.offsets_size = data.ipcObjectsCount()*sizeof(size_t);
        tr.data.ptr.offsets = data.ipcObjects();
    } else if (statusBuffer) {
        tr.flags |= TF_STATUS_CODE;
        *statusBuffer = err;
        tr.data_size = sizeof(status_t);
        tr.data.ptr.buffer = statusBuffer;
        tr.offsets_size = 0;
        tr.data.ptr.offsets = NULL;
    } else {
        return (mLastError = err);
    }
    
    mOut.writeInt32(cmd);
    mOut.write(&tr, sizeof(tr));
    
    return NO_ERROR;
}
结构体binder_transaction_data在上一篇文章Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析已经介绍过,这里不再累述,这个结构体是用来描述要传输的参数的内容的。这里着重描述一下将要传输的参数tr里面的内容,handle = 0,code =CHECK_SERVICE_TRANSACTION,cmd =BC_TRANSACTION,data里面的数据分别为:

writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);
writeString16("android.os.IServiceManager");
writeString16("media.player");
这是在BpServiceManager::checkService函数里面写进去的,其中前两个是RPC头,Service Manager在收到这个请求时会验证这两个参数是否正确,这点前面也提到了。IPCThread->getStrictModePolicy默认返回0,STRICT_MODE_PENALTY_GATHER定义为:

// Note: must be kept in sync with android/os/StrictMode.java's PENALTY_GATHER
#define STRICT_MODE_PENALTY_GATHER 0x100
我们不关心这个参数的含义,这不会影响我们分析下面的源代码,有兴趣的读者可以研究一下。这里要注意的是,要传输的参数不包含有Binder对象,因此tr.offsets_size = 0。要传输的参数最后写入到IPCThreadState的成员变量mOut中,包括cmd和tr两个数据。

回到IPCThread::transact函数中,由于(flags & TF_ONE_WAY) == 0为true,即这是一个同步请求,并且reply != NULL,最终调用:

err = waitForResponse(reply);
进入到waitForResponse函数中:

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    int32_t cmd;
    int32_t err;

    while (1) {
        if ((err=talkWithDriver()) < NO_ERROR) break;
        err = mIn.errorCheck();
        if (err < NO_ERROR) break;
        if (mIn.dataAvail() == 0) continue;
        
        cmd = mIn.readInt32();
        
        IF_LOG_COMMANDS() {
            alog << "Processing waitForResponse Command: "
                << getReturnString(cmd) << endl;
        }

        switch (cmd) {
        case BR_TRANSACTION_COMPLETE:
            if (!reply && !acquireResult) goto finish;
            break;
        
        case BR_DEAD_REPLY:
            err = DEAD_OBJECT;
            goto finish;

        case BR_FAILED_REPLY:
            err = FAILED_TRANSACTION;
            goto finish;
        
        case BR_ACQUIRE_RESULT:
            {
                LOG_ASSERT(acquireResult != NULL, "Unexpected brACQUIRE_RESULT");
                const int32_t result = mIn.readInt32();
                if (!acquireResult) continue;
                *acquireResult = result ? NO_ERROR : INVALID_OPERATION;
            }
            goto finish;
        
        case BR_REPLY:
            {
                binder_transaction_data tr;
                err = mIn.read(&tr, sizeof(tr));
                LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");
                if (err != NO_ERROR) goto finish;

                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 size_t*>(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t),
                            freeBuffer, this);
                    } else {
                        err = *static_cast<const status_t*>(tr.data.ptr.buffer);
                        freeBuffer(NULL,
                            reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(size_t), this);
                    }
                } else {
                    freeBuffer(NULL,
                        reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                        tr.data_size,
                        reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                        tr.offsets_size/sizeof(size_t), this);
                    continue;
                }
            }
            goto finish;

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

finish:
    if (err != NO_ERROR) {
        if (acquireResult) *acquireResult = err;
        if (reply) reply->setError(err);
        mLastError = err;
    }
    
    return err;
}
这个函数通过IPCThreadState::talkWithDriver与驱动程序进行交互:

status_t IPCThreadState::talkWithDriver(bool doReceive)
{
	LOG_ASSERT(mProcess->mDriverFD >= 0, "Binder driver is not opened");

	binder_write_read bwr;

	// Is the read buffer empty?
	const bool needRead = mIn.dataPosition() >= mIn.dataSize();

	// We don't want to write anything if we are still reading
	// from data left in the input buffer and the caller
	// has requested to read the next data.
	const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;

	bwr.write_size = outAvail;
	bwr.write_buffer = (long unsigned int)mOut.data();

	// This is what we'll read.
	if (doReceive && needRead) {
		bwr.read_size = mIn.dataCapacity();
		bwr.read_buffer = (long unsigned int)mIn.data();
	} else {
		bwr.read_size = 0;
	}

	......

	// Return immediately if there is nothing to do.
	if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;

	bwr.write_consumed = 0;
	bwr.read_consumed = 0;
	status_t err;
	do {
		......
#if defined(HAVE_ANDROID_OS)
		if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
			err = NO_ERROR;
		else
			err = -errno;
#else
		err = INVALID_OPERATION;
#endif
		......
	} while (err == -EINTR);

	......

	if (err >= NO_ERROR) {
		if (bwr.write_consumed > 0) {
			if (bwr.write_consumed < (ssize_t)mOut.dataSize())
				mOut.remove(0, bwr.write_consumed);
			else
				mOut.setDataSize(0);
		}
		if (bwr.read_consumed > 0) {
			mIn.setDataSize(bwr.read_consumed);
			mIn.setDataPosition(0);
		}

		......

		return NO_ERROR;
	}

	return err;
}
这里的needRead为true,因此,bwr.read_size大于0;outAvail也大于0,因此,bwr.write_size也大于0。函数最后通过:

ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)
进入到Binder驱动程序的binder_ioctl函数中。注意,这里的mProcess->mDriverFD是在我们前面调用defaultServiceManager函数获得Service Manager远程接口时,打开的设备文件/dev/binder的文件描述符,mProcess是IPCSThreadState的成员变量。

Binder驱动程序的binder_ioctl函数中,我们只关注BINDER_WRITE_READ命令相关的逻辑:

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
	int ret;
	struct binder_proc *proc = filp->private_data;
	struct binder_thread *thread;
	unsigned int size = _IOC_SIZE(cmd);
	void __user *ubuf = (void __user *)arg;

	/*printk(KERN_INFO "binder_ioctl: %d:%d %x %lx\n", proc->pid, current->pid, cmd, arg);*/

	ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
	if (ret)
		return ret;

	mutex_lock(&binder_lock);
	thread = binder_get_thread(proc);
	if (thread == NULL) {
		ret = -ENOMEM;
		goto err;
	}

	switch (cmd) {
	case BINDER_WRITE_READ: {
		struct binder_write_read bwr;
		if (size != sizeof(struct binder_write_read)) {
			ret = -EINVAL;
			goto err;
		}
		if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
			ret = -EFAULT;
			goto err;
		}
		if (binder_debug_mask & BINDER_DEBUG_READ_WRITE)
			printk(KERN_INFO "binder: %d:%d write %ld at %08lx, read %ld at %08lx\n",
			proc->pid, thread->pid, bwr.write_size, bwr.write_buffer, bwr.read_size, bwr.read_buffer);
		if (bwr.write_size > 0) {
			ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
			if (ret < 0) {
				bwr.read_consumed = 0;
				if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
					ret = -EFAULT;
				goto err;
			}
		}
		if (bwr.read_size > 0) {
			ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);
			if (!list_empty(&proc->todo))
				wake_up_interruptible(&proc->wait);
			if (ret < 0) {
				if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
					ret = -EFAULT;
				goto err;
			}
		}
		if (binder_debug_mask & BINDER_DEBUG_READ_WRITE)
			printk(KERN_INFO "binder: %d:%d wrote %ld of %ld, read return %ld of %ld\n",
			proc->pid, thread->pid, bwr.write_consumed, bwr.write_size, bwr.read_consumed, bwr.read_size);
		if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
			ret = -EFAULT;
			goto err;
		}
		break;
							}
	......
	default:
		ret = -EINVAL;
		goto err;
	}
	ret = 0;
err:
	......
	return ret;
}
这里的filp->private_data的值是在defaultServiceManager函数创建ProcessState对象时,在ProcessState构造函数通过open文件操作函数打开设备文件/dev/binder时设置好的,它表示的是调用open函数打开设备文件/dev/binder的进程上下文信息,这里将它取出来保存在proc本地变量中。

这里的thread本地变量表示当前线程上下文信息,通过binder_get_thread函数获得。在前面执行ProcessState构造函数时,也会通过ioctl文件操作函数进入到这个函数,那是第一次进入到binder_ioctl这里,因此,调用binder_get_thread时,表示当前进程上下文信息的proc变量还没有关于当前线程的上下文信息,因此,会为proc创建一个表示当前线程上下文信息的thread,会保存在proc->threads表示的红黑树结构中。这里调用binder_get_thread就可以直接从proc找到并返回了。

进入到BINDER_WRITE_READ相关的逻辑。先看看BINDER_WRITE_READ的定义:

#define BINDER_WRITE_READ   		_IOWR('b', 1, struct binder_write_read)
这里可以看出,BINDER_WRITE_READ命令的参数类型为struct binder_write_read:

struct binder_write_read {
	signed long	write_size;	/* bytes to write */
	signed long	write_consumed;	/* bytes consumed by driver */
	unsigned long	write_buffer;
	signed long	read_size;	/* bytes to read */
	signed long	read_consumed;	/* bytes consumed by driver */
	unsigned long	read_buffer;
};
这个结构体的含义可以参考浅谈Service Manager成为Android进程间通信(IPC)机制Binder守护进程之路一文。这里首先是通过copy_from_user函数把用户传进来的参数的内容拷贝到本地变量bwr中。
从上面的调用过程,我们知道,这里bwr.write_size是大于0的,因此进入到binder_thread_write函数中,我们只关注BC_TRANSACTION相关的逻辑:

int
binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
					void __user *buffer, int size, signed long *consumed)
{
	uint32_t cmd;
	void __user *ptr = buffer + *consumed;
	void __user *end = buffer + size;

	while (ptr < end && thread->return_error == BR_OK) {
		if (get_user(cmd, (uint32_t __user *)ptr))
			return -EFAULT;
		ptr += sizeof(uint32_t);
		if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
			binder_stats.bc[_IOC_NR(cmd)]++;
			proc->stats.bc[_IOC_NR(cmd)]++;
			thread->stats.bc[_IOC_NR(cmd)]++;
		}
		switch (cmd) {
		......
		case BC_TRANSACTION:
		case BC_REPLY: {
			struct binder_transaction_data tr;

			if (copy_from_user(&tr, ptr, sizeof(tr)))
				return -EFAULT;
			ptr += sizeof(tr);
			binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
			break;
					   }
		......
		default:
			printk(KERN_ERR "binder: %d:%d unknown command %d\n", proc->pid, thread->pid, cmd);
			return -EINVAL;
		}
		*consumed = ptr - buffer;
	}
	return 0;
}
这里再次把用户传出来的参数拷贝到本地变量tr中,tr的类型为struct binder_transaction_data,这个就是前面我们在IPCThreadState::writeTransactionData写入的内容了。

接着进入到binder_transaction函数中,不相关的代码我们忽略掉:

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;
	size_t *offp, *off_end;
	struct binder_proc *target_proc;
	struct binder_thread *target_thread = NULL;
	struct binder_node *target_node = NULL;
	struct list_head *target_list;
	wait_queue_head_t *target_wait;
	struct binder_transaction *in_reply_to = NULL;
	struct binder_transaction_log_entry *e;
	uint32_t return_error;

	.......

	if (reply) {
		......
	} else {
		if (tr->target.handle) {
			......
		} else {
			target_node = binder_context_mgr_node;
			if (target_node == NULL) {
				return_error = BR_DEAD_REPLY;
				goto err_no_context_mgr_node;
			}
		}
		......
		target_proc = target_node->proc;
		if (target_proc == NULL) {
			return_error = BR_DEAD_REPLY;
			goto err_dead_binder;
		}
		if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
			......
		}
	}
	if (target_thread) {
		......
	} else {
		target_list = &target_proc->todo;
		target_wait = &target_proc->wait;
	}
	......

	/* TODO: reuse incoming transaction for reply */
	t = kzalloc(sizeof(*t), GFP_KERNEL);
	if (t == NULL) {
		return_error = BR_FAILED_REPLY;
		goto err_alloc_t_failed;
	}
	binder_stats.obj_created[BINDER_STAT_TRANSACTION]++;

	tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
	if (tcomplete == NULL) {
		return_error = BR_FAILED_REPLY;
		goto err_alloc_tcomplete_failed;
	}
	binder_stats.obj_created[BINDER_STAT_TRANSACTION_COMPLETE]++;

	t->debug_id = ++binder_last_id;
	
	......


	if (!reply && !(tr->flags & TF_ONE_WAY))
		t->from = thread;
	else
		t->from = NULL;
	t->sender_euid = proc->tsk->cred->euid;
	t->to_proc = target_proc;
	t->to_thread = target_thread;
	t->code = tr->code;
	t->flags = tr->flags;
	t->priority = task_nice(current);
	t->buffer = binder_alloc_buf(target_proc, tr->data_size,
		tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
	if (t->buffer == NULL) {
		return_error = BR_FAILED_REPLY;
		goto err_binder_alloc_buf_failed;
	}
	t->buffer->allow_user_free = 0;
	t->buffer->debug_id = t->debug_id;
	t->buffer->transaction = t;
	t->buffer->target_node = target_node;
	if (target_node)
		binder_inc_node(target_node, 1, 0, NULL);

	offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

	if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
		......
		return_error = BR_FAILED_REPLY;
		goto err_copy_data_failed;
	}

