详解App保活技术实现

Class<?> amnCls = Class.forName("android.app.ActivityManagerNative");
amn = activityManagerNative.getMethod("getDefault").invoke(amnCls);
Field mRemoteField = amn.getClass().getDeclaredField("mRemote");
mRemoteField.setAccessible(true);
mRemote = (IBinder) mRemoteField.get(amn);

启动Service的Intent:

Intent intent = new Intent();
ComponentName component = new ComponentName(context.getPackageName(), serviceName);
intent.setComponent(component);

Parcel mServiceData = Parcel.obtain();
mServiceData.writeInterfaceToken("android.app.IActivityManager");
mServiceData.writeStrongBinder(null);
mServiceData.writeInt(1);
intent.writeToParcel(mServiceData, 0);
mServiceData.writeString(null); // resolvedType
mServiceData.writeInt(0);
mServiceData.writeString(context.getPackageName()); // callingPackage
mServiceData.writeInt(0);

mRemote.transact(transactCode, mServiceData, null, 1);

Parcel mServiceData = Parcel.obtain();
mServiceData.writeInterfaceToken("android.app.IActivityManager");
mServiceData.writeStrongBinder(null);
mServiceData.writeInt(1);
intent.writeToParcel(mServiceData, 0);
mServiceData.writeString(null); // resolvedType
mServiceData.writeInt(0);
mServiceData.writeString(context.getPackageName()); // callingPackage
mServiceData.writeInt(0);

private long mNativePtr; // used by native code
// android4.4 mNativePtr是int类型

private static long getNativePtr(Parcel parcel) {
    try {
        Field ptrField = parcel.getClass().getDeclaredField("mNativePtr");
        ptrField.setAccessible(true);
        return (long) ptrField.get(parcel);
    } catch (Exception e) {
        e.printStackTrace();
    }
    return 0;
}

Parcel *parcel = (Parcel *) parcel_ptr;

# 链接binder_libs目录下的所有so库
link_directories(binder_libs/${CMAKE_ANDROID_ARCH_ABI})
# 引入binder相关的头文件
include_directories(${CMAKE_CURRENT_SOURCE_DIR}/include/)
# libbinder.so libcutils.so libutils.so libc.so等库链接到libkeep_alive.so
target_link_libraries(
        keep_alive
        ${log-lib} binder cutils utils c)

Parcel *parcel = (Parcel *) parcel_ptr;
size_t data_size = parcel->dataSize();
int fd[2];
// 创建管道
if (pipe(fd) < 0) {return;}

pid_t pid;
// 创建子进程
if ((pid = fork()) < 0) {
  exit(-1);
} else if (pid == 0) {//第一个子进程
  if ((pid = fork()) < 0) {
    exit(-1);
  } else if (pid > 0) {
    // 托孤
    exit(0);
  }
  
   uint8_t data[data_size];
   // 托孤的子进程，读取管道中的数据
   int result = read(fd[0], data, data_size);
}

// 父进程向管道中写数据
int result = write(fd[1], parcel->data(), data_size);

Parcel parcel;
parcel.setData(data, data_size);

// 获取ServiceManager
sp<IServiceManager> sm = defaultServiceManager();
// 获取ActivityManager binder
sp<IBinder> binder = sm->getService(String16("activity"));
// 传输parcel
int result = binder.get()->transact(code, parcel, NULL, 0);

int ioctl(int fd, int request, …);

作用：通过IOCTL函数实现指令的传递

• fd 是用户程序打开设备时使用open函数返回的文件描述符
• request是用户程序对设备的控制命令
• 后面的省略号是一些补充参数，和cmd的意义相关

应用程序在调用ioctl进行设备控制时，最后会调用到设备注册struct file_operations结构体对象时的unlocked_ioctl或者compat_ioctl两个钩子上，例如Binder驱动的这两个钩子是挂到了binder_ioctl方法上：

static const struct file_operations binder_fops = {
  .owner = THIS_MODULE,
  .poll = binder_poll,
  .unlocked_ioctl = binder_ioctl,
  .compat_ioctl = binder_ioctl,
  .mmap = binder_mmap,
  .open = binder_open,
  .flush = binder_flush,
  .release = binder_release,
};

