android so壳入口浅析

本文转自http://www.9hao.info/pages/2014/08/android-soke-ru-kou-q

前言

开年来开始接触一些加固样本，基本都对了so进行了处理，拖入ida一看，要么没有JNI_OnLoad,要么JNI_OnLoad汇编代码羞涩难懂，让人无法下手。JNI_OnLoad是真正入口么？

先看看几个文档

1 摘自属性服务一节（《深入理解Android卷1》）

 利用gcc的constructor属性，这个属性指明了一个__libc_prenit函数(这个函数内部就将完成共享内存到本地进程的映射工作)。用法：当bionic libc库被加载时，将自动调用__libc_prenit函数。这样在bionic libc动态库被装载时，系统属性缓冲区地址就被确定了，后续的API调用就能找对位置了。/* We flag the __libc_preinit function as a constructor to ensure * that its address is listed in libc.so's .init_array section. * This ensures that the function is called by the dynamic linker * as soon as the shared library is loaded. */ //constructor属性指示加载器加载该库之后，首先调用__libc_prenit函数。这一点和windows上的动态库的DllMain函数类似void __attribute__((constructor)) __libc_prenit(void);

从英文说明里面提到到.init_array section，我们可以搜索一下这一节的说明

2 .init_array section

.init_array contains pointers to blocks of code that need to be executed when an application is being initialized (before main() is called). It is used for a number of things, but the primary use is in C++ for running static constructors; a secondary use that is sometimes used is to initialize IO systems in the C library.If you are not using C++ you may (depending on your C library) be able to live without it entirely; but you’d need to hack your startup code to deal with this..init_array probably ends up in ram because its marked read/write — that happens because in a dynamic linking environment the dynamic linker has to fix up all the pointers it contains before it can be used. In a static environment you might be able to get away with forcing it into a read-only section.来源： <http://blog.sina.com.cn/s/blog_a9303fd901019kvq.html>

3 摘自dlopen小结(《程序员的自我修养》)

动态连接器在加载模块时，会执行".init"段的代码,用以完成模块的初始化工作，dlopen的加载过程基本跟动态连接器一致，在完成装载、映射和重定向以后，就会执行".init"段的代码然后返回

看完这个3段资料，我们可以知道在系统加载so，在完成装载、映射和重定向以后，就首先执行.init和.init_array段的代码.

探本溯源，在源码中追踪

我们先从System.loadLibrary->Runtime.loadLibrary

  public void loadLibrary(String libName) {        loadLibrary(libName, VMStack.getCallingClassLoader());   }    /*     * Loads and links a library without security checks.     */   void loadLibrary(String libraryName, ClassLoader loader) {        代码略...        String error = nativeLoad(filename, loader);               代码略... }

->nativeLoad

static void Dalvik_java_lang_Runtime_nativeLoad(const u4* args,  JValue* pResult){   代码略...   StringObject* fileNameObj = (StringObject*) args[0];   success = dvmLoadNativeCode(fileName, classLoader, &reason);   代码略...}

来源： <http://androidxref.com/4.1.2/xref/dalvik/vm/native/java_lang_Runtime.cpp#72>

->dvmLoadNativeCode

bool dvmLoadNativeCode(const char* pathName, Object* classLoader,        char** detail){         代码略...        handle = dlopen(pathName, RTLD_LAZY);         代码略...        vonLoad = dlsym(handle, "JNI_OnLoad");        if (vonLoad == NULL) {            ALOGD("No JNI_OnLoad found in %s %p, skipping init",                pathName, classLoader);        } else {            /*             * Call JNI_OnLoad.  We have to override the current class             * loader, which will always be "null" since the stuff at the             * top of the stack is around Runtime.loadLibrary().  (See             * the comments in the JNI FindClass function.)             */            OnLoadFunc func = (OnLoadFunc)vonLoad;            Object* prevOverride = self->classLoaderOverride;            self->classLoaderOverride = classLoader;            oldStatus = dvmChangeStatus(self, THREAD_NATIVE);            if (gDvm.verboseJni) {                ALOGI("[Calling JNI_OnLoad for \"%s\"]", pathName);            }            version = (*func)(gDvmJni.jniVm, NULL);            dvmChangeStatus(self, oldStatus);           self->classLoaderOverride = prevOverride;    }    代码略...}

来源： <http://androidxref.com/4.1.2/xref/dalvik/vm/Native.cpp#318>

通过dvmLoadNativeCode函数我们知道系统用dlopen加载so完成后，会查看有没有JNI_OnLoad函数，有的话就调用.

