pre-verified问题

现象

DexClassLoader加载patch.dex.咱们试试跑在Android4.4及如下，结果报错了。java

java.lang.IllegalAccessError: Class ref in pre-verified class resolved to unexpected implementation at com.a.android_sample.MainActivity.onCreate(MainActivity.java:16) at android.app.Activity.perfromCreate(Activity.java:5266) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1313) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3733) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3939)  复制代码

出错代码android

String str = M.a();
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缘由分析

简单来讲

1.假如类A及其引用类都在同一个dex中，则类A会被提早验证和优化，并被标记CLASS_ISPREVERIFIED 这里，MainActivity就会被标记上。 2.当咱们调用M.a()时，须要加载类M，此时虚拟机会去校验M和MainActivity是否属于同一个dex。很明显不在，这就报错了。git

不了解，Dalvik类加载机制，这个缘由是分析不出来的。咱们算是站在巨人的肩膀上，有迹可循，而不是小马过河。github

具体代码抛错处

Android4.4 dalvik/vm/oo/Resolve.cppapache

//省略了部分代码
ClassObject* dvmResolveClass(const ClassObject* referrer, u4 classIdx,
    bool fromUnverifiedConstant){
    DvmDex* pDvmDex = referrer->pDvmDex;
    ClassObject* resClass;
    const char* className;
    
    //不用重复解析
    resClass = dvmDexGetResolvedClass(pDvmDex, classIdx);
    if (resClass != NULL)  return resClass;
    ....
    //这里的resClass是 com.a.fix.M,
    //referrer是com.a.
    resClass = dvmFindClassNoInit(className, referrer->classLoader);
	//....
    if (resClass != NULL) {
        /* * If the referrer was pre-verified, the resolved class must come * from the same DEX or from a bootstrap class. */
        if (!fromUnverifiedConstant &&
            IS_CLASS_FLAG_SET(referrer, CLASS_ISPREVERIFIED)) {
            ClassObject* resClassCheck = resClass;
            if (referrer->pDvmDex != resClassCheck->pDvmDex &&
                resClassCheck->classLoader != NULL){
                dvmThrowIllegalAccessError(
                    "Class ref in pre-verified class resolved to unexpected "
                    "implementation");
                return NULL;
            }
        }
        //存一下，
        dvmDexSetResolvedClass(pDvmDex, classIdx, resClass);
    }
    .....
    return resClass;
}
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调用链路

这部分能够折叠不看。bootstrap

M.a()

AndroidStudio安装插件java2smali，看看MainActivity编译后的产物。 MainActivity.smali 部分代码安全

.class public Lcom/a/android_sample/MainActivity;
.source "MainActivity.java"

.method protected onCreate(Landroid/os/Bundle;)V .registers 4 #执行到这一行出错了。 .line 16 invoke-static {}, Lcom/a/fix/M;->a()Ljava/lang/String;

    .line 17
    ...
    invoke-virtual {v1, v0}, Landroid/widget/TextView;->setText(Ljava/lang/CharSequence;)V
    ...
.end method
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invoke-static

代码在Android4.4源码 dalvik/vm/mterp/out/InterpC-portable.cppmarkdown

GOTO_TARGET(invokeStatic, bool methodCallRange)

    methodToCall = dvmDexGetResolvedMethod(methodClassDex, ref);
    if (methodToCall == NULL) {
        //还没解析过，就去解析它
        methodToCall = dvmResolveMethod(curMethod->clazz, ref, METHOD_STATIC);
    }
    GOTO_invokeMethod(methodCallRange, methodToCall, vsrc1, vdst);
GOTO_TARGET_END
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dvmResolveMethod

Android4.4源码 dalvik/vm/oo/Resolve.cpp 解析Method前，先解析其所在的classapp

/* * Find the method corresponding to "methodRef". * If this is a static method, we ensure that the method's class is * initialized. */
//省略了部分代码
Method* dvmResolveMethod(const ClassObject* referrer, u4 methodIdx,
    MethodType methodType){
    ClassObject* resClass;
    const DexMethodId* pMethodId;
    pMethodId = dexGetMethodId(pDvmDex->pDexFile, methodIdx);

    //这里就开始调用到咱们上一节提到的具体代码抛错处了。
    resClass = dvmResolveClass(referrer, pMethodId->classIdx, false);
    if (resClass == NULL) {
        /* can't find the class that the method is a part of */
        assert(dvmCheckException(dvmThreadSelf()));
        return NULL;
    }
    ....
}
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dex文件验证优化

回头在来看dex文件优化，咱们就放上调用

//libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java
BaseDexClassLoader(dexPath,optimizedDirectory,libraryPath,parent)