它的实现如下：

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    /*根据不同的命令，调用不同的处理函数进行处理*/
    switch (cmd) {
    case BINDER_WRITE_READ:
        /*读写命令,数据传输,binder IPC通信的核心逻辑*/
        ret = **binder_ioctl_write_read**(filp, cmd, arg, thread);
        break;
    case BINDER_SET_MAX_THREADS:
        /*设置最大线程数，直接将值设置到proc结构的max_threads域中。*/
        break;
    case BINDER_SET_CONTEXT_MGR:
        /*设置Context manager，即将自己设置为ServiceManager，详见3.3*/
        break;
    case BINDER_THREAD_EXIT:
        /*binder线程退出命令，释放相关资源*/
        break;
    case BINDER_VERSION: {
        /*获取binder驱动版本号，在kernel4.4版本中，32位该值为7，64位版本该值为8*/
        break;
    }
   return ret;
}

具体内核层的实现，我们就不关心了。到这里我们了解到，Binder在Android系统中会有一个设备节点，调用ioctl控制这个节点时，实际上会调用到内核态的binder_ioctl方法。

为了利用ioctl启动Android Service，必然是需要用ioctl向binder驱动写数据，而这个控制命令就是BINDER_WRITE_READ。binder驱动层的一些细节我们在这里就不关心了。那么在什么地方会用ioctl 向binder写数据呢？

IPCThreadState.talkWithDriver

阅读Gityuan的Binder系列6—获取服务(getService)一节，在binder模块下IPCThreadState.cpp中有这样的实现(源码目录：frameworks/native/libs/binder/IPCThreadState.cpp)：

status_t IPCThreadState::talkWithDriver(bool doReceive) {
    ...
    binder_write_read bwr;
    bwr.write_buffer = (uintptr_t)mOut.data();
    status_t err;
    do {
        //通过ioctl不停的读写操作，跟Binder Driver进行通信
        if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
            err = NO_ERROR;
        ...
    } while (err == -EINTR); //当被中断，则继续执行
    ...
    return err;
}

可以看到ioctl跟binder driver交互很简单，一个参数是mProcess->mDriverFD，一个参数是BINDER_WRITE_READ，另一个参数是binder_write_read结构体，很幸运的是，NDK中提供了linux/android/binder.h这个头文件，里面就有binder_write_read这个结构体，以及BINDER_WRITE_READ常量的定义。

#include<linux/android/binder.h>
struct binder_write_read {
  binder_size_t write_size;
  binder_size_t write_consumed;
  binder_uintptr_t write_buffer;
  binder_size_t read_size;
  binder_size_t read_consumed;
  binder_uintptr_t read_buffer;
};
#define BINDER_WRITE_READ _IOWR('b', 1, struct binder_write_read)

这意味着，这些结构体和宏定义很可能是版本兼容的。

那我们只需要到时候把数据揌到binder_write_read结构体里面，就可以进行ioctl系统调用了！

/dev/binder

再来看看mProcess->mDriverFD是什么东西。mProcess也就是ProcessState.cpp(源码目录：frameworks/native/libs/binder/ProcessState.cpp)：

ProcessState::ProcessState(const char *driver)
    : mDriverName(String8(driver))
    , mDriverFD(open_driver(driver))
    , ... 
{}

从ProcessState的构造函数中得知，mDriverFD由open_driver方法初始化。

static int open_driver(const char *driver) {
    int fd = open(driver, O_RDWR | O_CLOEXEC);
    if (fd >= 0) {
        int vers = 0;
        status_t result = ioctl(fd, BINDER_VERSION, &vers);
    }
    return fd;
}

ProcessState在哪里实例化呢？

sp<ProcessState> ProcessState::self() {
    if (gProcess != nullptr) {
        return gProcess;
    }
    gProcess = new ProcessState(kDefaultDriver);
    return gProcess;
}

