Xposed原理分析

安卓系统启动

什么zygote？

init是内核启动的第一个用户级进程，zygote是由init进程通过解析init.zygote.rc文件而创建的，zygote所对应的具体可执行程序是app_process，所对应的源文件是App_main.cpp，进程名称为zygote。

init.zygote.rc：

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart media
    onrestart restart netd
    writepid /dev/cpuset/foreground/tasks

安卓应用运行？

在ART模式下，zygote被init进程创建出来，用来孵化和启动其他App。zygote进程具有App所需要的所有核心库。

新的App进程在生成后，就会加载本App的程序代码（apk中的dex文件）

Xposed介绍

Xposed是安卓系统上能够修改系统或三方应用信息的框架。

Xposed构成

名称	介绍
Xposed	Xposed框架Native部分
XposedBridge	Xposed向开发者提供的API与相应工具类库
XposedInstaller	Xposed框架Android端本地管理，环境框架，以及第三方module资源下载的工具

Xposed初始化大体工作流程

（1）xposed的主要接口在XposedBrigde.jar中，核心功能在替换的虚拟机中实现。

（2）app_process是Android App的启动程序（具体形式是zygote fork() 调用app_process作为Android app的载体）。

源码分析

初始化

app_process有两个对应源文件，Android.mk会在编译时根据sdk版本选择对应源文件作为入口（app_main.cpp或app_main2.cpp）

...

ifeq (1,$(strip $(shell expr $(PLATFORM_SDK_VERSION) >= 21)))
  LOCAL_SRC_FILES := app_main2.cpp
  LOCAL_MULTILIB := both
  LOCAL_MODULE_STEM_32 := app_process32_xposed
  LOCAL_MODULE_STEM_64 := app_process64_xposed
else
  LOCAL_SRC_FILES := app_main.cpp
  LOCAL_MODULE_STEM := app_process_xposed
endif
...
ifeq (1,$(strip $(shell expr $(PLATFORM_SDK_VERSION) >= 21)))
  include frameworks/base/cmds/xposed/ART.mk
else
  include frameworks/base/cmds/xposed/Dalvik.mk
endif

app_main#main

在系统开机时，会通过app_process去创建zygote虚拟机，就会调用到app_main2.cpp中的main函数。

main函数中主要做两件事：（1）初始化xposed；（2）创建虚拟机

int main(int argc, char* const argv[])
{
    if (xposed::handleOptions(argc, argv))
        return 0;
    
    //代码省略...

    runtime.mParentDir = parentDir;
    // 初始化xposed，主要是将jar包添加至Classpath中
    isXposedLoaded = xposed::initialize(zygote, startSystemServer, className, argc, argv);
    if (zygote) {
        // 如果xposed初始化成功，将zygoteInit 替换为 de.robv.android.xposed.XposedBridge，然后创建虚拟机
        runtime.start(isXposedLoaded ? XPOSED_CLASS_DOTS_ZYGOTE : "com.android.internal.os.ZygoteInit",
                startSystemServer ? "start-system-server" : "");
    } 
    ...
}

app_main#initialize

初始化xposed

（1）初始化xposed内相关变量

（2）调用addJarToClasspath将XposedBridge.jar添加至系统目录。

bool initialize(bool zygote, bool startSystemServer, const char* className, int argc, char* const argv[]) {
    ...
    // 初始化xposed的相关变量
    xposed->zygote = zygote;
    xposed->startSystemServer = startSystemServer;
    xposed->startClassName = className;
    xposed->xposedVersionInt = xposedVersionInt;
    ...
    // 打印 release、sdk、manufacturer、model、rom、fingerprint、platform相关数据
    printRomInfo();

    // 主要在于将jar包加入Classpath
    return addJarToClasspath();
}

frameworks.base.core.jni.AndroidRuntime#start

创建对应虚拟机

start做了4件事：

（1）创建虚拟机

（2）初始化虚拟机

（3）传入调用类de.robv.android.xposed.XposedBridge

（4）初始化XposedBridge

/*
 * Start the Android runtime.  This involves starting the virtual machine
 * and calling the "static void main(String[] args)" method in the class
 * named by "className".
 *
 * Passes the main function two arguments, the class name and the specified
 * options string.
 */
void AndroidRuntime::start(const char* className, const Vector<String8>& options)
{
    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    //创建虚拟机
    if (startVm(&mJavaVM, &env) != 0) {
        return;
    }
    
    // 初始化虚拟机，xposed对虚拟机进行修改
    onVmCreated(env);
   