//libcore/dalvik/src/main/java/dalvik/system/DexPathList.java
DexPathList.loadDexFile(file, optimizedDirectory);

//libcore/dalvik/src/main/java/dalvik/system/DexFile.java
DexFile.loadDex(file.getPath(), optimizedPath, 0);

//dalvik/vm/native/dalvik_system_DexFile.cpp
Dalvik_dalvik_system_DexFile_openDexFileNative(const u4* args, JValue* pResult)

//dalvik/vm/RawDexFile.cpp
dvmRawDexFileOpen(sourceName, outputName, &pRawDexFile, false)
    
//dalvik/vm/analysis/DexPrepare.cpp 
dvmOptimizeDexFile(optFd, dexOffset, fileSize,fileName,....)
    
//建立进程 /system/bing/dexopt
//dalvik/dexopt/OptMain.cpp
int main(int argc, char* const argv[]) fromDex(int argc, char* const argv[]) dvmContinueOptimization(fd, offset, length...) //dalvik/vm/analysis/DexPrepare.cpp  rewriteDex(addr, int len,doVerify,doOpt,..) verifyAndOptimizeClasses(pDvmDex->pDexFile, doVerify, doOpt) verifyAndOptimizeClass(pDexFile, clazz, pClassDef, doVerify, doOpt) dvmVerifyClass(clazz)//Set the "is preverified" flag in the DexClassDef  复制代码

dvmVerifyClass

//dalvik/vm/analysis/DexPrepare.cpp 
if (dvmVerifyClass(clazz)) {
/* Set the "is preverified" flag in the DexClassDef. */
  ((DexClassDef*)pClassDef)->accessFlags |= CLASS_ISPREVERIFIED;
   verified = true;
}

//dalvik/vm/analysis/DexVerify.cpp 
bool dvmVerifyClass(ClassObject* clazz) bool verifyMethod(method) bool dvmVerifyCodeFlow(VerifierData* vdata) //dalvik/vm/analysis/CodeVerify.cpp  bool doCodeVerification() ... 复制代码

参考

深刻理解Java虚拟机：JVM高级特性与最佳实践（第3版）周志明.pdf 深刻理解Dalvik虚拟机系统源码(AOSP) github地址连接，下载你想要的。或者这个官网连接安卓App热补丁动态修复技术介绍 android热修复的pre-verify问题详解及实践 05-DALVIK加载和解析DEX过程

pre-verified解决

方案分析

咱们在把代码抄过来，发现有三个条件同时知足才会报错

//省略了部分代码
ClassObject* dvmResolveClass(const ClassObject* referrer, u4 classIdx,
    bool fromUnverifiedConstant){
    
    resClass = dvmDexGetResolvedClass(pDvmDex, classIdx);
    if (resClass != NULL) return resClass;
    
    if (!fromUnverifiedConstant &&
            IS_CLASS_FLAG_SET(referrer, CLASS_ISPREVERIFIED)) {
            ClassObject* resClassCheck = resClass;
            if (referrer->pDvmDex != resClassCheck->pDvmDex &&
                resClassCheck->classLoader != NULL){
                dvmThrowIllegalAccessError(
                    "Class ref in pre-verified class resolved to unexpected "
                    "implementation");
                return NULL;
            }
        }
    }
    return resClass;
}
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根据上述代码，解决方案大体上有如下四种。

禁止dexopt过程打上CLASS_ISPREVERIFIED标记

Q-zone插桩方案突破了此限制，可是致使preverify失效，损失了性能。

修改fromUnverfiedConstant=true

须要经过 native hook 拦截系统方法，更改方法的入口参数，将 fromUnverifiedConstant 统一改成 true，风险大，几乎无人采用。Cydia native hook

使dvmDexGetResolvedClass返回不为null，直接返回

QFix采用此方案，

补丁类与引用类放在同一个dex中

Tinker等全量合成方案突破了此限制。

Q-zone插桩方案

方案分析

经过字节码技术，在每一个类的构造方法中插入一段引用 HackCode.class的代码,使得MainActivity引用到hack.dex中的Hack.class,致使verify不经过。此时方案分红两部分

单独打包HackCode.class
MainActivity引用HackCode.class。

package com.a.hack;
public class HackCode {}
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实际代码执行处。

//dalvik/vm/analysis/CodeVerify.cpp
case OP_CONST_CLASS:
	 //给它整失败了，会把错误值给failure，后面判断下失败，就返回失败了，就不标记了。
      resClass = dvmOptResolveClass(meth->clazz, decInsn.vB, &failure);