可以看到，ProcessState的gProcess是一个全局单例对象，这意味着，在当前进程中，open_driver只会执行一次，得到的 mDriverFD 会一直被使用。

const char* kDefaultDriver = "/dev/binder";

而open函数操作的这个设备节点就是/dev/binder。

纳尼？在应用层直接操作设备节点？Gityuan大佬不会骗我吧？一般来说，Android系统在集成SELinux的安全机制之后，普通应用甚至是系统应用，都不能直接操作一些设备节点，除非有SELinux规则，给应用所属的域或者角色赋予了那样的权限。

看看文件权限：

➜  ~ adb shell

chiron:/ $ ls -l /dev/binder

crw-rw-rw- 1 root root 10,  49 1972-07-03 18:46 /dev/binder

可以看到，/dev/binder设备对所有用户可读可写。

再看看，SELinux权限：

chiron:/ $ ls -Z /dev/binder

u:object_r:binder_device:s0 /dev/binder

查看源码中对binder_device角色的SELinux规则描述：

allow domain binder_device:chr_file rw_file_perms;

也就是所有domain对binder的字符设备有读写权限，而普通应用属于domain。

写个Demo试一下

验证一下上面的想法，看看ioctl给binder driver发数据好不好使。

1、打开设备

int fd = open("/dev/binder", O_RDWR | O_CLOEXEC);
if (fd < 0) {
    LOGE("Opening '%s' failed: %s\n", "/dev/binder", strerror(errno));
} else {
    LOGD("Opening '%s' success %d: %s\n", "/dev/binder", fd, strerror(errno));
}

2、ioctl

Parcel *parcel = new Parcel;
parcel->writeString16(String16("test"));
binder_write_read bwr;
bwr.write_size = parcel->dataSize();
bwr.write_buffer = (binder_uintptr_t) parcel->data();
int ret = ioctl(fd, BINDER_WRITE_READ, bwr);
LOGD("ioctl result is %d: %s\n", ret, strerror(errno));

3、查看日志

D/KeepAlive: Opening '/dev/binder' success, fd is 35

D/KeepAlive: ioctl result is -1: Invalid argument

打开设备节点成功了，耶✌️！但是ioctl失败了🤔，失败原因是Invalid argument，也就是说可以通信，但是Parcel数据有问题。来看看数据应该是什么样的。

binder_write_read结构体数据封装

IPCThreadState.talkWithDriver方法中，bwr.write_buffer指针指向了mOut.data()，显然mOut是一个Parcel对象。

binder_write_read bwr;
bwr.write_buffer = (uintptr_t)mOut.data();

再来看看什么时候会向mOut中写数据：

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.data.ptr.buffer = data.ipcData();
    ...
    mOut.writeInt32(cmd);
    mOut.write(&tr, sizeof(tr));
    return NO_ERROR;
}

writeTransactionData方法中，会往mOut中写入一个binder_transaction_data结构体数据，binder_transaction_data结构体中又包含了作为参数传进来的data Parcel对象。

writeTransactionData方法会被transact方法调用：

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

    // 默认情况下,都是采用非oneway的方式, 也就是需要等待服务端的返回结果
    if ((flags & TF_ONE_WAY) == 0) {
        if (reply) {
            //等待回应事件
            err = waitForResponse(reply);
        }else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
    } else {
        err = waitForResponse(NULL, NULL);
    }
    return err;
}

IPCThreadState是跟binder driver真正进行交互的类。每个线程都有一个IPCThreadState，每个IPCThreadState中都有一个mIn、一个mOut。成员变量mProcess保存了ProcessState变量(每个进程只有一个)。