	......

	if (reply) {
		......
	} else if (!(t->flags & TF_ONE_WAY)) {
		BUG_ON(t->buffer->async_transaction != 0);
		t->need_reply = 1;
		t->from_parent = thread->transaction_stack;
		thread->transaction_stack = t;
	} else {
		......
	}

	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;

    ......
}
注意,这里的参数reply = 0,表示这是一个BC_TRANSACTION命令。
前面我们提到,传给驱动程序的handle值为0,即这里的tr->target.handle = 0,表示请求的目标Binder对象是Service Manager,因此有:

target_node = binder_context_mgr_node;
target_proc = target_node->proc;
target_list = &target_proc->todo;
target_wait = &target_proc->wait;

其中binder_context_mgr_node是在Service Manager通知Binder驱动程序它是守护过程时创建的。

接着创建一个待完成事项tcomplete,它的类型为struct binder_work,这是等一会要保存在当前线程的todo队列去的,表示当前线程有一个待完成的事务。紧跟着创建一个待处理事务t,它的类型为struct binder_transaction,这是等一会要存在到Service Manager的todo队列去的,表示Service Manager当前有一个事务需要处理。同时,这个待处理事务t也要存放在当前线程的待完成事务transaction_stack列表中去:

t->from_parent = thread->transaction_stack;
thread->transaction_stack = t;
这样表明当前线程还有事务要处理。

继续往下看,就是分别把tcomplete和t放在当前线程thread和Service Manager进程的todo队列去了:

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);
最后,Service Manager有事情可做了,就要唤醒它了:

wake_up_interruptible(target_wait);
前面我们提到,此时Service Manager正在等待Client的请求,也就是Service Manager此时正在进入到Binder驱动程序的binder_thread_read函数中,并且休眠在target->wait上,具体参考浅谈Service Manager成为Android进程间通信(IPC)机制Binder守护进程之路一文。
这里,我们暂时忽略Service Manager被唤醒之后的情景,继续看当前线程的执行。
函数binder_transaction执行完成之后,就一路返回到binder_ioctl函数里去了。函数binder_ioctl从binder_thread_write函数调用处返回后,发现bwr.read_size大于0,于是就进入到binder_thread_read函数去了:

static int
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,
				   void  __user *buffer, int size, signed long *consumed, int non_block)
{
	void __user *ptr = buffer + *consumed;
	void __user *end = buffer + size;

	int ret = 0;
	int wait_for_proc_work;

	if (*consumed == 0) {
		if (put_user(BR_NOOP, (uint32_t __user *)ptr))
			return -EFAULT;
		ptr += sizeof(uint32_t);
	}

retry:
	wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);

	......
	
	if (wait_for_proc_work) {
		......
	} else {
		if (non_block) {
			if (!binder_has_thread_work(thread))
				ret = -EAGAIN;
		} else
			ret = wait_event_interruptible(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;

		if (!list_empty(&thread->todo))
			w = list_first_entry(&thread->todo, struct binder_work, entry);
		else if (!list_empty(&proc->todo) && wait_for_proc_work)
			w = list_first_entry(&proc->todo, struct binder_work, entry);
		else {
			if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
				goto retry;
			break;
		}

		if (end - ptr < sizeof(tr) + 4)
			break;

		switch (w->type) {
		......
		case BINDER_WORK_TRANSACTION_COMPLETE: {
			cmd = BR_TRANSACTION_COMPLETE;
			if (put_user(cmd, (uint32_t __user *)ptr))
				return -EFAULT;
			ptr += sizeof(uint32_t);

			binder_stat_br(proc, thread, cmd);
			if (binder_debug_mask & BINDER_DEBUG_TRANSACTION_COMPLETE)
				printk(KERN_INFO "binder: %d:%d BR_TRANSACTION_COMPLETE\n",
				proc->pid, thread->pid);

			list_del(&w->entry);
			kfree(w);
			binder_stats.obj_deleted[BINDER_STAT_TRANSACTION_COMPLETE]++;
											   } break;
		......
		}

		if (!t)
			continue;

		......
	}

done:
	......
	return 0;
}
函数首先是写入一个操作码BR_NOOP到用户传进来的缓冲区中去。

回忆一下上面的binder_transaction函数,这里的thread->transaction_stack != NULL,并且thread->todo也不为空,所以线程不会进入休眠状态。

进入while循环中,首先是从thread->todo队列中取回待处理事项w,w的类型为BINDER_WORK_TRANSACTION_COMPLETE,这也是在binder_transaction函数里面设置的。对BINDER_WORK_TRANSACTION_COMPLETE的处理也很简单,只是把一个操作码BR_TRANSACTION_COMPLETE写回到用户传进来的缓冲区中去。这时候,用户传进来的缓冲区就包含两个操作码了,分别是BR_NOOP和BINDER_WORK_TRANSACTION_COMPLETE。

binder_thread_read执行完之后,返回到binder_ioctl函数中,将操作结果写回到用户空间中去:

if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
	ret = -EFAULT;
	goto err;
}
最后就返回到IPCThreadState::talkWithDriver函数中了。

IPCThreadState::talkWithDriver函数从下面语句:

ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)
返回后,首先是清空之前写入Binder驱动程序的内容:

if (bwr.write_consumed > 0) {
     if (bwr.write_consumed < (ssize_t)mOut.dataSize())
          mOut.remove(0, bwr.write_consumed);
     else
          mOut.setDataSize(0);
}
接着是设置从Binder驱动程序读取的内容:

if (bwr.read_consumed > 0) {
     mIn.setDataSize(bwr.read_consumed);
     mIn.setDataPosition(0);
}
然后就返回到IPCThreadState::waitForResponse去了。IPCThreadState::waitForResponse函数的处理也很简单,就是处理刚才从Binder驱动程序读入内容了。从前面的分析中,我们知道,从Binder驱动程序读入的内容就是两个整数了,分别是BR_NOOP和BR_TRANSACTION_COMPLETE。对BR_NOOP的处理很简单,正如它的名字所示,什么也不做;而对BR_TRANSACTION_COMPLETE的处理,就分情况了,如果这个请求是异步的,那个整个BC_TRANSACTION操作就完成了,如果这个请求是同步的,即要等待回复的,也就是reply不为空,那么还要继续通过IPCThreadState::talkWithDriver进入到Binder驱动程序中去等待BC_TRANSACTION操作的处理结果。

