android hook 框架 xposed 如何实现挂钩

浏览数：74 / 时间：2015年06月11日

前面知道，安装xposed框架后，系统启动，执行init, init 再调用 app_process 程序，由于这时候 app_process 已经被换了，所以app_process 启动后先进入的是 xposedbridge.class 的 main 函数，这个函数最后才进入标准的 zygoteInit.class 的 main 函数，在进入 zygote 之前，它调用了几个函数，初始化了xposed框架，下面逐个分析。

一. initNative

Xposed.cpp (xposed): {"initNative", "()Z", (void*)de_robv_android_xposed_XposedBridge_initNative},
XposedBridge.java (xposedbridge\src\de\robv\android\xposed):    if (initNative()) {
XposedBridge.java (xposedbridge\src\de\robv\android\xposed):    private native static boolean initNative();

XposedBridge.class 的 initNative 是一个 native 函数，真正的实现在 Xposed.cpp 里的 de_robv_android_xposed_XposedBridge_initNative

1. de_robv_android_xposed_XposedBridge_initNative(JNIEnv* env, jclass clazz)

 xposedHandleHookedMethod = (Method*) env->GetStaticMethodID(xposedClass, "handleHookedMethod",
        "(Ljava/lang/reflect/Member;ILjava/lang/Object;Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;");

首先，从xposedClass(即XposedBridge.class)类获取函数 handleHookedMethod，这个函数是java函数，这里获取其对应的 Method 结构体指针，这样就可以在 native 里调用（jni的原理）。那么，这个 handleHookedMethod 是干嘛的呢，这个函数就是最终执行的挂钩函数，后面会分析。

Method* xposedInvokeOriginalMethodNative = (Method*) env->GetStaticMethodID(xposedClass, "invokeOriginalMethodNative",
        "(Ljava/lang/reflect/Member;I[Ljava/lang/Class;Ljava/lang/Class;Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;");
dvmSetNativeFunc(xposedInvokeOriginalMethodNative, de_robv_android_xposed_XposedBridge_invokeOriginalMethodNative, NULL);

其次，从xposedClass(即XposedBridge.class)类获取函数 invokeOriginalMethodNative 函数的 Method 结构体指针，然后调用 dvmSetNativeFunc 为这个java函数设置其 jni 实现 de_robv_android_xposed_XposedBridge_invokeOriginalMethodNative ，这样，调用 invokeOriginalMethodNative 函数其实执行的是后者。

objectArrayClass = dvmFindArrayClass("[Ljava/lang/Object;", NULL);
xresourcesClass = env->FindClass(XRESOURCES_CLASS);
    xresourcesClass = reinterpret_cast<jclass>(env->NewGlobalRef(xresourcesClass));
if (register_android_content_res_XResources(env) != JNI_OK) {}
xresourcesTranslateResId = env->GetStaticMethodID(xresourcesClass, "translateResId",
        "(ILandroid/content/res/XResources;Landroid/content/res/Resources;)I");
xresourcesTranslateAttrId = env->GetStaticMethodID(xresourcesClass, "translateAttrId",
        "(Ljava/lang/String;Landroid/content/res/XResources;)I");

最后，获取其他一些java类或函数的标识

二， initXbridgeZygote

XposedBridge.class main 函数第二个重要的函数是 initXbridgeZygote

// normal process initialization (for new Activity, Service, BroadcastReceiver etc.)
        findAndHookMethod(ActivityThread.class, "handleBindApplication", "android.app.ActivityThread.AppBindData", new XC_MethodHook() {
            protected void beforeHookedMethod(MethodHookParam param) throws Throwable {
            。。。
}

首先，挂钩了 ActivityThread 类的 handleBindApplication 函数，这个函数是在android ams 系统创建新进程成功后在新进程内部调用的，挂钩这个函数，可以在新进程创建后做一些事情

这里调用了一个函数，实现了挂钩 findAndHookMethod 。这个函数定义在 XposedHelpers.java

XposedHelpers.class

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");