    // 虚拟机初始化完成后，会调用传入的de.robv.android.xposed.XposedBridge类，初始化java层XposedBridge.jar
    char* slashClassName = toSlashClassName(className);
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        ...
    } else {
        
        jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
        ...
    }
}

Xposed.cpp#onVmCreated

xposed重写了onVmCreated。

onVmCreated做了什么：

1、xposedInitLib->onVmCreatedCommon->initXposedBridge，初始化XposedBridge

（1）将register_natives_XposedBridge中的函数注册为Native方法

2、xposedInitLib->onVmCreatedCommon->onVmCreated，为xposed_callback_class与xposed_callback_method赋值；

（1）xposed_callback_class和xposed_callback_method变量赋值

3、 de.robv.android.xposed.XposedBridge#main，初始化java层XposedBridge.jar

（1）hook 住系统资源相关的方法；

（2）hook 住zygote 的相关方法；

（3）加载系统中已经安装的xposed 模块。

void onVmCreated(JNIEnv* env) {
    // Determine the currently active runtime
    ...

    // Load the suitable libxposed_*.so for it 通过dlopen加载libxposed_art.so
    void* xposedLibHandle = dlopen(xposedLibPath, RTLD_NOW);
    ...

    // Initialize the library  初始化xposed相关库
    bool (*xposedInitLib)(XposedShared* shared) = NULL;
    // 根据动态链接库操作句柄与符号，返回符号对应的地址
    *(void **) (&xposedInitLib) = dlsym(xposedLibHandle, "xposedInitLib");
    if (!xposedInitLib)  {
        ALOGE("Could not find function xposedInitLib");
        return;
    }
    ...
    // xposedInitLib -> onVmCreatedCommon -> initXposedBridge -> 注册Xposed相关Native方法
    if (xposedInitLib(xposed)) {
        xposed->onVmCreated(env);
    }
}

libxposed_art.cpp#xposedInitLib

/** Called by Xposed's app_process replacement. */
bool xposedInitLib(XposedShared* shared) {
    xposed = shared;
    xposed->onVmCreated = &onVmCreatedCommon;
    return true;
}

libxposed_common.cpp#onVmCreatedCommon

void onVmCreatedCommon(JNIEnv* env) {
    if (!initXposedBridge(env) || !initZygoteService(env)) {
        return;
    }

    if (!onVmCreated(env)) {
        return;
    }

    xposedLoadedSuccessfully = true;
    return;
}

libxposed_common.cpp#initXposedBridge

bool initXposedBridge(JNIEnv* env) {
    classXposedBridge = env->FindClass(CLASS_XPOSED_BRIDGE);
    ...
    classXposedBridge = reinterpret_cast<jclass>(env->NewGlobalRef(classXposedBridge));
    ALOGI("Found Xposed class '%s', now initializing", CLASS_XPOSED_BRIDGE);
    // 将register_natives_XposedBridge中的函数注册为Native方法
    if (register_natives_XposedBridge(env, classXposedBridge) != JNI_OK) {
        ALOGE("Could not register natives for '%s'", CLASS_XPOSED_BRIDGE);
        logExceptionStackTrace();
        env->ExceptionClear();
        return false;
    }

    // 获取XposedBridge.jar中的handleHookedMethod方法,并将该方法赋值给methodXposedBridgeHandleHookedMethod，后续会赋值至全局变量中
    methodXposedBridgeHandleHookedMethod = env->GetStaticMethodID(classXposedBridge, "handleHookedMethod",
        "(Ljava/lang/reflect/Member;ILjava/lang/Object;Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;");
    ...
    return true;
}

libxposed_art.cpp#onVmCreated

/** Called very early during VM startup. */
bool onVmCreated(JNIEnv*) {
    // TODO: Handle CLASS_MIUI_RESOURCES?
    ArtMethod::xposed_callback_class = classXposedBridge;
    ArtMethod::xposed_callback_method = methodXposedBridgeHandleHookedMethod;
    return true;
}

de.robv.android.xposed.XposedBridge#main

虚拟机初始化完成后，会调用传入的de.robv.android.xposed.XposedBridge类，初始化java层XposedBridge.jar，调用main函数

（1）hook 系统资源相关的方法；

（2）hook zygote 的相关方法；

（3）加载系统中已经安装的xposed 模块。

protected static void main(String[] args) {
        // Initialize the Xposed framework and modules
        try {
            if (!hadInitErrors()) {
                initXResources();