这里属于后一种情况,于是再次通过IPCThreadState::talkWithDriver进入到Binder驱动程序的binder_ioctl函数中。不过这一次在binder_ioctl函数中,bwr.write_size等于0,而bwr.read_size大于0,于是再次进入到binder_thread_read函数中。这时候thread->transaction_stack仍然不为NULL,不过thread->todo队列已经为空了,因为前面我们已经处理过thread->todo队列的内容了,于是就通过下面语句:

ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
进入休眠状态了,等待Service Manager的唤醒。

现在,我们终于可以回到Service Manager被唤醒之后的过程了。前面我们说过,Service Manager此时正在binder_thread_read函数中休眠中:

static int
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,
				   void  __user *buffer, int size, signed long *consumed, int non_block)
{
	void __user *ptr = buffer + *consumed;
	void __user *end = buffer + size;

	int ret = 0;
	int wait_for_proc_work;

	if (*consumed == 0) {
		if (put_user(BR_NOOP, (uint32_t __user *)ptr))
			return -EFAULT;
		ptr += sizeof(uint32_t);
	}

retry:
	wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);

	......

	if (wait_for_proc_work) {
		......
		if (non_block) {
			if (!binder_has_proc_work(proc, thread))
				ret = -EAGAIN;
		} else
			ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));
	} else {
		......
	}
	
	......

	while (1) {
		uint32_t cmd;
		struct binder_transaction_data tr;
		struct binder_work *w;
		struct binder_transaction *t = NULL;

		if (!list_empty(&thread->todo))
			w = list_first_entry(&thread->todo, struct binder_work, entry);
		else if (!list_empty(&proc->todo) && wait_for_proc_work)
			w = list_first_entry(&proc->todo, struct binder_work, entry);
		else {
			if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
				goto retry;
			break;
		}

		if (end - ptr < sizeof(tr) + 4)
			break;

		switch (w->type) {
		case BINDER_WORK_TRANSACTION: {
			t = container_of(w, struct binder_transaction, work);
									  } break;
		......
		}

		if (!t)
			continue;

		BUG_ON(t->buffer == NULL);
		if (t->buffer->target_node) {
			struct binder_node *target_node = t->buffer->target_node;
			tr.target.ptr = target_node->ptr;
			tr.cookie =  target_node->cookie;
			t->saved_priority = task_nice(current);
			if (t->priority < target_node->min_priority &&
				!(t->flags & TF_ONE_WAY))
				binder_set_nice(t->priority);
			else if (!(t->flags & TF_ONE_WAY) ||
				t->saved_priority > target_node->min_priority)
				binder_set_nice(target_node->min_priority);
			cmd = BR_TRANSACTION;
		} else {
			......
		}
		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;
			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 *));

		if (put_user(cmd, (uint32_t __user *)ptr))
			return -EFAULT;
		ptr += sizeof(uint32_t);
		if (copy_to_user(ptr, &tr, sizeof(tr)))
			return -EFAULT;
		ptr += sizeof(tr);

		......

		list_del(&t->work.entry);
		t->buffer->allow_user_free = 1;
		if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
			t->to_parent = thread->transaction_stack;
			t->to_thread = thread;
			thread->transaction_stack = t;
		} else {
			......
		}
		break;
	}

done:

	*consumed = ptr - buffer;
	......
	return 0;
}
这里就是从语句中唤醒了:

ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));
Service Manager唤醒过来看,继续往下执行,进入到while循环中。首先是从proc->todo中取回待处理事项w。这个事项w的类型是BINDER_WORK_TRANSACTION,这是上面调用binder_transaction的时候设置的,于是通过w得到待处理事务t:

t = container_of(w, struct binder_transaction, work);
接下来的内容,就把cmd和t->buffer的内容拷贝到用户传进来的缓冲区去了,这里就是Service Manager从用户空间传进来的缓冲区了:

if (put_user(cmd, (uint32_t __user *)ptr))
	return -EFAULT;
ptr += sizeof(uint32_t);
if (copy_to_user(ptr, &tr, sizeof(tr)))
	return -EFAULT;
ptr += sizeof(tr);
注意,这里先是把t->buffer的内容拷贝到本地变量tr中,再拷贝到用户空间缓冲区去。关于t->buffer内容的拷贝,请参考Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析一文,它的一个关键地方是Binder驱动程序和Service Manager守护进程共享了同一个物理内存的内容,拷贝的只是这个物理内存在用户空间的虚拟地址回去:

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 *));
对于Binder驱动程序这次操作来说,这个事项就算是处理完了,就要从todo队列中删除了:

list_del(&t->work.entry);
紧接着,还不放删除这个事务,因为它还要等待Service Manager处理完成后,再进一步处理,因此,放在thread->transaction_stack队列中:

t->to_parent = thread->transaction_stack;
t->to_thread = thread;
thread->transaction_stack = t;
还要注意的一个地方是,上面写入的cmd =BR_TRANSACTION,告诉Service Manager守护进程,它要做什么事情,后面我们会看到相应的分析。

这样,binder_thread_read函数就处理完了,回到binder_ioctl函数中,同样是操作结果写回到用户空间的缓冲区中去:

if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
    ret = -EFAULT;
    goto err;
}
最后,就返回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数去了:

void binder_loop(struct binder_state *bs, binder_handler func)
{
    int res;
    struct binder_write_read bwr;
    unsigned readbuf[32];

    bwr.write_size = 0;
    bwr.write_consumed = 0;
    bwr.write_buffer = 0;
    
    readbuf[0] = BC_ENTER_LOOPER;
    binder_write(bs, readbuf, sizeof(unsigned));

    for (;;) {
        bwr.read_size = sizeof(readbuf);
        bwr.read_consumed = 0;
        bwr.read_buffer = (unsigned) readbuf;

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);

        if (res < 0) {
            LOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
            break;
        }

        res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func);
        if (res == 0) {
            LOGE("binder_loop: unexpected reply?!\n");
            break;
        }
        if (res < 0) {
            LOGE("binder_loop: io error %d %s\n", res, strerror(errno));
            break;
        }
    }
}
这里就是从下面的语句:

res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
返回来了。接着就进入binder_parse函数处理从Binder驱动程序里面读取出来的数据:

int binder_parse(struct binder_state *bs, struct binder_io *bio,
				 uint32_t *ptr, uint32_t size, binder_handler func)
{
	int r = 1;
	uint32_t *end = ptr + (size / 4);

	while (ptr < end) {
		uint32_t cmd = *ptr++;
		switch(cmd) {
		......
		case BR_TRANSACTION: {
			struct binder_txn *txn = (void *) ptr;
			......
			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);
				res = func(bs, txn, &msg, &reply);
				binder_send_reply(bs, &reply, txn->data, res);
			}
			ptr += sizeof(*txn) / sizeof(uint32_t);
			break;
							 }
		......
		default:
			LOGE("parse: OOPS %d\n", cmd);
			return -1;
		}
	}

	return r;
}
前面我们说过,Binder驱动程序写入到用户空间的缓冲区中的cmd为BR_TRANSACTION,因此,这里我们只关注BR_TRANSACTION相关的逻辑。