////dalvik/vm/analysis/Optimize.cpp
/* * Performs access checks on every resolve, * and refuses to acknowledge the existence of classes * defined in more than one DEX file. * 不认可定义在多个dex中的类 */
ClassObject* dvmOptResolveClass(ClassObject* referrer, u4 classIdx, VerifyError* pFailure){
    ...
    const char* className = dexStringByTypeIdx(pDvmDex->pDexFile, classIdx);
    //referrer是全部引用类包括MainAcitivityClass，resClass的Hack.class
    //referrer的dex中固然没有Hack.class
	resClass = dvmFindClassNoInit(className, referrer->classLoader);
    if (resClass == NULL) {
         *pFailure = VERIFY_ERROR_NO_CLASS;
         ...			
     }
    ...
}
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引用hackCode.class

apk源码不能包含HackCode.class,咱们经过字节码插入引用。编写自定义Gradle插件，使用javassist字节码技术自定义Gradle插件参考 Gradle系列一 -- Groovy、Gradle和自定义Gradle插件 javassist参考 javassist使用全解析关键代码，有点长

class HackTransform extends Transform {

    def pool = ClassPool.default
    def project
    ....	
    @Override
    void transform(TransformInvocation transformInvocation) throws javax.xml.crypto.dsig.TransformException, InterruptedException, IOException {
        super.transform(transformInvocation)

        project.android.bootClasspath.each {
            pool.appendClassPath(it.absolutePath)
        }
        //这一行要注意，不然编译不经过哦
        pool.makeClass("com.a.hack.HackCode")

        transformInvocation.inputs.each {

            it.jarInputs.each {
                pool.insertClassPath(it.file.absolutePath)
                // 重命名输出文件（同目录copyFile会冲突）
                def jarName = it.name
                def md5Name = DigestUtils.md5Hex(it.file.getAbsolutePath())
                if (jarName.endsWith(".jar")) {
                    jarName = jarName.substring(0, jarName.length() - 4)
                }
                def dest = transformInvocation.outputProvider.getContentLocation(
                        jarName + md5Name, it.contentTypes, it.scopes, Format.JAR)
                org.apache.commons.io.FileUtils.copyFile(it.file, dest)
            }

            it.directoryInputs.each {
                def inputDir = it.file.absolutePath
                pool.insertClassPath(inputDir)
                findTarget(it.file, inputDir)
                def dest = transformInvocation.outputProvider.getContentLocation(
                        it.name, it.contentTypes, it.scopes, Format.DIRECTORY)
                org.apache.commons.io.FileUtils.copyDirectory(it.file, dest)
            }
        }
    }

    private void findTarget(File fileOrDir, String inputDir) {
        if (fileOrDir.isDirectory()) {
            fileOrDir.listFiles().each {
                findTarget(it, inputDir)
            }
        } else {
            modify(fileOrDir, inputDir)

        }
    }

    private void modify(File file, String fileName) {
        def filePath = file.absolutePath

        if (!filePath.endsWith(SdkConstants.DOT_CLASS)
           ||filePath.contains('R$') 
           || filePath.contains('R.class')
           || filePath.contains("BuildConfig.class")) {
            return
        }
        def className = filePath.replace(fileName, "")
        		.replace("\\", ".").replace("/", ".")
        def name = className.replace(SdkConstants.DOT_CLASS, "").substring(1)
        CtClass ctClass = pool.get(name)
        //咱们的自定义的Application是初始类，加载完dex之后的类，才能插入Hakcode引用。
        if (ctClass.getSuperclass() != null
                && ctClass.getSuperclass().name == "android.app.Application") {
            return
        }
       
        //真正执行插入字节码的地方
        ctClass.defrost()
        CtConstructor[] constructors = ctClass.getDeclaredConstructors()
        if (constructors != null && constructors.length > 0) {
            CtConstructor constructor = constructors[0]
            def body = "android.util.Log.e(\"alvin\",\"${constructor.name} constructor\" + com.a.hack.HackCode.class);"
            constructor.insertBefore(body)
        }
        ctClass.writeFile(fileName)
        ctClass.detach()
    }
}
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生成hack.dex

参考patch.dex的生成方式。编写app/main/java/com/a/hack/HackCode.java,单独编译成dex，生成后，能够删掉此java文件。

package com.a.hack;
public class HackCode {}
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//来到java源码目录下，
cd app/main/java
//.class文件
javac com/a/hack/HackCode.java   
//生成hack.dex
dx --dex --output com/a/hack/hack.dex com/a/hack/HackCode.class
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加载hack.dex

