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...

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接口之路和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&,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& IMediaDeathNotifier::getMediaPlayerService() { LOGV("getMediaPlayerService"); Mutex::Autolock _l(sServiceLock); if (sMediaPlayerService.get() == 0) { sp sm = defaultServiceManager(); sp 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(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 sm = defaultServiceManager(); 相当于是:

sp 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 { ...... virtual sp getService(const String16& name) const { unsigned n; for (n = 0; n < 5; n++){ sp svc = checkService(name); if (svc != NULL) return svc; LOGI("Waiting for service %s...\n", String8(name).string()); sleep(1); } return NULL; } virtual sp 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(tr.data.ptr.buffer), tr.data_size, reinterpret_cast(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), freeBuffer, this); } else { err = *static_cast(tr.data.ptr.buffer); freeBuffer(NULL, reinterpret_cast(tr.data.ptr.buffer), tr.data_size, reinterpret_cast(tr.data.ptr.offsets), tr.offsets_size/sizeof(size_t), this); } } else { freeBuffer(NULL, reinterpret_cast(tr.data.ptr.buffer), tr.data_size, reinterpret_cast(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(tr.data.ptr.buffer), tr.data_size, reinterpret_cast(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(tr.data.ptr.buffer), tr.data_size, reinterpret_cast(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(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(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 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 Parcel::readStrongBinder() const { sp 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& proc, const Parcel& in, sp* out) { const flat_binder_object* flat = in.readObject(false); if (flat) { switch (flat->type) { case BINDER_TYPE_BINDER: *out = static_cast(flat->cookie); return finish_unflatten_binder(NULL, *flat, in); case BINDER_TYPE_HANDLE: *out = proc->getStrongProxyForHandle(flat->handle); return finish_unflatten_binder( static_cast(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 ProcessState::getStrongProxyForHandle(int32_t handle) { sp 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(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::asInterface(const android::sp& obj) { android::sp intr; if (obj != NULL) { intr = static_cast( 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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