linker源码解析，脱壳点，在分析过程中产生的反调试思路

前言

linker是Android系统动态库so的加载器和链接器，也是Android脱壳一重要脱壳点，这里介绍一下此部分的Android源码，并介绍几个脱壳点，及分析过程中产生的反调试手段，学习Linker的加载和启动原理，又需要介绍so的加载和启动。

系统 :Android4.4-r1

linker源码的位置 : Android/bionic/linker

0x00 加载与启动so

1、Java层中声明加载某个so文件以共享，则可在Java层声明代码：

static{
System.loadLibrary("libhello.so")
}

其对应的执行流程如下：

1、 定位到文件Dalvik/vm/native/java_lang_Runtime.cpp

2、 调用Dalvik_java_lang_Runtime_nativeLoad -> Dalvik/vm/Native.cpp:dvmLoadNativeCode

具体代码如下，我略过一些错误判断代码，这段代码以/* 说明 */的形式省略

bool dvmLoadNativeCode(const char* pathName, Object* classLoader,
char** detail)
{
SharedLib* pEntry;
void* handle;
bool verbose;

/*********根据path查找so文件**********/
pEntry = findSharedLibEntry(pathName);

/*********加载指定so文件，并延迟加载*******/
handle = dlopen(pathName, RTLD_LAZY);
dvmChangeStatus(self, oldStatus);

/* create a new entry */
SharedLib* pNewEntry;
/*.......*/

/* try to add it to the list */
/***当执行addShareLibEntry()方法的时候，如果还有线程B同时在加载该so，***/
/***并且B线程先执行到了这里，那么就说明该so的信息已经添加过了，我们就不需要再执行添加pNewEntry的操作***/
SharedLib* pActualEntry = addSharedLibEntry(pNewEntry);
if (pNewEntry != pActualEntry) {
ALOGI("WOW: we lost a race to add a shared lib (%s CL=%p)",
pathName, classLoader);
freeSharedLibEntry(pNewEntry);
return checkOnLoadResult(pActualEntry);
} else {
if (verbose)
ALOGD("Added shared lib %s %p", pathName, classLoader);

bool result = false;
void* vonLoad;
int version;
/***定位JNI_OnLoad()方法***/
vonLoad = dlsym(handle, "JNI_OnLoad");//
/***如果找不到则延迟加载，说明是用javah风格的代码**/
if (vonLoad == NULL) {
ALOGD("No JNI_OnLoad found in %s %p, skipping init", pathName, classLoader);
result = true;
} 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.)
*/
/***这里省略了重写相应类加载器的代码，如上面鸡肠文所示功能,不重要**/
/***gDvm是一个全局变量，功能后面再补***/
if (gDvm.verboseJni) {
ALOGI("[Calling JNI_OnLoad for \"%s\"]", pathName);
}
/***执行JNI_OnLoad***/
version = (*func)(gDvmJni.jniVm, NULL);

/***省略一大段代码。。。***/
dvmUnlockMutex(&pNewEntry->onLoadLock);
return result;
}
}

从上面的代码我们可看出

1、Android系统加载so文件时使用了dlopen函数；

2、定位JNI_OnLoad()方法，则dlsym(handle, “JNI_OnLoad”)；

3、执行JNI_OnLoad()方法：(*func)(gDvmJni.jniVm, NULL)

我们查找加载so文件的函数dlopen在bionic/linker/dlfcn.c中，而此函数有主要调用了do_dlopen函数，这里对dlopen函数不详细赘述，主要解析一下do_dlopen函数，而此关键函数正是在Linker中。接下来详细分析Linker源码。

0x01 Linker源码总览

0x02 do_dlopen

先贴上代码：

soinfo* si = find_library(name);//**完成so的加载到内存的工作
if (si != NULL) {
si->CallConstructors();//**完成so及本身的构造函数的调用。完成so文件的加载
}

嗯，注释简单明了，接下来解析find_library(name)与si->CallConstructors()。

0x03 续0 find_library

作用：完成so的加载到内存的工作,成为是否加载过该so的重要依据

static soinfo* find_library(const char* name) {//成为是否加载过该so的重要依据
soinfo* si = find_library_internal(name);//寻找相应的so信息
if (si != NULL) {
si->ref_count++;
}
return si;