        XC_MethodHook callback = (XC_MethodHook) parameterTypesAndCallback[parameterTypesAndCallback.length-1];
        Method m = findMethodExact(clazz, methodName, getParameterClasses(clazz.getClassLoader(), parameterTypesAndCallback));

        return XposedBridge.hookMethod(m, callback);
    }

这个函数找到类 clazz 的函数 methodName 所对应的 Method结构体，然后调用 XposedBridge 的 hookMethod 函数挂钩它

XposedBridge.class

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

        boolean newMethod = false;
        CopyOnWriteSortedSet<XC_MethodHook> callbacks;
        synchronized (sHookedMethodCallbacks) {
            callbacks = sHookedMethodCallbacks.get(hookMethod);
            if (callbacks == null) {
                callbacks = new CopyOnWriteSortedSet<XC_MethodHook>();
                sHookedMethodCallbacks.put(hookMethod, callbacks);
                newMethod = true;
            }
        }
        callbacks.add(callback);  // 先将被挂钩的函数 hookMethod 及挂钩它的函数保存起来
        if (newMethod) {
            Class<?> declaringClass = hookMethod.getDeclaringClass();
            int slot = (int) 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;
            }

            AdditionalHookInfo additionalInfo = new AdditionalHookInfo(callbacks, parameterTypes, returnType);
            hookMethodNative(hookMethod, declaringClass, slot, additionalInfo); // 最终调用 hookMethodNative 函数

} return callback.new Unhook(hookMethod); }

这个函数先将被挂钩的函数 hookMethod 及挂钩它的 XC_MethodHook结构体保存起来，然后调用 hookMethodNative 函数

{"hookMethodNative", "(Ljava/lang/reflect/Member;Ljava/lang/Class;ILjava/lang/Object;)V", (void*)de_robv_android_xposed_XposedBridge_hookMethodNative},

这个函数定义在 xposed.cpp 里

static void de_robv_android_xposed_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
    ClassObject* declaredClass = (ClassObject*) dvmDecodeIndirectRef(dvmThreadSelf(), declaredClassIndirect);
    Method* method = dvmSlotToMethod(declaredClass, slot);
    if (method == NULL) {
        dvmThrowNoSuchMethodError("could not get internal representation for method");
        return;
    }
    
    if (xposedIsHooked(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
    SET_METHOD_FLAG(method, ACC_NATIVE);
    method->nativeFunc = &xposedCallHandler;
    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);
        }
    }
}

这个过程跟ADBI框架类似，先获取要挂钩的java函数的 Method 结构体指针，然后检查一下是否已经被挂钩了，如果是直接返回，否则，通过对 Method 结构体进行赋值的方式，完成挂钩

method->nativeFunc = &xposedCallHandler;
    method->insns = (const u2*) hookInfo;
    method->registersSize = method->insSize;
    method->outsSize = 0;

这里挂钩函数全部使用 xposedCallHandler 这个函数。挂钩的详细信息保存在 Method结构体的 insns 成员里

xposed.cpp

static void xposedCallHandler(const u4* args, JValue* pResult, const Method* method, ::Thread* self) {
。。。
JValue result;
    dvmCallMethod(self, xposedHandleHookedMethod, NULL, &result,
        originalReflected, (int) original, additionalInfo, thisObject, argsArray);
。。。
}

可以看到，最终执行xposedHandleHookedMethod这个native函数，这个native函数前面 initNative 执行时，已经获取了它的java实现，真正的实现在

XposedBridge.java

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

        if (disableHooks) {
            try {
                return invokeOriginalMethodNative(method, originalMethodId, additionalInfo.parameterTypes,
                        additionalInfo.returnType, thisObject, args);
            } catch (InvocationTargetException e) {
                throw e.getCause();
            }
        }

        Object[] callbacksSnapshot = additionalInfo.callbacks.getSnapshot();
        final int callbacksLength = callbacksSnapshot.length;
        if (callbacksLength == 0) {
            try {
                return invokeOriginalMethodNative(method, originalMethodId, additionalInfo.parameterTypes,
                        additionalInfo.returnType, thisObject, args);
            } catch (InvocationTargetException e) {
                throw e.getCause();
            }
        }

        MethodHookParam param = new MethodHookParam();
        param.method = method;
        param.thisObject = thisObject;
        param.args = args;