这里用到的两个数据结构struct binder_txn和struct binder_io可以参考前面一篇文章Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析,这里就不复述了。

接着往下看,函数调bio_init来初始化reply变量:

void bio_init(struct binder_io *bio, void *data,
              uint32_t maxdata, uint32_t maxoffs)
{
    uint32_t n = maxoffs * sizeof(uint32_t);

    if (n > maxdata) {
        bio->flags = BIO_F_OVERFLOW;
        bio->data_avail = 0;
        bio->offs_avail = 0;
        return;
    }

    bio->data = bio->data0 = data + n;
    bio->offs = bio->offs0 = data;
    bio->data_avail = maxdata - n;
    bio->offs_avail = maxoffs;
    bio->flags = 0;
}
接着又调用bio_init_from_txn来初始化msg变量:

void bio_init_from_txn(struct binder_io *bio, struct binder_txn *txn)
{
    bio->data = bio->data0 = txn->data;
    bio->offs = bio->offs0 = txn->offs;
    bio->data_avail = txn->data_size;
    bio->offs_avail = txn->offs_size / 4;
    bio->flags = BIO_F_SHARED;
}
最后,真正进行处理的函数是从参数中传进来的函数指针func,这里就是定义在frameworks/base/cmds/servicemanager/service_manager.c文件中的svcmgr_handler函数:

int svcmgr_handler(struct binder_state *bs,
                   struct binder_txn *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    unsigned len;
    void *ptr;
    uint32_t strict_policy;

//    LOGI("target=%p code=%d pid=%d uid=%d\n",
//         txn->target, txn->code, txn->sender_pid, txn->sender_euid);

    if (txn->target != svcmgr_handle)
        return -1;

    // Equivalent to Parcel::enforceInterface(), reading the RPC
    // header with the strict mode policy mask and the interface name.
    // Note that we ignore the strict_policy and don't propagate it
    // further (since we do no outbound RPCs anyway).
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,"invalid id %s\n", str8(s));
        return -1;
    }

    switch(txn->code) {
    case SVC_MGR_GET_SERVICE:
    case SVC_MGR_CHECK_SERVICE:
        s = bio_get_string16(msg, &len);
        ptr = do_find_service(bs, s, len);
        if (!ptr)
            break;
        bio_put_ref(reply, ptr);
        return 0;

    ......
    }
    default:
        LOGE("unknown code %d\n", txn->code);
        return -1;
    }

    bio_put_uint32(reply, 0);
    return 0;
}
这里, Service Manager要处理的code是SVC_MGR_CHECK_SERVICE,这是在前面的BpServiceManager::checkService函数里面设置的。

回忆一下,在BpServiceManager::checkService时,传给Binder驱动程序的参数为:

writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);  
writeString16("android.os.IServiceManager");  
writeString16("media.player");  
这里的语句:

strict_policy = bio_get_uint32(msg);  
s = bio_get_string16(msg, &len);  
s = bio_get_string16(msg, &len); 
其中,会验证一下传进来的第二个参数,即"android.os.IServiceManager"是否正确,这个是验证RPC头,注释已经说得很清楚了。

最后,就是调用do_find_service函数查找是存在名称为"media.player"的服务了。回忆一下前面一篇文章Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析,MediaPlayerService已经把一个名称为"media.player"的服务注册到Service Manager中,所以这里一定能找到。我们看看do_find_service这个函数:

void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len)
{
    struct svcinfo *si;
    si = find_svc(s, len);

//    LOGI("check_service('%s') ptr = %p\n", str8(s), si ? si->ptr : 0);
    if (si && si->ptr) {
        return si->ptr;
    } else {
        return 0;
    }
}
这里又调用了find_svc函数:

struct svcinfo *find_svc(uint16_t *s16, unsigned len)
{
    struct svcinfo *si;

    for (si = svclist; si; si = si->next) {
        if ((len == si->len) &&
            !memcmp(s16, si->name, len * sizeof(uint16_t))) {
            return si;
        }
    }
    return 0;
}
就是在svclist列表中查找对应名称的svcinfo了。

然后返回到do_find_service函数中。回忆一下前面一篇文章Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析,这里的si->ptr就是指MediaPlayerService这个Binder实体在Service Manager进程中的句柄值了。

回到svcmgr_handler函数中,调用bio_put_ref函数将这个Binder引用写回到reply参数。我们看看bio_put_ref的实现:

void bio_put_ref(struct binder_io *bio, void *ptr)
{
    struct binder_object *obj;

    if (ptr)
        obj = bio_alloc_obj(bio);
    else
        obj = bio_alloc(bio, sizeof(*obj));

    if (!obj)
        return;

    obj->flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
    obj->type = BINDER_TYPE_HANDLE;
    obj->pointer = ptr;
    obj->cookie = 0;
}
这里很简单,就是把一个类型为BINDER_TYPE_HANDLE的binder_object写入到reply缓冲区中去。这里的binder_object就是相当于是flat_binder_obj了,具体可以参考Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析一文。

再回到svcmgr_handler函数中,最后,还写入一个0值到reply缓冲区中,表示操作结果码:

bio_put_uint32(reply, 0);
最后返回到binder_parse函数中,调用binder_send_reply函数将操作结果反馈给Binder驱动程序:

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

    data.cmd_free = BC_FREE_BUFFER;
    data.buffer = buffer_to_free;
    data.cmd_reply = BC_REPLY;
    data.txn.target = 0;
    data.txn.cookie = 0;
    data.txn.code = 0;
    if (status) {
        data.txn.flags = TF_STATUS_CODE;
        data.txn.data_size = sizeof(int);
        data.txn.offs_size = 0;
        data.txn.data = &status;
        data.txn.offs = 0;
    } else {
        data.txn.flags = 0;
        data.txn.data_size = reply->data - reply->data0;
        data.txn.offs_size = ((char*) reply->offs) - ((char*) reply->offs0);
        data.txn.data = reply->data0;
        data.txn.offs = reply->offs0;
    }
    binder_write(bs, &data, sizeof(data));
}
注意,这里的status参数为0。从这里可以看出,binder_send_reply告诉Binder驱动程序执行BC_FREE_BUFFER和BC_REPLY命令,前者释放之前在binder_transaction分配的空间,地址为buffer_to_free,buffer_to_free这个地址是Binder驱动程序把自己在内核空间用的地址转换成用户空间地址再传给Service Manager的,所以Binder驱动程序拿到这个地址后,知道怎么样释放这个空间;后者告诉Binder驱动程序,它的SVC_MGR_CHECK_SERVICE操作已经完成了,要查询的服务的句柄值也是保存在data.txn.data,操作结果码是0,也是保存在data.txn.data中。
再来看binder_write函数:

int binder_write(struct binder_state *bs, void *data, unsigned len)
{
    struct binder_write_read bwr;
    int res;
    bwr.write_size = len;
    bwr.write_consumed = 0;
    bwr.write_buffer = (unsigned) data;
    bwr.read_size = 0;
    bwr.read_consumed = 0;
    bwr.read_buffer = 0;
    res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
    if (res < 0) {
        fprintf(stderr,"binder_write: ioctl failed (%s)\n",
                strerror(errno));
    }
    return res;
}
这里可以看出,只有写操作,没有读操作,即read_size为0。
这里又是一个ioctl的BINDER_WRITE_READ操作。直入到驱动程序的binder_ioctl函数后,执行BINDER_WRITE_READ命令,这里就不累述了。
最后,从binder_ioctl执行到binder_thread_write函数,首先是执行BC_FREE_BUFFER命令,这个命令的执行在前面一篇文章Android系统进程间通信(IPC)机制Binder中的Server启动过程源代码分析已经介绍过了,这里就不再累述了。

我们重点关注BC_REPLY命令的执行:

int  
binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,  
                    void __user *buffer, int size, signed long *consumed)  
{  
    uint32_t cmd;  
    void __user *ptr = buffer + *consumed;  
    void __user *end = buffer + size;  
  
    while (ptr < end && thread->return_error == BR_OK) {  
        if (get_user(cmd, (uint32_t __user *)ptr))  
            return -EFAULT;  
        ptr += sizeof(uint32_t);  
        if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {  
            binder_stats.bc[_IOC_NR(cmd)]++;  
            proc->stats.bc[_IOC_NR(cmd)]++;  
            thread->stats.bc[_IOC_NR(cmd)]++;  
        }  
        switch (cmd) {  
        ......  
        case BC_TRANSACTION:  
        case BC_REPLY: {  
            struct binder_transaction_data tr;  
  
            if (copy_from_user(&tr, ptr, sizeof(tr)))  
                return -EFAULT;  
            ptr += sizeof(tr);  
            binder_transaction(proc, thread, &tr, cmd == BC_REPLY);  
            break;  
                       }  
  
        ......  
        *consumed = ptr - buffer;  
    }  
    return 0;  
} 
又再次进入到binder_transaction函数:

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;
	size_t *offp, *off_end;
	struct binder_proc *target_proc;
	struct binder_thread *target_thread = NULL;
	struct binder_node *target_node = NULL;
	struct list_head *target_list;
	wait_queue_head_t *target_wait;
	struct binder_transaction *in_reply_to = NULL;
	struct binder_transaction_log_entry *e;
	uint32_t return_error;

	......

	if (reply) {
		in_reply_to = thread->transaction_stack;
		if (in_reply_to == NULL) {
			......
			return_error = BR_FAILED_REPLY;
			goto err_empty_call_stack;
		}
		......
		thread->transaction_stack = in_reply_to->to_parent;
		target_thread = in_reply_to->from;
		......
		target_proc = target_thread->proc;
	} else {
		......
	}
	if (target_thread) {
		e->to_thread = target_thread->pid;
		target_list = &target_thread->todo;
		target_wait = &target_thread->wait;
	} else {
		......
	}
	

	/* TODO: reuse incoming transaction for reply */
	t = kzalloc(sizeof(*t), GFP_KERNEL);
	if (t == NULL) {
		return_error = BR_FAILED_REPLY;
		goto err_alloc_t_failed;
	}
	binder_stats.obj_created[BINDER_STAT_TRANSACTION]++;

	tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
	if (tcomplete == NULL) {
		return_error = BR_FAILED_REPLY;
		goto err_alloc_tcomplete_failed;
	}
	......

	if (!reply && !(tr->flags & TF_ONE_WAY))
		t->from = thread;
	else
		t->from = NULL;
	t->sender_euid = proc->tsk->cred->euid;
	t->to_proc = target_proc;
	t->to_thread = target_thread;
	t->code = tr->code;
	t->flags = tr->flags;
	t->priority = task_nice(current);
	t->buffer = binder_alloc_buf(target_proc, tr->data_size,
		tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
	if (t->buffer == NULL) {
		return_error = BR_FAILED_REPLY;
		goto err_binder_alloc_buf_failed;
	}
	t->buffer->allow_user_free = 0;
	t->buffer->debug_id = t->debug_id;
	t->buffer->transaction = t;
	t->buffer->target_node = target_node;
	if (target_node)
		binder_inc_node(target_node, 1, 0, NULL);

	offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

	if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
		binder_user_error("binder: %d:%d got transaction with invalid "
			"data ptr\n", proc->pid, thread->pid);
		return_error = BR_FAILED_REPLY;
		goto err_copy_data_failed;
	}
	if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
		binder_user_error("binder: %d:%d got transaction with invalid "
			"offsets ptr\n", proc->pid, thread->pid);
		return_error = BR_FAILED_REPLY;
		goto err_copy_data_failed;
	}
	......

	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);
		switch (fp->type) {
		......
		case BINDER_TYPE_HANDLE:
		case BINDER_TYPE_WEAK_HANDLE: {
			struct binder_ref *ref = binder_get_ref(proc, fp->handle);
			if (ref == NULL) {
				......
				return_error = BR_FAILED_REPLY;
				goto err_binder_get_ref_failed;
			}
			if (ref->node->proc == target_proc) {
				......
			} else {
				struct binder_ref *new_ref;
				new_ref = binder_get_ref_for_node(target_proc, ref->node);
				if (new_ref == NULL) {
					return_error = BR_FAILED_REPLY;
					goto err_binder_get_ref_for_node_failed;
				}
				fp->handle = new_ref->desc;
				binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
				......
			}
		} break;

		......
		}
	}

	if (reply) {
		BUG_ON(t->buffer->async_transaction != 0);
		binder_pop_transaction(target_thread, in_reply_to);
	} else if (!(t->flags & TF_ONE_WAY)) {
		......
	} else {
		......
	}

	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;

    ......
}
这次进入binder_transaction函数的情形和上面介绍的binder_transaction函数的情形基本一致,只是这里的proc、thread和target_proc、target_thread调换了角色,这里的proc和thread指的是Service Manager进程,而target_proc和target_thread指的是刚才请求SVC_MGR_CHECK_SERVICE的进程。