嗯，还是很清晰，里面的调用的方法先放下先，我们现分析如下方法。

0x04 si->CallConstructors()

完成so及本身的构造函数的调用。完成so文件的加载

/***省略一大堆代码,下面是主要函数****/
if (dynamic != NULL) {
for (Elf32_Dyn* d = dynamic; d->d_tag != DT_NULL; ++d) {
if (d->d_tag == DT_NEEDED) {//调动依赖库的构造函数
const char* library_name = strtab + d->d_un.d_val;
TRACE("\"%s\": calling constructors in DT_NEEDED \"%s\"", name, library_name);
find_loaded_library(library_name)->CallConstructors();
}
// DT_INIT should be called before DT_INIT_ARRAY if both are present.
TRACE("\"%s\": calling constructors", name);
CallFunction("DT_INIT", init_func);//调用自己的一系列构造函数后返回,这里是so文件加壳的脱壳点
CallArray("DT_INIT_ARRAY", init_array, init_array_count, false);//dex文件的脱壳点

这主要是完成so文件的加载，然后遍历所有动态节，再根据标签d_tag ==DT_NEEDED调用依赖库的构造函数，再调用自己的一系列构造函数，以及init_arry函数，其中后面两个函数分别是so文件和dex文件的脱壳点，这样就结束了so文件的加载，但分析远远没有结束，我们需要回过头来解析find_library方法。

0x05 find_library 之find_library_internal()

操作：寻找相应的so信息

static soinfo* find_library_internal(const char* name) {
if (name == NULL) {
return somain;
}

soinfo* si = find_loaded_library(name);
if (si != NULL) {
if (si->flags & FLAG_LINKED) {
return si;
}
DL_ERR("OOPS: recursive link to \"%s\"", si->name);
return NULL;
}

TRACE("[ '%s' has not been loaded yet.  Locating...]", name);
si = load_library(name);//真正加载so文件的函数
if (si == NULL) {
return NULL;
}

// At this point we know that whatever is loaded @ base is a valid ELF
// shared library whose segments are properly mapped in.
TRACE("[ init_library base=0x%08x sz=0x%08x name='%s' ]",
si->base, si->size, si->name);

if (!soinfo_link_image(si)) {//进行重定位
munmap(reinterpret_cast<void*>(si->base), si->size);
soinfo_free(si);
return NULL;
}

注释很明白，整理一下基本流程：

1、find_loaded_library()：寻找相应的so信息

2、load_library()：真正加载so文件的函数

3、soinfo_link_image():处理动态节dynamic section,初始化动态节dynamic section的属性

0x06 find_library_internal() 之 find_loaded_library()

//寻找相应的so信息
static soinfo *find_loaded_library(const char *name)
{
soinfo *si;
const char *bname;

// TODO: don't use basename only for determining libraries
// http://code.google.com/p/android/issues/detail?id=6670 
bname = strrchr(name, '/');//查找一个字符"/"在另一个字符串name中末次出现的位置并返回这个位置的地址
bname = bname ? bname + 1 : name;

for (si = solist; si != NULL; si = si->next) {//判断是否有加载这个so
if (!strcmp(bname, si->name)) {
return si;
}
}
return NULL;
}

0x07 find_library_internal() 之load_library()

操作：真正加载so文件的函数

static soinfo* load_library(const char* name) {
// Open the file.
int fd = open_library(name);
if (fd == -1) {
DL_ERR("library \"%s\" not found", name);
return NULL;
}

// Read the ELF header and load the segments.
ElfReader elf_reader(name, fd);
if (!elf_reader.Load()) {//读取elf的操作，源码中看出只读取了Program 段
return NULL;
}
const char* bname = strrchr(name, '/');
//在so文件加载完以后，接着就会调用soinfo_alloc函数为so分配soinfo
soinfo* si = soinfo_alloc(bname ? bname + 1 : name);
if (si == NULL) {
return NULL;
}//利用装载结果初始化soinfo对象
si->base = elf_reader.load_start();
si->size = elf_reader.load_size();
si->load_bias = elf_reader.load_bias();
si->flags = 0;
si->entry = 0;
si->dynamic = NULL;
si->phnum = elf_reader.phdr_count();
si->phdr = elf_reader.loaded_phdr();
return si;
}