        // 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();
    }

这个函数查找被挂钩函数的挂钩 XC_MethodHook 结构体，然后执行里边的 beforeHookedMethod 函数，再通过 invokeOriginalMethodNative 执行挂钩前的原始函数，最后再执行 afterHookedMethod 函数。

findAndHookMethod 的实现就分析完了，本质上仍然是寻找被挂钩函数的 Method 结构体，将Method属性改为native ,然后对其成员 nativeFunc， registersize 等进行赋值，其中 insns 成员保存了挂钩的详细信息。所有被挂钩的函数，其nativeFunc都赋值为 xposedCallHandler 函数，该函数最终执行 XposedBridge.class 里的 handleHookedMethod 。 handleHookedMethod 寻找 xposed模块及xposed框架调用 findAndHookMethod 注册的 before,after 函数，如果有，就执行，再通过 invokeOriginalMethodNative 执行挂钩前函数。

回到 initXbridgeZygote 函数，xposed 框架预先挂钩的函数，除了 handleBindApplication 外，还有

com.android.server.ServerThread  系统thread创建时触发
hookAllConstructors(LoadedApk.class, new XC_MethodHook(){。。。}  apk 包加载时触发这个挂钩
findAndHookMethod("android.app.ApplicationPackageManager", null, "getResourcesForApplication", 。。。 

3.  loadModules

/**
     * Try to load all modules defined in <code>BASE_DIR/conf/modules.list</code>
     */
    private static void loadModules(String startClassName) throws IOException {
        BufferedReader apks = new BufferedReader(new FileReader(BASE_DIR + "conf/modules.list"));
        String apk;
        while ((apk = apks.readLine()) != null) {
            loadModule(apk, startClassName);
        }
        apks.close();
    }

xposed框架本身挂钩的函数很少，真正的挂钩由具体的xposed模块实现，xposed模块也是正常的app，安装的时候注册其挂钩信息到xposed框架的 modules.list 等配置里。xposed框架初始化的时候，加载这些模块，并完成挂钩函数的挂钩

/**
     * Load a module from an APK by calling the init(String) method for all classes defined
     * in <code>assets/xposed_init</code>.
     */
    @SuppressWarnings("deprecation")
    private static void loadModule(String apk, String startClassName) {
      。。。。
             while ((moduleClassName = moduleClassesReader.readLine()) != null) {// call the init(String) method of the module
                    final Object moduleInstance = moduleClass.newInstance();
                    if (startClassName == null) {
                        if (moduleInstance instanceof IXposedHookZygoteInit) {
                            IXposedHookZygoteInit.StartupParam param = new IXposedHookZygoteInit.StartupParam();
                            param.modulePath = apk;
                            ((IXposedHookZygoteInit) moduleInstance).initZygote(param);
                        }

                        if (moduleInstance instanceof IXposedHookLoadPackage)
                            hookLoadPackage(new IXposedHookLoadPackage.Wrapper((IXposedHookLoadPackage) moduleInstance));

                        if (moduleInstance instanceof IXposedHookInitPackageResources)
                            hookInitPackageResources(new IXposedHookInitPackageResources.Wrapper((IXposedHookInitPackageResources) moduleInstance));
                    } else {
                        if (moduleInstance instanceof IXposedHookCmdInit) {
                            IXposedHookCmdInit.StartupParam param = new IXposedHookCmdInit.StartupParam();
                            param.modulePath = apk;
                            param.startClassName = startClassName;
                            ((IXposedHookCmdInit) moduleInstance).initCmdApp(param);
                        }
                    }
                }
}

loadModule 函数从配置文件里读取所有的模块，实例化模块类，xposed 提供了几个接口类供xposed模块继承，不同的接口类对应不同的hook时机

IXposedHookZygoteInit  zygote 初始化前就执行挂钩，即loadModule执行时就挂钩了
IXposedHookLoadPackage  apk包加载的时候执行挂钩，先将挂钩函数保存起来，等加载apk函数执行后触发callback （这里的callback是xposed框架自己挂钩的函数），再执行模块注册的挂钩函数
IXposedHookInitPackageResources apk资源实例化时执行挂钩，同上

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android hook 框架 xposed 如何实现挂钩