Xposed学习二：实现机制

在上一篇我们学习了如何在AS中创建Xposed模块，本篇来分析下官方教程中redClock的实现原理。本系列文章基于version-51

public void handleLoadPackage(XC_LoadPackage.LoadPackageParam lpparam) throws Throwable {

        if(!lpparam.packageName.equals("com.android.systemui")) return;

        XposedBridge.log("we are in systemui !");

        findAndHookMethod("com.android.systemui.statusbar.policy.Clock", lpparam.classLoader, "updateClock", new XC_MethodHook() {

            @Override

            protected void beforeHookedMethod(MethodHookParam param) throws Throwable {

                super.beforeHookedMethod(param);

            }

            @Override

            protected void afterHookedMethod(MethodHookParam param) throws Throwable {

                TextView tv = (TextView) param.thisObject;

                String text = tv.getText().toString();

                tv.setText(text + ")");

                tv.setTextColor(Color.RED);

            }

        });

    }

上面的代码可以将原先在状态栏的时钟文本颜色变成红色，且在后面加")"。看下图：

主要的实现代码在findAndHookMethod函数中,查看函数定义:

findAndHookMethod:

——>findMethodExact(clazz,methodName,paramterClasses);

——>XposedBridge.hookMethod(method，callback);

先看findMethodExact，

代码很简单就是要得到methodName在android中对应的函数对象，根据findAndHookMethod的参数得到字符串sb(格式参考注释行)，用sb在methodCache这个hashMap查找有没有对应的method；若没有则根据methodName和parameterTypes利用getDeclaredMethod得到对应的method。

public static Method findMethodExact(Class<?> clazz, String methodName, Class... parameterTypes) {

        StringBuilder sb = new StringBuilder(clazz.getName());

        sb.append('#');

        sb.append(methodName);

        sb.append(getParametersString(parameterTypes));

        sb.append("#exact");

    // sb = com.android.systemui.statusbar.policy.Clock#updateClock(参数1，参数2)#exact

        String fullMethodName = sb.toString();

        Method e;

    //methodCache键值对存放fullMethodName,method对象

        if(methodCache.containsKey(fullMethodName)) {

            e = (Method)methodCache.get(fullMethodName);

            if(e == null) {

                throw new NoSuchMethodError(fullMethodName);

            } else {

                return e;

            }

        } else {

            try {

                e = clazz.getDeclaredMethod(methodName, parameterTypes);

                e.setAccessible(true);

                methodCache.put(fullMethodName, e);

                return e;

            } catch (NoSuchMethodException var6) {

                methodCache.put(fullMethodName, (Object)null);

                throw new NoSuchMethodError(fullMethodName);

            }

        }

    }

　　看来重点在XposedBridge.hookMethod：

public static Unhook hookMethod(Member hookMethod, XC_MethodHook callback) {

        if(!(hookMethod instanceof Method) && !(hookMethod instanceof Constructor)) {

            throw new IllegalArgumentException("Only methods and constructors can be hooked: " + hookMethod.toString());

        } else if(hookMethod.getDeclaringClass().isInterface()) {

            throw new IllegalArgumentException("Cannot hook interfaces: " + hookMethod.toString());

        } else if(Modifier.isAbstract(hookMethod.getModifiers())) {

            throw new IllegalArgumentException("Cannot hook abstract methods: " + hookMethod.toString());

        } else {

    //上面代码分步检查hookMethod的类型

    //else中的代码得到hookMethod对应的键值对

            boolean newMethod = false;

            Map declaringClass = sHookedMethodCallbacks;

            XposedBridge.CopyOnWriteSortedSet callbacks;

            synchronized(sHookedMethodCallbacks) {

                callbacks = (XposedBridge.CopyOnWriteSortedSet)sHookedMethodCallbacks.get(hookMethod);

                if(callbacks == null) {

                    callbacks = new XposedBridge.CopyOnWriteSortedSet();

                    sHookedMethodCallbacks.put(hookMethod, callbacks);

                    newMethod = true;

                }

            }

            callbacks.add(callback);

    //替换hookMehod的callbacks为callback，其实callback是存放的是hookMethod所有的callback，看定义：</span></span>

    final XposedBridge.CopyOnWriteSortedSet<XC_MethodHook> callbacks;

    //以上代码就是为hookMethod建立对应的callback list

    //sHookedMethodCallbacks存放hookMethod和callback

            if(newMethod) {

                Class declaringClass1 = hookMethod.getDeclaringClass();

                int slot = XposedHelpers.getIntField(hookMethod, "slot");

                Class[] parameterTypes;

                Class returnType;

                if(hookMethod instanceof Method) {

                    parameterTypes = ((Method)hookMethod).getParameterTypes();

                    returnType = ((Method)hookMethod).getReturnType();