这里着重强调一下elf_reader.Load()方法：

bool ElfReader::Load() {
return ReadElfHeader() &&
VerifyElfHeader() &&
ReadProgramHeader() &&
ReserveAddressSpace() &&
LoadSegments() &&
FindPhdr();
}

可以看出读取elf的操作，源码中看出只读取了Program 段，这也是很多加固进行抹头操作的原因：IDA只通过加载section Header段，来读取so文件，而实际上源码只读取了Program段，如果只是将section Header段抹头了，IDA便无法正常解析so文件，而Android系统却可以正常解析so文件，这也是一种反调试手段。

再后来就进行一系列对so文件的初始化操作：

si->base = elf_reader.load_start();
si->size = elf_reader.load_size();
si->load_bias = elf_reader.load_bias();
si->flags = 0;
si->entry = 0;
si->dynamic = NULL;
si->phnum = elf_reader.phdr_count();
si->phdr = elf_reader.loaded_phdr();
return si;

其中si结构体如下（可略）：

struct soinfo {
public:
char name[SOINFO_NAME_LEN];
const Elf32_Phdr* phdr;
size_t phnum;
Elf32_Addr entry;
Elf32_Addr base;
unsigned size;

uint32_t unused1;  // DO NOT USE, maintained for compatibility.

Elf32_Dyn* dynamic;

uint32_t unused2; // DO NOT USE, maintained for compatibility
uint32_t unused3; // DO NOT USE, maintained for compatibility

soinfo* next;
unsigned flags;

const char* strtab;
Elf32_Sym* symtab;

size_t nbucket;
size_t nchain;
unsigned* bucket;
unsigned* chain;

unsigned* plt_got;

Elf32_Rel* plt_rel;
size_t plt_rel_count;

Elf32_Rel* rel;
size_t rel_count;

linker_function_t* preinit_array;
size_t preinit_array_count;

linker_function_t* init_array;
size_t init_array_count;
linker_function_t* fini_array;
size_t fini_array_count;

linker_function_t init_func;
linker_function_t fini_func;

#if defined(ANDROID_ARM_LINKER)
// ARM EABI section used for stack unwinding.
unsigned* ARM_exidx;
size_t ARM_exidx_count;
#elif defined(ANDROID_MIPS_LINKER)
unsigned mips_symtabno;
unsigned mips_local_gotno;
unsigned mips_gotsym;
#endif

size_t ref_count;
link_map_t link_map;

bool constructors_called;

// When you read a virtual address from the ELF file, add this
// value to get the corresponding address in the process' address space.
Elf32_Addr load_bias;

bool has_text_relocations;
bool has_DT_SYMBOLIC;

void CallConstructors();
void CallDestructors();
void CallPreInitConstructors();

private:
void CallArray(const char* array_name, linker_function_t* functions, size_t count, bool reverse);//dex文件脱壳点
void CallFunction(const char* function_name, linker_function_t function);//so文件脱壳点
};

0x07 find_library_internal() 之soinfo_link_image(si)

这一部分参考了雪一梦师傅的分析，并作了一些补充：

http://blog.csdn.net/feibabeibei_beibei/article/details/53004525

在si = load_library(name)获得了so文件的info之后，就开始进行一系列操作：

1、定位动态节；

2、解析动态节；

3、加载动态节

4、重定位

1、定位动态节：

phdr_table_get_dynamic_section(const Elf32_Phdr* phdr_table,
int               phdr_count,
Elf32_Addr        load_bias,
Elf32_Dyn**       dynamic,
size_t*           dynamic_count,
Elf32_Word*       dynamic_flags)
{
const Elf32_Phdr* phdr = phdr_table;
const Elf32_Phdr* phdr_limit = phdr + phdr_count;

for (phdr = phdr_table; phdr < phdr_limit; phdr++) {
if (phdr->p_type != PT_DYNAMIC) {//遍历phdr寻找DYNAMIC段，存放了字符串，方法等偏移地址
continue;
}

*dynamic = reinterpret_cast<Elf32_Dyn*>(load_bias + phdr->p_vaddr);
if (dynamic_count) {
*dynamic_count = (unsigned)(phdr->p_memsz / 8);
}
if (dynamic_flags) {
*dynamic_flags = phdr->p_flags;
}
return;
}
*dynamic = NULL;
if (dynamic_count) {
*dynamic_count = 0;
}
}