                SELinuxHelper.initOnce();
                SELinuxHelper.initForProcess(null);

                runtime = getRuntime();
                XPOSED_BRIDGE_VERSION = getXposedVersion();

                if (isZygote) {
                    XposedInit.hookResources();
                    XposedInit.initForZygote();
                }

                XposedInit.loadModules();
            } else {
                Log.e(TAG, "Not initializing Xposed because of previous errors");
            }
        } 

        // Call the original startup code
        if (isZygote) {
            ZygoteInit.main(args);
        } else {
            RuntimeInit.main(args);
        }
    }

初始化结束。

例子

static final String TAG = "XposedTest001";
    //final XC_MethodReplacement replacementTrue = XC_MethodReplacement.returnConstant(true);

    public CheckSNHook(ClassLoader cl) {
        super();

        XposedBridge.log("hooking checkSN.");
        try {
            Class clz = (Class<?>) XposedHelpers.findClass("com.droider.crackme0201.MainActivity", cl);
            //XposedBridge.hookAllMethods(clz, "checkSN", replacementTrue);
            Log.d(TAG, "hooking clz");
            XposedHelpers.findAndHookMethod(clz,
                    "checkSN",
                    String.class, String.class,
                    new XC_MethodHook() {

                        @Override
                        protected void afterHookedMethod(MethodHookParam param)
                                throws Throwable {
                            XposedBridge.log("1CheckSN afterHookedMethod called.");
                            String s1 = (String) param.args[0];
                            String s2 = (String) param.args[1];

                            Log.d(TAG, "s1:" + s1);
                            Log.d(TAG, "s2:" + s2);
                            param.setResult(true);

                            super.afterHookedMethod(param);
                        }
                    });

        } catch (Exception e) {
            e.printStackTrace();
        }
        XposedBridge.log("1hook checkSN done.");
    }

Hook原理分析

XposedBridge#findAndHookMethod

1、根据函数名获取对应Method对象

2、调用XposedBridge.hookMethod函数

public static XC_MethodHook.Unhook findAndHookMethod(Class<?> clazz, String methodName, Object... parameterTypesAndCallback) {
        if (parameterTypesAndCallback.length == 0 || !(parameterTypesAndCallback[parameterTypesAndCallback.length-1] instanceof XC_MethodHook))
            throw new IllegalArgumentException("no callback defined");
        // 封装回调函数
        XC_MethodHook callback = (XC_MethodHook) parameterTypesAndCallback[parameterTypesAndCallback.length-1];
        // 主要函数Method method = clazz.getDeclaredMethod(methodName, parameterTypes);
        Method m = findMethodExact(clazz, methodName, getParameterClasses(clazz.getClassLoader(), parameterTypesAndCallback));
        // 核心函数
        return XposedBridge.hookMethod(m, callback);
    }

XposedBridge#hookMethod

1、将回调函数、参数类型、返回类型记录到AdditionalHookInfo中

2、拦截指定函数调用，并使用其他函数替代（native函数）

public static XC_MethodHook.Unhook hookMethod(Member hookMethod, XC_MethodHook callback) {
        ...
            // 将回调函数、参数类型、返回类型记录到AdditionalHookInfo中
            AdditionalHookInfo additionalInfo = new AdditionalHookInfo(callbacks, parameterTypes, returnType);
            // 拦截指定函数调用，并使用其他函数替代
            hookMethodNative(hookMethod, declaringClass, slot, additionalInfo);
        }

        return callback.new Unhook(hookMethod);
    }

private native synchronized static void hookMethodNative(Member method, Class<?> declaringClass, int slot, Object additionalInfo);

libxposed.cpp#hookMethodNative

1、查找我们需要hook的java Method对应的ArtMethod （每一个java层函数在ART下都有一个对应的ArtMethod）

void XposedBridge_hookMethodNative(JNIEnv* env, jclass, jobject javaReflectedMethod,
            jobject, jint, jobject javaAdditionalInfo) {
    ...
        
    // 获取Java层Method对应native层的ArtMethod指针,将java函数描述为ArtMethod，查找我们需要hook的java Method对应的ArtMethod 
    ArtMethod* artMethod = ArtMethod::FromReflectedMethod(soa, javaReflectedMethod);

    // Hook the method
    artMethod->EnableXposedHook(soa, javaAdditionalInfo);
}

EnableXposedHook

1、创建原函数备份

2、创建 XposedHookInfo 保存原函数、before函数、after函数

3、设置机器指令入口地址，此时跳入到GetQuickProxyInvokeHandler()地址

void ArtMethod::EnableXposedHook(ScopedObjectAccess& soa, jobject additional_info) {
  ...
      