那么,这次是如何找到target_proc和target_thread呢。首先,我们注意到,这里的reply等于1,其次,上面我们提到,Binder驱动程序在唤醒Service Manager,告诉它有一个事务t要处理时,事务t虽然从Service Manager的todo队列中删除了,但是仍然保留在transaction_stack中。因此,这里可以从thread->transaction_stack找回这个等待回复的事务t,然后通过它找回target_proc和target_thread:

in_reply_to = thread->transaction_stack;
target_thread = in_reply_to->from;
target_list = &target_thread->todo;
target_wait = &target_thread->wait;
再接着往下看,由于Service Manager返回来了一个Binder引用,所以这里要处理一下,就是中间的for循环了。这是一个BINDER_TYPE_HANDLE类型的Binder引用,这是前面设置的。先把t->buffer->data的内容转换为一个struct flat_binder_object对象fp,这里的fp->handle值就是这个Service在Service Manager进程里面的引用值了。接通过调用binder_get_ref函数得到Binder引用对象struct binder_ref类型的对象ref:

struct binder_ref *ref = binder_get_ref(proc, fp->handle);
这里一定能找到,因为前面MediaPlayerService执行IServiceManager::addService的时候把自己添加到Service Manager的时候,会在Service Manager进程中创建这个Binder引用,然后把这个Binder引用的句柄值返回给Service Manager用户空间。

这里面的ref->node->proc不等于target_proc,因为这个Binder实体是属于创建MediaPlayerService的进程的,而不是请求这个服务的远程接口的进程的,因此,这里调用binder_get_ref_for_node函数为这个Binder实体在target_proc创建一个引用:

struct binder_ref *new_ref;
new_ref = binder_get_ref_for_node(target_proc, ref->node);
然后增加引用计数:

binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
这样,返回数据中的Binder对象就处理完成了。注意,这里会把fp->handle的值改为在target_proc中的引用值:

fp->handle = new_ref->desc;
这里就相当于是把t->buffer->data里面的Binder对象的句柄值改写了。因为这是在另外一个不同的进程里面的Binder引用,所以句柄值当然要用新的了。这个值最终是要拷贝回target_proc进程的用户空间去的。

再往下看:

if (reply) {
     BUG_ON(t->buffer->async_transaction != 0);
     binder_pop_transaction(target_thread, in_reply_to);
} else if (!(t->flags & TF_ONE_WAY)) {
     ......
} else {
     ......
}
这里reply等于1,执行binder_pop_transaction函数把当前事务in_reply_to从target_thread->transaction_stack队列中删掉,这是上次调用binder_transaction函数的时候设置的,现在不需要了,所以把它删掉。

再往后的逻辑就跟前面执行binder_transaction函数时候一样了,这里不再介绍。最后的结果就是唤醒请求SVC_MGR_CHECK_SERVICE操作的线程:

if (target_wait)
     wake_up_interruptible(target_wait);
这样,Service Manger回复调用SVC_MGR_CHECK_SERVICE请求就算完成了,重新回到frameworks/base/cmds/servicemanager/binder.c文件中的binder_loop函数等待下一个Client请求的到来。事实上,Service Manger回到binder_loop函数再次执行ioctl函数时候,又会再次进入到binder_thread_read函数。这时个会发现thread->todo不为空,这是因为刚才我们调用了:
list_add_tail(&tcomplete->entry, &thread->todo);
把一个工作项tcompelete放在了在thread->todo中,这个tcompelete的type为BINDER_WORK_TRANSACTION_COMPLETE,因此,Binder驱动程序会执行下面操作:

switch (w->type) {  
case BINDER_WORK_TRANSACTION_COMPLETE: {  
    cmd = BR_TRANSACTION_COMPLETE;  
    if (put_user(cmd, (uint32_t __user *)ptr))  
        return -EFAULT;  
    ptr += sizeof(uint32_t);  
  
    list_del(&w->entry);  
    kfree(w);  
      
    } break;  
    ......  
}  
binder_loop函数执行完这个ioctl调用后,才会在下一次调用ioctl进入到Binder驱动程序进入休眠状态,等待下一次Client的请求。
上面讲到调用请求SVC_MGR_CHECK_SERVICE操作的线程被唤醒了,于是,重新执行binder_thread_read函数:
static int  
binder_thread_read(struct binder_proc *proc, struct binder_thread *thread,  
                   void  __user *buffer, int size, signed long *consumed, int non_block)  
{  
    void __user *ptr = buffer + *consumed;  
    void __user *end = buffer + size;  
  
    int ret = 0;  
    int wait_for_proc_work;  
  
    if (*consumed == 0) {  
        if (put_user(BR_NOOP, (uint32_t __user *)ptr))  
            return -EFAULT;  
        ptr += sizeof(uint32_t);  
    }  
  
retry:  
    wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo);  
  
    ......  
  
    if (wait_for_proc_work) {  
        ......  
    } else {  
        if (non_block) {  
            if (!binder_has_thread_work(thread))  
                ret = -EAGAIN;  
        } else  
            ret = wait_event_interruptible(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;  
  
        if (!list_empty(&thread->todo))  
            w = list_first_entry(&thread->todo, struct binder_work, entry);  
        else if (!list_empty(&proc->todo) && wait_for_proc_work)  
            w = list_first_entry(&proc->todo, struct binder_work, entry);  
        else {  
            if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */  
                goto retry;  
            break;  
        }  
  
        ......  
  
        switch (w->type) {  
        case BINDER_WORK_TRANSACTION: {  
            t = container_of(w, struct binder_transaction, work);  
                                      } break;  
        ......  
        }  
  
        if (!t)  
            continue;  
  
        BUG_ON(t->buffer == NULL);  
        if (t->buffer->target_node) {  
            ......  
        } else {  
            tr.target.ptr = NULL;  
            tr.cookie = NULL;  
            cmd = BR_REPLY;  
        }  
        tr.code = t->code;  
        tr.flags = t->flags;  
        tr.sender_euid = t->sender_euid;  
  
        if (t->from) {  
            ......  
        } else {  
            tr.sender_pid = 0;  
        }  
  
        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 *));  
  
        if (put_user(cmd, (uint32_t __user *)ptr))  
            return -EFAULT;  
        ptr += sizeof(uint32_t);  
        if (copy_to_user(ptr, &tr, sizeof(tr)))  
            return -EFAULT;  
        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);  
            binder_stats.obj_deleted[BINDER_STAT_TRANSACTION]++;  
        }  
        break;  
    }  
  
done:  
    ......  
    return 0;  
}  
就是从下面这个调用:

ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
被唤醒过来了。在while循环中,从thread->todo得到w,w->type为BINDER_WORK_TRANSACTION,于是,得到t。从上面可以知道,Service Manager返回来了一个Binder引用和一个结果码0回来,写在t->buffer->data里面,现在把t->buffer->data加上proc->user_buffer_offset,得到用户空间地址,保存在tr.data.ptr.buffer里面,这样用户空间就可以访问这个数据了。由于cmd不等于BR_TRANSACTION,这时就可以把t删除掉了,因为以后都不需要用了。
执行完这个函数后,就返回到binder_ioctl函数,执行下面语句,把数据返回给用户空间:

if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {  
    ret = -EFAULT;  
    goto err;  
}  
接着返回到用户空间IPCThreadState::talkWithDriver函数,最后返回到IPCThreadState::waitForResponse函数,最终执行到下面语句:

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)  
{  
    int32_t cmd;  
    int32_t err;  
  
    while (1) {  
        if ((err=talkWithDriver()) < NO_ERROR) break;  
          
        ......  
  
        cmd = mIn.readInt32();  
  
        ......  
  
        switch (cmd) {  
        ......  
        case BR_REPLY:  
            {  
                binder_transaction_data tr;  
                err = mIn.read(&tr, sizeof(tr));  
                LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");  
                if (err != NO_ERROR) goto finish;  
  
                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 size_t*>(tr.data.ptr.offsets),  
                            tr.offsets_size/sizeof(size_t),  
                            freeBuffer, this);  
                    } else {  
                        ......
                    }  
                } else {  
                    ...... 
                }  
            }  
            goto finish;  
  
        ......  
        }  
    }  
  
finish:  
    ......  
    return err;  
}  
注意,这里的tr.flags等于0,这个是在上面的binder_send_reply函数里设置的。接着就把结果保存在reply了:

reply->ipcSetDataReference(  
       reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),  
       tr.data_size,  
       reinterpret_cast<const size_t*>(tr.data.ptr.offsets),  
       tr.offsets_size/sizeof(size_t),  
       freeBuffer, this);  
我们简单看一下Parcel::ipcSetDataReference函数的实现:

void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize,
    const size_t* objects, size_t objectsCount, release_func relFunc, void* relCookie)
{
    freeDataNoInit();
    mError = NO_ERROR;
    mData = const_cast<uint8_t*>(data);
    mDataSize = mDataCapacity = dataSize;
    //LOGI("setDataReference Setting data size of %p to %lu (pid=%d)\n", this, mDataSize, getpid());
    mDataPos = 0;
    LOGV("setDataReference Setting data pos of %p to %d\n", this, mDataPos);
    mObjects = const_cast<size_t*>(objects);
    mObjectsSize = mObjectsCapacity = objectsCount;
    mNextObjectHint = 0;
    mOwner = relFunc;
    mOwnerCookie = relCookie;
    scanForFds();
}
上面提到,返回来的数据中有一个Binder引用,因此,这里的mObjectSize等于1,这个Binder引用对应的位置记录在mObjects成员变量中。

从这里层层返回,最后回到BpServiceManager::checkService函数中:

virtual sp<IBinder> BpServiceManager::checkService( const String16& name) const
{
    Parcel data, reply;
    data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
    data.writeString16(name);
    remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
    return reply.readStrongBinder();
}
这里就是从:

remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
返回来了。我们接着看一下reply.readStrongBinder函数的实现:

sp<IBinder> Parcel::readStrongBinder() const
{
    sp<IBinder> val;
    unflatten_binder(ProcessState::self(), *this, &val);
    return val;
}
这里调用了unflatten_binder函数来构造一个Binder对象:

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:
                *out = static_cast<IBinder*>(flat->cookie);
                return finish_unflatten_binder(NULL, *flat, in);
            case BINDER_TYPE_HANDLE:
                *out = proc->getStrongProxyForHandle(flat->handle);
                return finish_unflatten_binder(
                    static_cast<BpBinder*>(out->get()), *flat, in);
        }        
    }
    return BAD_TYPE;
}
这里的flat->type是BINDER_TYPE_HANDLE,因此调用ProcessState::getStrongProxyForHandle函数:

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

    AutoMutex _l(mLock);

    handle_entry* e = lookupHandleLocked(handle);

    if (e != NULL) {
        // We need to create a new BpBinder if there isn't currently one, OR we
        // are unable to acquire a weak reference on this current one.  See comment
        // in getWeakProxyForHandle() for more info about this.
        IBinder* b = e->binder;
        if (b == NULL || !e->refs->attemptIncWeak(this)) {
            b = new BpBinder(handle); 
            e->binder = b;
            if (b) e->refs = b->getWeakRefs();
            result = b;
        } else {
            // This little bit of nastyness is to allow us to add a primary
            // reference to the remote proxy when this team doesn't have one
            // but another team is sending the handle to us.
            result.force_set(b);
            e->refs->decWeak(this);
        }
    }

    return result;
}
这里我们可以看到,ProcessState会把使用过的Binder远程接口(BpBinder)缓存起来,这样下次从Service Manager那里请求得到相同的句柄(Handle)时就可以直接返回这个Binder远程接口了,不用再创建一个出来。这里是第一次使用,因此,e->binder为空,于是创建了一个BpBinder对象:

b = new BpBinder(handle); 
e->binder = b;
if (b) e->refs = b->getWeakRefs();
result = b;
最后,函数返回到IMediaDeathNotifier::getMediaPlayerService这里,从这个语句返回:

binder = sm->getService(String16("media.player"));
这里,就相当于是:

binder = new BpBinder(handle);
最后,函数调用:

sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
到了这里,我们可以参考一下前面一篇文章浅谈Android系统进程间通信(IPC)机制Binder中的Server和Client获得Service Manager,就会知道,这里的interface_cast实际上最终调用了IMediaPlayerService::asInterface函数:

android::sp<IMediaPlayerService> IMediaPlayerService::asInterface(const android::sp<android::IBinder>& obj)
{
	android::sp<IServiceManager> intr;
	if (obj != NULL) {             
		intr = static_cast<IMediaPlayerService*>( 
			obj->queryLocalInterface(IMediaPlayerService::descriptor).get());
		if (intr == NULL) {
			intr = new BpMediaPlayerService(obj);
		}
	}
	return intr; 
}
这里的obj就是BpBinder,而BpBinder::queryLocalInterface返回NULL,因此就创建了一个BpMediaPlayerService对象:

intr = new BpMediaPlayerService(new BpBinder(handle));
因此,我们最终就得到了一个BpMediaPlayerService对象,达到我们最初的目标。

有了这个BpMediaPlayerService这个远程接口之后,MediaPlayer就可以调用MediaPlayerService的服务了。

至此,Android系统进程间通信(IPC)机制Binder中的Client如何通过Service Manager的getService函数获得Server远程接口的过程就分析完了,Binder机制的学习就暂告一段落了。

不过,细心的读者可能会发现,我们这里介绍的Binder机制都是基于C/C++语言实现的,但是我们在编写应用程序都是基于Java语言的,那么,我们如何使用Java语言来使用系统的Binder机制来进行进程间通信呢?这就是下一篇文章要介绍的内容了,敬请关注。

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