值得注意的是，这里源码加载动态节的时候只加载了第一个动态节，后面的动态节都没有加载，因此我们可以自己自定义多个programm段中的动态节区，然后在section header中也改变相应的数据，这样IDA解析的信息是有所偏差的，因为Android系统实际上只读取了第一个动态节。

2、解析动态节。

解析代码如下：

for (Elf32_Dyn* d = si->dynamic; d->d_tag != DT_NULL; ++d) {
DEBUG("d = %p, d[0](tag) = 0x%08x d[1](val) = 0x%08x", d, d->d_tag, d->d_un.d_val);
switch(d->d_tag){/初始化动态节dynamic section的属性
case DT_HASH:
si->nbucket = ((unsigned *) (base + d->d_un.d_ptr))[0];
si->nchain = ((unsigned *) (base + d->d_un.d_ptr))[1];
si->bucket = (unsigned *) (base + d->d_un.d_ptr + 8);
si->chain = (unsigned *) (base + d->d_un.d_ptr + 8 + si->nbucket * 4);
break;
case DT_STRTAB:
si->strtab = (const char *) (base + d->d_un.d_ptr);
break;
case DT_SYMTAB:
si->symtab = (Elf32_Sym *) (base + d->d_un.d_ptr);
break;
case DT_PLTREL:
if (d->d_un.d_val != DT_REL) {
DL_ERR("unsupported DT_RELA in \"%s\"", si->name);
return false;
}
break;
case DT_JMPREL:
si->plt_rel = (Elf32_Rel*) (base + d->d_un.d_ptr);
break;
case DT_PLTRELSZ:
si->plt_rel_count = d->d_un.d_val / sizeof(Elf32_Rel);
break;
case DT_REL:
si->rel = (Elf32_Rel*) (base + d->d_un.d_ptr);
break;
case DT_RELSZ:
si->rel_count = d->d_un.d_val / sizeof(Elf32_Rel);
break;
case DT_PLTGOT:
/* Save this in case we decide to do lazy binding. We don't yet. */
si->plt_got = (unsigned *)(base + d->d_un.d_ptr);
break;
case DT_DEBUG:
// Set the DT_DEBUG entry to the address of _r_debug for GDB
// if the dynamic table is writable
if ((dynamic_flags & PF_W) != 0) {
d->d_un.d_val = (int) &_r_debug;
}
break;
case DT_RELA:
DL_ERR("unsupported DT_RELA in \"%s\"", si->name);
return false;
case DT_INIT:
si->init_func = reinterpret_cast<linker_function_t>(base + d->d_un.d_ptr);
DEBUG("%s constructors (DT_INIT) found at %p", si->name, si->init_func);
break;
case DT_FINI:
si->fini_func = reinterpret_cast<linker_function_t>(base + d->d_un.d_ptr);
DEBUG("%s destructors (DT_FINI) found at %p", si->name, si->fini_func);
break;
case DT_INIT_ARRAY:
si->init_array = reinterpret_cast<linker_function_t*>(base + d->d_un.d_ptr);
DEBUG("%s constructors (DT_INIT_ARRAY) found at %p", si->name, si->init_array);
break;
case DT_INIT_ARRAYSZ:
si->init_array_count = ((unsigned)d->d_un.d_val) / sizeof(Elf32_Addr);
break;
case DT_FINI_ARRAY:
si->fini_array = reinterpret_cast<linker_function_t*>(base + d->d_un.d_ptr);
DEBUG("%s destructors (DT_FINI_ARRAY) found at %p", si->name, si->fini_array);
break;
case DT_FINI_ARRAYSZ:
si->fini_array_count = ((unsigned)d->d_un.d_val) / sizeof(Elf32_Addr);
break;
case DT_PREINIT_ARRAY:
si->preinit_array = reinterpret_cast<linker_function_t*>(base + d->d_un.d_ptr);
DEBUG("%s constructors (DT_PREINIT_ARRAY) found at %p", si->name, si->preinit_array);
break;
case DT_PREINIT_ARRAYSZ:
si->preinit_array_count = ((unsigned)d->d_un.d_val) / sizeof(Elf32_Addr);
break;
case DT_TEXTREL:
si->has_text_relocations = true;
break;
case DT_SYMBOLIC:
si->has_DT_SYMBOLIC = true;
break;
case DT_NEEDED:
++needed_count;
break;
#if defined DT_FLAGS
// TODO: why is DT_FLAGS not defined?
case DT_FLAGS:
if (d->d_un.d_val & DF_TEXTREL) {
si->has_text_relocations = true;
}
if (d->d_un.d_val & DF_SYMBOLIC) {
si->has_DT_SYMBOLIC = true;
}
break;
#endif
#if defined(ANDROID_MIPS_LINKER)
case DT_STRSZ:
case DT_SYMENT:
case DT_RELENT:
break;
case DT_MIPS_RLD_MAP:
// Set the DT_MIPS_RLD_MAP entry to the address of _r_debug for GDB.
{
r_debug** dp = (r_debug**) d->d_un.d_ptr;
*dp = &_r_debug;
}
break;
case DT_MIPS_RLD_VERSION:
case DT_MIPS_FLAGS:
case DT_MIPS_BASE_ADDRESS:
case DT_MIPS_UNREFEXTNO:
break;