  // 创建原函数备份
  auto* cl = Runtime::Current()->GetClassLinker();
  auto* linear_alloc = cl->GetAllocatorForClassLoader(GetClassLoader());
  ArtMethod* backup_method = cl->CreateRuntimeMethod(linear_alloc);
  backup_method->CopyFrom(this, cl->GetImagePointerSize());
  // 设置标识符kAccXposedOriginalMethod
  backup_method->SetAccessFlags(backup_method->GetAccessFlags() | kAccXposedOriginalMethod);

  // Create a Method/Constructor object for the backup ArtMethod object
  mirror::AbstractMethod* reflected_method;
  if (IsConstructor()) {
    reflected_method = mirror::Constructor::CreateFromArtMethod(soa.Self(), backup_method);
  } else {
    reflected_method = mirror::Method::CreateFromArtMethod(soa.Self(), backup_method);
  }
  reflected_method->SetAccessible<false>(true);

  // 创建 XposedHookInfo 保存原函数、before函数、after函数（reflected_method：被hook的函数，XposedHookInfo包含回调函数）
  XposedHookInfo* hook_info = reinterpret_cast<XposedHookInfo*>(linear_alloc->Alloc(soa.Self(), sizeof(XposedHookInfo)));
  hook_info->reflected_method = soa.Vm()->AddGlobalRef(soa.Self(), reflected_method);
  hook_info->additional_info = soa.Env()->NewGlobalRef(additional_info);
  hook_info->original_method = backup_method;
  ...
  //将entry_point_from_jni_指针指向hook信息（目的是存储），hook信息包括原函数、before函数、after函数
  SetEntryPointFromJniPtrSize(reinterpret_cast<uint8_t*>(hook_info), sizeof(void*));
  
  // 设置机器指令入口地址，此时跳入到GetQuickProxyInvokeHandler()地址
  SetEntryPointFromQuickCompiledCode(GetQuickProxyInvokeHandler());
  SetCodeItemOffset(0);

  // Adjust access flags.
  // 进行标志位清除，此时这个ArtMethod对象对应是Hook后的方法，这个方法的实现不是native的
  const uint32_t kRemoveFlags = kAccNative | kAccSynchronized | kAccAbstract | kAccDefault | kAccDefaultConflict;
  SetAccessFlags((GetAccessFlags() & ~kRemoveFlags) | kAccXposedHookedMethod);

  MutexLock mu(soa.Self(), *Locks::thread_list_lock_);
  Runtime::Current()->GetThreadList()->ForEach(StackReplaceMethodAndInstallInstrumentation, this);
}

artQuickProxyInvokeHandler

extern "C" uint64_t artQuickProxyInvokeHandler(
            ArtMethod* proxy_method, mirror::Object* receiver, Thread* self, ArtMethod** sp)
            const bool is_xposed = proxy_method->IsXposedHookedMethod();//判断 GetAccessFlags() 的kAccXposedHookedMethod 字段
            ......
            if (is_xposed) {
                jmethodID proxy_methodid = soa.EncodeMethod(proxy_method);
                self->EndAssertNoThreadSuspension(old_cause);
                JValue result = InvokeXposedHandleHookedMethod(soa, shorty, rcvr_jobj, proxy_methodid, args);
                local_ref_visitor.FixupReferences();
                return result.GetJ();
            }
            ......
        }

InvokeXposedHandleHookedMethod

JValue InvokeXposedHandleHookedMethod(ScopedObjectAccessAlreadyRunnable& soa, const char* shorty, jobject rcvr_jobj, jmethodID method, std::vector<jvalue>& args) {
            //获取ArtMethod 的 hookinfo 信息，该信息是EntryPointFromJniPtrSize所指向的信息
            const XposedHookInfo* hookInfo = soa.DecodeMethod(method)->GetXposedHookInfo();
            //将hookinfo 转为一个数组，以便和java 层进行通信调用
            jvalue invocation_args[5];
            invocation_args[0].l = hookInfo->reflectedMethod;
            invocation_args[1].i = 1;
            invocation_args[2].l = hookInfo->additionalInfo;
            invocation_args[3].l = rcvr_jobj;
            invocation_args[4].l = args_jobj;
            //通过CallStaticObjectMethodA 调用 xposed_callback_class 类里面 xposed_callback_method 的方法
            //xposed_callback_class： XposedBridge.java
            //xposed_callback_method: handleHookedMethod 方法
            //ArtMethod 的这两个值，在系统开机时 在 onVmCreated 进行赋值的
            jobject result = soa.Env()->CallStaticObjectMethodA(ArtMethod::xposed_callback_class,
                                       ArtMethod::xposed_callback_method,
                                       invocation_args);
        }