我们再到dlopen函数探个究竟:

void *dlopen(const char *filename, int flag){    soinfo *ret;    pthread_mutex_lock(&dl_lock);    /*find_library 会判断so是否已经加载，如果没有加载，对so进行加载，完成一些初始化工作,有兴趣的读者可自行分析 */    ret = find_library(filename);    if (unlikely(ret == NULL)) {        set_dlerror(DL_ERR_CANNOT_LOAD_LIBRARY);    } else {        call_constructors_recursive(ret);        ret->refcount++;    }    pthread_mutex_unlock(&dl_lock);    return ret;}

->call_constructors_recursive

void call_constructors_recursive(soinfo *si){    if (si->constructors_called)        return;    // Set this before actually calling the constructors, otherwise it doesn't    // protect against recursive constructor calls. One simple example of    // constructor recursion is the libc debug malloc, which is implemented in    // libc_malloc_debug_leak.so:    // 1. The program depends on libc, so libc's constructor is called here.    // 2. The libc constructor calls dlopen() to load libc_malloc_debug_leak.so.    // 3. dlopen() calls call_constructors_recursive() with the newly created    //    soinfo for libc_malloc_debug_leak.so.    // 4. The debug so depends on libc, so call_constructors_recursive() is    //    called again with the libc soinfo. If it doesn't trigger the early-    //    out above, the libc constructor will be called again (recursively!).    si->constructors_called = 1;    if (si->flags & FLAG_EXE) {        TRACE("[ %5d Calling preinit_array @ 0x%08x [%d] for '%s' ]\n",              pid, (unsigned)si->preinit_array, si->preinit_array_count,              si->name);        call_array(si->preinit_array, si->preinit_array_count, 0);        TRACE("[ %5d Done calling preinit_array for '%s' ]\n", pid, si->name);    } else {        if (si->preinit_array) {            DL_ERR("%5d Shared library '%s' has a preinit_array table @ 0x%08x."                  " This is INVALID.", pid, si->name,                   (unsigned)si->preinit_array);       }   }    代码略...    if (si->init_func) {        TRACE("[ %5d Calling init_func @ 0x%08x for '%s' ]\n", pid,             (unsigned)si->init_func, si->name);       si->init_func();       TRACE("[ %5d Done calling init_func for '%s' ]\n", pid, si->name);  }  if (si->init_array) {    TRACE("[ %5d Calling init_array @ 0x%08x [%d] for '%s' ]\n", pid,            (unsigned)si->init_array, si->init_array_count, si->name);    //遍历函数数组并执行    call_array(si->init_array, si->init_array_count, 0);       TRACE("[ %5d Done calling init_array for '%s' ]\n", pid, si->name);  //ps:看到这么多TRACE这么多调试信息，我们把调试开关打开，是不是能拿到诸多信息？ }}来源： <http://androidxref.com/4.1.2/xref/bionic/linker/linker.c#1519>

通过可以函数我们知道si->init_func和si->init_array存在的时候，会执行指向的函数
(不知道大家注意到么si->flags & FLAG_EXE时，还有si->preinit_array？以后会不会有这方面的东西？)
再找下si->init_func和si->init_array的赋值

 case DT_INIT:           si->init_func = (void (*)(void))(si->base + *d);           DEBUG("%5d %s constructors (init func) found at %p\n",                  pid, si->name, si->init_func); case DT_INIT_ARRAY:            si->init_array = (unsigned *)(si->base + *d);      DEBUG("%5d %s constructors (init_array) found at %p\n",                  pid, si->name, si->init_array);           break;

DEBUG里面说明了constructors (init func)和constructors (init_array)。
我们再看看一份文档Android Dynamic Linker Design Notes

DT_INIT      Points to the address of an initialization function      that must be called when the file is loaded.DT_INIT_ARRAY      Points to an array of function addresses that must be      called, in-order, to perform initialization. Some of      the entries in the array can be 0 or -1, and should      be ignored.      Note: this is generally stored in a .init_array section

通过层层分析，我们很清楚知道了系统加载so，在完成装载、映射和重定向以后，就首先执行.init和.init_array段的代码.

前面有一篇文章我已经对so加壳进行简单说明

把源码的dlopen复制出来修改，在把自己so加载起来的时候 ,把自己内存里面某部分地址解密后,用自己的dlopen打开返回一个soinfo结构体 然后把当前soinfo结构体替换原来的soinfo结构体

小结

系统加载so，在完成装载、映射和重定向以后，就首先执行.init和.init_array段的代码，之后如果存在JNI_OnLoad就调用该函数.我们要对一个so进行分析，需要先看看有没有.init_array section和.init section，so加壳一般会在初始化函数进行脱壳操作。

如何在.init和.init_array段添加我们的函数

[1] 共享构造函数，在函数声明时加上"__attribute__((constructor))"属性    void __attribute__((constructor)) init_function(void);    对应有共享虚构函数，在程序exit()或者dlclose()返回前执行    void __attribute__((destructor)) fini_function(void);[2]c++ 静态构造函数

在.init和.init_array下断点

init_array  用ida可以看到，  可以对里面的函数数组下断点init ida有时没识别出来,可用readelf查看入口点xxx@xx:~$ readelf -a '/home/xxx/桌面/libsecexe.so' 0x00000010 (SYMBOLIC)                   0x0 0x0000000c (INIT)                       0x11401 0x00000019 (INIT_ARRAY)                 0x28ca4 0x0000001b (INIT_ARRAYSZ)               8 (bytes) 0x0000001a (FINI_ARRAY)                 0x28cac 0x0000001c (FINI_ARRAYSZ)               12 (bytes) 0x00000004 (HASH)                       0xf4我们看到 INIT 入口为    0x11401(ps:有时你在0x11401是数据，你需要make code，由于对齐关系，要从0x11401+1开始）样本：梆梆 爱加密

参考：
[1] 《深入理解Android卷1》
[2] 《程序员的自我修养-链接、装载与库》
[3]android linker 浅析
[4]Android Dynamic Linker Design Notes

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