接着看一下一次Binder调用的时序图：

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

// 获取ServiceManager
sp<IServiceManager> sm = defaultServiceManager();
// 获取ActivityManager binder
sp<IBinder> binder = sm->getService(String16("activity"));
// 传输parcel
int result = binder.get()->transact(code, parcel, NULL, 0);

sp<IServiceManager> defaultServiceManager() {
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;
    while (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast<IServiceManager>(
                ProcessState::self()->getContextObject(NULL));
        }
    }
    return gDefaultServiceManager;
}

sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& /*caller*/) {
    sp<IBinder> context = getStrongProxyForHandle(0);
    return context;
}

sp<IBinder> binder = serviceManager->getService(String16("activity"));

virtual sp<IBinder> getService(const String16& name) const {
	sp<IBinder> svc = checkService(name);
	if (svc != NULL) return svc;
	return NULL;
}

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

sp<IBinder> Parcel::readStrongBinder() const {
    sp<IBinder> val;
    readNullableStrongBinder(&val);
    return val;
}
status_t Parcel::readNullableStrongBinder(sp<IBinder>* val) const {
    return unflattenBinder(val);
}

status_t Parcel::unflattenBinder(sp<IBinder>* out) const {
    const flat_binder_object* flat = readObject(false);
    if (flat) {
        ...
        sp<IBinder> binder =
            ProcessState::self()->getStrongProxyForHandle(flat->handle);
    }
    return BAD_TYPE;
}

Parcel data, reply;
// data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
// IServiceManager::getInterfaceDescriptor()的值是android.app.IActivityManager
data.writeInterfaceToken(String16("android.app.IActivityManager"));
data.writeString16(String16("activity"));
IPCThreadState::self()->transact(
	0/*ServiceManger的mHandle句柄恒为0*/, 
CHECK_SERVICE_TRANSACTION, data, reply, 0);

const flat_binder_object* flat = reply->readObject(false);
return flat->handle;

Parcel data;
// 把Service相关信息写到parcel中
writeService(data, packageName, serviceName, sdk_version);
IPCThreadState::self()->transact(
	1/*上一步获取的ActivityManger的mHandle句柄值是1*/, 
	CHECK_SERVICE_TRANSACTION, data, reply, 
	1/*TF_ONE_WAY*/);

Intent intent = new Intent();
ComponentName component = new ComponentName(context.getPackageName(), serviceName);
intent.setComponent(component);

Parcel mServiceData = Parcel.obtain();
mServiceData.writeInterfaceToken("android.app.IActivityManager");
mServiceData.writeStrongBinder(null);
mServiceData.writeInt(1);
intent.writeToParcel(mServiceData, 0);
mServiceData.writeString(null); // resolvedType
mServiceData.writeInt(0);
mServiceData.writeString(context.getPackageName()); // callingPackage
mServiceData.writeInt(0);

void writeIntent(Parcel &out, const char *mPackage, const char *mClass) {
    // mAction
    out.writeString16(NULL, 0);
    // uri mData
    out.writeInt32(0);
    // mType
    out.writeString16(NULL, 0);
//    // mIdentifier
    out.writeString16(NULL, 0);
    // mFlags
    out.writeInt32(0);
    // mPackage
    out.writeString16(NULL, 0);
    // mComponent
    out.writeString16(String16(mPackage));
    out.writeString16(String16(mClass));
    // mSourceBounds
    out.writeInt32(0);
    // mCategories
    out.writeInt32(0);
    // mSelector
    out.writeInt32(0);
    // mClipData
    out.writeInt32(0);
    // mContentUserHint
    out.writeInt32(-2);
    // mExtras
    out.writeInt32(-1);
}

// 打开binder设备
int fd = open("/dev/binder", O_RDWR | O_CLOEXEC);
Parcel data, reply;
// data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
// IServiceManager::getInterfaceDescriptor()的值是android.app.IActivityManager
data.writeInterfaceToken(String16("android.app.IActivityManager"));
data.writeString16(String16("activity"));
IPCThreadState::self()->transact(
	0/*ServiceManger的mHandle句柄恒为0*/, 
CHECK_SERVICE_TRANSACTION, data, reply, 0);

const flat_binder_object *flat = reply->readObject(false);
if (flat) {
    LOGD("write_transact handle is:%llu", flat->handle);
}else {
    LOGD("write_transact failed, error=%d", status);
}