case DT_MIPS_SYMTABNO:
si->mips_symtabno = d->d_un.d_val;
break;

case DT_MIPS_LOCAL_GOTNO:
si->mips_local_gotno = d->d_un.d_val;
break;

case DT_MIPS_GOTSYM:
si->mips_gotsym = d->d_un.d_val;
break;

default:
DEBUG("Unused DT entry: type 0x%08x arg 0x%08x", d->d_tag, d->d_un.d_val);
break;
#endif
}
}

3、加载依赖库

//********加载依赖库 (NEEDED)       Shared library: [liblog.so]
for (Elf32_Dyn* d = si->dynamic; d->d_tag != DT_NULL; ++d) {
if (d->d_tag == DT_NEEDED) {
const char* library_name = si->strtab + d->d_un.d_val;
DEBUG("%s needs %s", si->name, library_name);
soinfo* lsi = find_library(library_name);
if (lsi == NULL) {
strlcpy(tmp_err_buf, linker_get_error_buffer(), sizeof(tmp_err_buf));
DL_ERR("could not load library \"%s\" needed by \"%s\"; caused by %s",
library_name, si->name, tmp_err_buf);
return false;
}
*pneeded++ = lsi;
}
}

4、重定位操作

if (si->has_text_relocations) {//重定位操作
/* Unprotect the segments, i.e. make them writable, to allow
* text relocations to work properly. We will later call
* phdr_table_protect_segments() after all of them are applied
* and all constructors are run.
*/
DL_WARN("%s has text relocations. This is wasting memory and is "
"a security risk. Please fix.", si->name);
if (phdr_table_unprotect_segments(si->phdr, si->phnum, si->load_bias) < 0) {
DL_ERR("can't unprotect loadable segments for \"%s\": %s",
si->name, strerror(errno));
return false;
}
}

if (si->plt_rel != NULL) {//修改数据达到重定位的目的
DEBUG("[ relocating %s plt ]", si->name );
if (soinfo_relocate(si, si->plt_rel, si->plt_rel_count, needed)) {
return false;//修改数据达到重定位的目的
}
}
if (si->rel != NULL) {
DEBUG("[ relocating %s ]", si->name );
if (soinfo_relocate(si, si->rel, si->rel_count, needed)) {
return false;
}
}

其中具体的意义参见雪一梦师傅的博客：

http://blog.csdn.net/feibabeibei_beibei/article/details/53004525

0xFF 收尾

1、dlopen执行完毕后

2、调用 vonLoad = dlsym(handle, “JNI_OnLoad”)定位JNI_Onload

3、再执行version = (*func)(gDvmJni.jniVm, NULL);执行JNI_Onload

期间可以有三个断点：

.init->.init_array->JNI_Onload->java_com_XX.

反调试思路：

1、section Header的抹头操作

2、自定义多个动态节

这里留个坑，就是未进行对全局变量gDvm的解析，师傅们多多指教哇~

参考链接：

雪一梦师傅的博客：

http://blog.csdn.net/feibabeibei_beibei/article/details/53004525

土豆大师傅：https://xianzhi.aliyun.com/forum/read/316.html

从乌云转载的文章：http://blog.csdn.net/maspchen/article/details/50568693