InvokeXposedHandleHookedMethod

（1）获取ArtMethod 的 hookinfo 信息，该信息是EntryPointFromJniPtrSize所指向的信息

（2）通过CallStaticObjectMethodA 调用 xposed_callback_class 类里面 xposed_callback_method 的方法

（3）此处xposed_callback_class，xposed_callback_method 是libxposed_art****.cpp#onVmCreated重写时做的事

const XposedHookInfo* GetXposedHookInfo() {
  DCHECK(IsXposedHookedMethod());
  // 前面存储EntryPointFromJniPtrSize指向的信息
  return reinterpret_cast<const XposedHookInfo*>(GetEntryPointFromJniPtrSize(sizeof(void*)));
}

GetXposedHookInfo：获取EntryPointFromJniPtrSize存储的信息

Xposed.java#handleHookedMethod

private static Object handleHookedMethod(Member method, int originalMethodId, Object additionalInfoObj,
            Object thisObject, Object[] args) throws Throwable {
        AdditionalHookInfo additionalInfo = (AdditionalHookInfo) additionalInfoObj;
        ...
        // call "before method" callbacks
        int beforeIdx = 0;
        do {
            try {
                ((XC_MethodHook) callbacksSnapshot[beforeIdx]).beforeHookedMethod(param);
            } catch (Throwable t) {
                XposedBridge.log(t);

                // reset result (ignoring what the unexpectedly exiting callback did)
                param.setResult(null);
                param.returnEarly = false;
                continue;
            }

            if (param.returnEarly) {
                // skip remaining "before" callbacks and corresponding "after" callbacks
                beforeIdx++;
                break;
            }
        } while (++beforeIdx < callbacksLength);

        // call original method if not requested otherwise
        if (!param.returnEarly) {
            try {
                param.setResult(invokeOriginalMethodNative(method, originalMethodId,
                        additionalInfo.parameterTypes, additionalInfo.returnType, param.thisObject, param.args));
            } catch (InvocationTargetException e) {
                param.setThrowable(e.getCause());
            }
        }

        // call "after method" callbacks
        int afterIdx = beforeIdx - 1;
        do {
            Object lastResult =  param.getResult();
            Throwable lastThrowable = param.getThrowable();

            try {
                ((XC_MethodHook) callbacksSnapshot[afterIdx]).afterHookedMethod(param);
            } catch (Throwable t) {
                XposedBridge.log(t);

                // reset to last result (ignoring what the unexpectedly exiting callback did)
                if (lastThrowable == null)
                    param.setResult(lastResult);
                else
                    param.setThrowable(lastThrowable);
            }
        } while (--afterIdx >= 0);

        // return
        if (param.hasThrowable())
            throw param.getThrowable();
        else
            return param.getResult();
    }

XposedBridge.java 类的handleHookedMethod 方法，真正去处理 before、Original、after 这三个方法的调用关系。

ART函数调用原理

每一个Java函数在ART（虚拟机）内部都由一个ArtMethod对象表示，ArtMethod对象中包含了函数名、参数类型、方法体代码入口地址等。

class ArtMethod {
 ...
 protect:
  HeapReference<Class> declaring_class_;
  HeapReference<ObjectArray<ArtMethod>> dex_cache_resolved_methods_;
  HeapReference<ObjectArray<Class>> dex_cache_resolved_types_;
  uint32_t access_flags_;
  uint32_t dex_code_item_offset_;
  uint32_t dex_method_index_;
  uint32_t method_index_;
  struct PACKED(4) PtrSizedFields {
    void* entry_point_from_interpreter_;
    // 用于存储jni函数信息,非jni函数的无用，所以经常被hook框架将原方法保存在entry_point_from_jni_
    void* entry_point_from_jni_;
    // ART HOOK常见的方法是替换入口点，执行hook的函数。（此处指向的是汇编代码，运行的是已经预处理过的机器码）
    void* entry_point_from_quick_compiled_code_;
 
#if defined(ART_USE_PORTABLE_COMPILER)
    void* entry_point_from_portable_compiled_code_;
#endif
  } ptr_sized_fields_;
  static GcRoot<Class> java_lang_reflect_ArtMethod_;
｝