                } else {

                    parameterTypes = ((Constructor)hookMethod).getParameterTypes();

                    returnType = null;

                }

        //以上代码得到method的参数和返回值，在AdditionalHookInfo下使用

        //把callback、method参数、method返回值汇总在AdditionalHookInfo类下

                XposedBridge.AdditionalHookInfo additionalInfo = new XposedBridge.AdditionalHookInfo(callbacks, parameterTypes, returnType, (XposedBridge.AdditionalHookInfo)null);

               //本地函数在libxposed_dalvik.cpp

　　　　　　 hookMethodNative(hookMethod, declaringClass1, slot, additionalInfo);

            }

            callback.getClass();

            return new Unhook(callback, hookMethod);

    //为callback绑定hookMthod

        }

    }

上面乱七八糟的走了这么多，登记了2个hashmap{(fullMethodName，method对象),(hookmethod，callback)}。看来java层只是管理这些结构并没有实质性的操作，进入native代码----Xposed.cpp：

//参数：reflectedMethodIndirect==>hookmethod，declaredClassIndirect==>hookmethod所在的类

//        slot==>slot，additionalInfoIndirect==>结构体包含callback、parameterTypes、returnType

void XposedBridge_hookMethodNative(JNIEnv* env, jclass clazz, jobject reflectedMethodIndirect,

            jobject declaredClassIndirect, jint slot, jobject additionalInfoIndirect) {

    // Usage errors?

    if (declaredClassIndirect == NULL || reflectedMethodIndirect == NULL) {

        dvmThrowIllegalArgumentException("method and declaredClass must not be null");

        return;

    }

    // Find the internal representation of the method

    //获得dalvik中的classObject对象

    ClassObject* declaredClass = (ClassObject*) dvmDecodeIndirectRef(dvmThreadSelf(), declaredClassIndirect);

    //获得dalvik中的Method，被dalvik执行的函数体不同于java层的Method，位于Object.h

    Method* method = dvmSlotToMethod(declaredClass, slot);

    if (method == NULL) {

        dvmThrowNoSuchMethodError("Could not get internal representation for method");

        return;

    }

    //若method已被hook则直接返回

    if (isMethodHooked(method)) {

        // already hooked

        return;

    }

    // Save a copy of the original method and other hook info

    XposedHookInfo* hookInfo = (XposedHookInfo*) calloc(1, sizeof(XposedHookInfo));

    memcpy(hookInfo, method, sizeof(hookInfo->originalMethodStruct));

    hookInfo->reflectedMethod = dvmDecodeIndirectRef(dvmThreadSelf(), env->NewGlobalRef(reflectedMethodIndirect));

    hookInfo->additionalInfo = dvmDecodeIndirectRef(dvmThreadSelf(), env->NewGlobalRef(additionalInfoIndirect));

    // Replace method with our own code  将method替换成我们自己的代码

    //设置method->accessFlags = ACC_NATIVE；表示method为native代码

    //下面几行代码都是为这行代码作补充

    //For a native method, we compute the size of the argument list,

    //and set "insSize" and "registerSize" equal to it.

    SET_METHOD_FLAG(method, ACC_NATIVE);

    //给method添加callback函数表示已被hooked

    method->nativeFunc = &hookedMethodCallback;

    //原本的insns中存放的是dex指令，现变为hookinfo为hookedMethodCallback准备

    method->insns = (const u2*) hookInfo;

    method->registersSize = method->insSize;

    method->outsSize = 0;

    if (PTR_gDvmJit != NULL) {

        // reset JIT cache

        char currentValue = *((char*)PTR_gDvmJit + MEMBER_OFFSET_VAR(DvmJitGlobals,codeCacheFull));

        if (currentValue == 0 || currentValue == 1) {

            MEMBER_VAL(PTR_gDvmJit, DvmJitGlobals, codeCacheFull) = true;

        } else {

            ALOGE("Unexpected current value for codeCacheFull: %d", currentValue);

代码主要的就是红色标注的，它将Method标为native code。dalvik虚拟机在执行Method时，则会直接调用其成员变量hookedMethodCallback执行。注意，这个时候已经改变了原本的Method的执行步骤了(Xposed在此刻觉醒啦啦啦)。看下面dalvik代码，/dalvik/vm/interp/Stack.c

void dvmCallMethodV(Thread* self, const Method* method, Object* obj,

    bool fromJni, JValue* pResult, va_list args)

{

    ......

    if (dvmIsNativeMethod(method)) {

        TRACE_METHOD_ENTER(self, method);

        /*

         * Because we leave no space for local variables, "curFrame" points

         * directly at the method arguments.

         */

        (*method->nativeFunc)(self->curFrame, pResult, method, self);

        TRACE_METHOD_EXIT(self, method);

    } else {</span>

    //这里是在Inter.cpp中直接解析method

        dvmInterpret(self, method, pResult);

    }

    ......