参考patch.dex的方式。

验证

android4.4上验证成功

Cydia NativeHook

须要经过 native hook 拦截系统方法，更改方法的入口参数，将 fromUnverifiedConstant 统一改成 true，

这里咱们采用Cydia Substrate，hook dvmResolveClass方法,步骤以下 Demo代码：hook具体实现与动态库下载，注意方案只在Android4.4上验证可行。

实现步骤

cydia so库和头文件

这里能够下载。 so库放到一个本身的目录底下好比

<moduleName>/src/main/jniLibs/armeabi-v7a/libsubstrate.so
<moduleName>/src/main/jniLibs/armeabi-v7a/libsubstrate-dvm.so
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导入头文件

<moduleName>/src/main/cpp/include/substrate.h
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hook代码实现

//<moduleName>/src/main/cpp/cydia-hook.cpp
#include "include/substrate.h"
#include <android/log.h>

#define TAG "alvin"
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, TAG, __VA_ARGS__) //旧函数指针，指向旧函数 void *(*oldDvmResolveClass)(void *referrer, unsigned int classIdx, bool fromUnverifiedConstant);

//新函数实现
void *newDvmResolveClass(void *referrer, unsigned int classIdx, bool fromUnverifiedConstant) {
    //这里，fromUnverifiedConstant 强制为true，就不会去check dex是否相等了。
    return oldDvmResolveClass(referrer, classIdx, true);
}

//指明要hook的lib，涉及到dvmResolveClass的so
MSConfig(MSFilterLibrary, "/system/lib/libdvm.so")
//指明要hook的应用
MSConfig(MSFilterExecutable, "com.a.dexload.cydia")

MSInitialize {
    MSImageRef image = MSGetImageByName("/system/lib/libdvm.so");
    if (image == NULL) {
        return;
    }
    void *resloveMethd = MSFindSymbol(image, "dvmResolveClass");
    if (resloveMethd == NULL) {
        return;
    }
    //具体的Hook实现
    MSHookFunction(resloveMethd, (void *) newDvmResolveClass, (void **) &oldDvmResolveClass);
}
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CMakeLists.txt

生成libcydiahook.so

cmake_minimum_required(VERSION 3.10.2)

add_library(cydiahook SHARED src/main/cpp/cydia-hook.cpp)
target_include_directories(cydiahook PRIVATE  ${CMAKE_SOURCE_DIR}/src/main/cpp/include)
find_library(log-lib log)
file(GLOB libs ${CMAKE_SOURCE_DIR}/src/main/jniLibs/armeabi-v7a/libsubstrate.so ${CMAKE_SOURCE_DIR}/src/main/jniLibs/armeabi-v7a/libsubstrate-dvm.so)
target_link_libraries( cydiahook   ${libs}  ${log-lib})
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libcydiahook.so加载

public class ApplicationApp extends Application {
    static {
        System.loadLibrary("cydiahook");
    }
}
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其余

ClassObject属性

如同Andfix，咱们能够引入DexFile.h头文件，能够把参数和结果转成实际的class对象，查看class的一些属性

//新函数实现
void *newDvmResolveClass(void *referrer, unsigned int classIdx, bool fromUnverifiedConstant) {
    
    void *res = oldDvmResolveClass(referrer, classIdx, true);

    ClassObject *referrerClass = reinterpret_cast<ClassObject *>(referrer);
	ClassObject *resClass = reinterpret_cast<ClassObject *>(res);
	if (resClass == NULL) {
        LOGE("newDvmResolveClass %s, %s", referrerClass->descriptor,
             "resClass is NULL");
    } else {
        LOGE("newDvmResolveClass %s, %s", referrerClass->descriptor,
             resClass->descriptor);
    }
    return res;
}
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风险

和 Andfix 相似,native hook 方式存在各类兼容性和稳定性问题，甚至安全性问题。同时，拦截的是一个涉及 dalvik 基础功能同时调用很频繁的方法，无疑风险会大不少。

QFix方案实现

可参考这篇文章QFix探索之路—手Q热补丁轻量级方案

热修复类加载之pre-verified问题

pre-verified问题

现象

缘由分析

简单来讲

具体代码抛错处

调用链路

M.a()

invoke-static

dvmResolveMethod

dex文件验证优化

参考

pre-verified解决

方案分析

Q-zone插桩方案

方案分析

引用hackCode.class

生成hack.dex

加载hack.dex

验证

Cydia NativeHook

实现步骤

cydia so库和头文件

hook代码实现

CMakeLists.txt

libcydiahook.so加载

其余

ClassObject属性

风险

QFix方案实现