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

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