    }

　　这一路走来感觉有点偏离主线类，回看下主题，在执行完 findAndHookMethod("com.android.systemui.statusbar.policy.Clock", lpparam.classLoader,"updateClock", new XC_MethodHook() );后。updateClock(hookmethod)的accessflag = ACC_NATIVE，dalvik在执行updateClock方法时发现其为native code，则执行nativeFunc 函数体即hookedMethodCallback。ok，找到"元凶"了，继续看代码：

/* This is called when a hooked method is executed. */

void hookedMethodCallback(const u4* args, JValue* pResult, const Method* method, ::Thread* self) {

    ......

    //call the Java handler function</span>

    JValue result;

    dvmCallMethod(self, xposedHandleHookedMethod, NULL, &result,

        originalReflected, (int) original, additionalInfo, thisObject, argsArray);

    ......

}

  hookedMethodCallback函数中主要是调用dvmCallMethod去执行xposedHandleHookedMethod，而xposedHandleHookedMethod是classXposedBridge里的handleHookedMethod方法。ok,重头戏来了

private static Object handleHookedMethod(Member method, int originalMethodId, Object additionalInfoObj, Object thisObject, Object[] args) throws Throwable {

        XposedBridge.AdditionalHookInfo additionalInfo = (XposedBridge.AdditionalHookInfo)additionalInfoObj;

        if(disableHooks) {

            try {

        //hook不使能，执行原method

                return invokeOriginalMethodNative(method, originalMethodId, additionalInfo.parameterTypes, additionalInfo.returnType, thisObject, args);

            ......

            }

        } else {

    /得到hookmethod的callback，在之前的XposedBridge.hookMethod中为callbacks添加了callback

            Object[] callbacksSnapshot = additionalInfo.callbacks.getSnapshot();

            int callbacksLength = callbacksSnapshot.length;

            if(callbacksLength == 0) {

        //hookmethod的callback为空，hooked无意义，执行原method

                try {

                    return invokeOriginalMethodNative(method, originalMethodId, additionalInfo.parameterTypes, additionalInfo.returnType, thisObject, args);

                ......

                }

            } else {

                ......

                do {

                    label65: {

                        try {

                            ((XC_MethodHook)callbacksSnapshot[beforeIdx]).beforeHookedMethod(param);

                        } catch (Throwable var18) {

                            log(var18);

                            param.setResult((Object)null);

                            param.returnEarly = false;

                            break label65;

                        }

                        if(param.returnEarly) {

                            ++beforeIdx;

                            break;

                        }

                    }

                    ++beforeIdx;

        //hookmethod有几个callback就循环几次

                } while(beforeIdx < callbacksLength);

                if(!param.returnEarly) {

                    try {

            //在beforeHookedMethod后执行原method

                        param.setResult(invokeOriginalMethodNative(method, originalMethodId, additionalInfo.parameterTypes, additionalInfo.returnType, param.thisObject, param.args));

                    } catch (InvocationTargetException var16) {

                        param.setThrowable(var16.getCause());

                    }

                }

                int afterIdx = beforeIdx - 1;

                do {

                    Object lastResult = param.getResult();

                    Throwable lastThrowable = param.getThrowable();

                    try {

                        ((XC_MethodHook)callbacksSnapshot[afterIdx]).afterHookedMethod(param);

                    } catch (Throwable var17) {

                        log(var17);

                        if(lastThrowable == null) {

                            param.setResult(lastResult);

                        } else {

                            param.setThrowable(lastThrowable);

                        }

                    }

                    --afterIdx;

                } while(afterIdx >= 0);

                ......

            }

        }

    }

别看handleHookedMethod代码老长了，其实它很单纯。

第一步：是否需要执行callback，否则直接执行原method，gameover；

第二步：执行callbacks里的beforeHookedMethod方法，有几个callback执行几次beforeHookedMethod；

第三步：执行原method；

第四步：执行callbacks里的afterHookedMethod方法，类同beforeHookedMethod。

需要注意的是如果method有多个callback，其beforeHookedMethod和afterHookedMethod执行顺序：

A1.before->A2.before->原method->A2.after->A1.after，也是蛮符合客观规律的嘛。

好，关于findAndHookMethod()函数也算是从上倒下看了个遍，但你上面添这么多代码是算怎么回事呢？下面就简单总结下罗：

好了，本篇就先这样吧，太长了也不好看。下篇再分析下其余枝节：

1 handleLoadPackage 怎么生效

2 dalvik层如何回到java层(数据类型如何回传)

3 XC_MethodHook中会执行原来的java函数体，如何执行；

其他想到再分析啰，大家也可以对上述执行流程提问，我们一起探讨。

参考资料：

1 Xposed框架Java部分

2 Dalvik虚拟机的运行过程分析

巴特西

Xposed学习二：实现机制

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