DYLD动态连接器

一、Dyld 概述

dynamic link editor，动态链接编辑器，或可叫做dynamic loader，动态加载器。是苹果操作系统一个重要组成部分，在系统内核做好程序准备工作之后，余下的工作交由dyld负责。它的源码是开源的。 源码地址：opensource.apple.com/tarballs/dy…

二、Dyld 的start函数

• 在某类的load方法中打断点，通过函数调用栈（bt），可以找到_dyld_start:（up）是函数调用栈的第一个函数，在此函数的汇编代码中可以看到（bl跳转指令下）dyldbootstrap::start(macho_header const*, int, char const**, long, macho_header const*, unsigned long*)，由此说明程序启动的开始是调用C++的方法，是在dyldbootstrap作用域内调用start函数的；打开APP时，系统内核开启一个进程，然后dyld 在通过调用start函数加载APP；
• dyld_start 函数

/**
* appsMachHeader MachO header 
* slide ASLR 防止缓存溢出攻击
*/
uintptr_t start(const struct macho_header* appsMachHeader, int argc, const char* argv[],intptr_t slide, const struct macho_header* dyldsMachHeader,uintptr_t* startGlue){
	// 1、计算ASLR的偏移
    slide = slideOfMainExecutable(dyldsMachHeader);
    bool shouldRebase = slide != 0;
#if __has_feature(ptrauth_calls)
    shouldRebase = true;
#endif
    if ( shouldRebase ) {
   //  2、重定向MachOHeader ，slider 
        rebaseDyld(dyldsMachHeader, slide);
    }

	// 3、初始化MachO allow dyld to use mach messaging
	mach_init();

	// kernel sets up env pointer to be just past end of agv array
	const char** envp = &argv[argc+1];
	
	// kernel sets up apple pointer to be just past end of envp array
	const char** apple = envp;
	while(*apple != NULL) { ++apple; }
	++apple;

	//4、防止栈溢出 set up random value for stack canary
	__guard_setup(apple);

#if DYLD_INITIALIZER_SUPPORT
	// 5、初始化dyld run all C++ initializers inside dyld
	runDyldInitializers(dyldsMachHeader, slide, argc, argv, envp, apple);
#endif

	//6、初始化完成dyld，调用dyld_main   now that we are done bootstrapping dyld, call dyld's main
	uintptr_t appsSlide = slideOfMainExecutable(appsMachHeader);
	return dyld::_main(appsMachHeader, appsSlide, argc, argv, envp, apple, startGlue);
}

三、Dyld 的main函数

• dyld::_main()是整个App启动的关键函数，此函数里面做了很多事情，代码如下：

// Entry point for dyld.  The kernel loads dyld and jumps to __dyld_start which
// sets up some registers and call this function.
//
// Returns address of main() in target program which __dyld_start jumps to
//
uintptr_t
_main(const macho_header* mainExecutableMH, uintptr_t mainExecutableSlide, 
        int argc, const char* argv[], const char* envp[], const char* apple[], 
        uintptr_t* startGlue)
{
    // Grab the cdHash of the main executable from the environment
    // 第一步、设置运行环境
    uint8_t mainExecutableCDHashBuffer[20];
    const uint8_t* mainExecutableCDHash = nullptr;
    if ( hexToBytes(_simple_getenv(apple, "executable_cdhash"), 40, mainExecutableCDHashBuffer) )
        // 获取主程序的hash
        mainExecutableCDHash = mainExecutableCDHashBuffer;
    // Trace dyld's load
    notifyKernelAboutImage((macho_header*)&__dso_handle, _simple_getenv(apple, "dyld_file"));
#if !TARGET_IPHONE_SIMULATOR
    // Trace the main executable's load
    notifyKernelAboutImage(mainExecutableMH, _simple_getenv(apple, "executable_file"));
#endif
    uintptr_t result = 0;
    // 获取主程序的macho_header结构
    sMainExecutableMachHeader = mainExecutableMH;
    // 获取主程序的slide值 ASLR  
    sMainExecutableSlide = mainExecutableSlide;
    CRSetCrashLogMessage("dyld: launch started");
    // 设置上下文信息
    setContext(mainExecutableMH, argc, argv, envp, apple);
    // Pickup the pointer to the exec path.
    // 获取主程序路径
    sExecPath = _simple_getenv(apple, "executable_path");
    // <rdar://problem/13868260> Remove interim apple[0] transition code from dyld
    if (!sExecPath) sExecPath = apple[0];
    if ( sExecPath[0] != '/' ) {
        // have relative path, use cwd to make absolute
        char cwdbuff[MAXPATHLEN];
        if ( getcwd(cwdbuff, MAXPATHLEN) != NULL ) {
            // maybe use static buffer to avoid calling malloc so early...
            char* s = new char[strlen(cwdbuff) + strlen(sExecPath) + 2];
            strcpy(s, cwdbuff);
            strcat(s, "/");
            strcat(s, sExecPath);
            sExecPath = s;
        }
    }
    // Remember short name of process for later logging
    // 获取进程名称
    sExecShortName = ::strrchr(sExecPath, '/');
    if ( sExecShortName != NULL )
        ++sExecShortName;
    else
        sExecShortName = sExecPath;
    
    // 配置进程受限模式
    configureProcessRestrictions(mainExecutableMH);
    // 检测环境变量
    checkEnvironmentVariables(envp);
    defaultUninitializedFallbackPaths(envp);
    // 如果设置了DYLD_PRINT_OPTS则调用printOptions()打印参数 主程序MachO的地址
    if ( sEnv.DYLD_PRINT_OPTS )
        printOptions(argv);
    // 如果设置了DYLD_PRINT_ENV则调用printEnvironmentVariables()打印环境变量
    if ( sEnv.DYLD_PRINT_ENV ) 
        printEnvironmentVariables(envp);
    // 获取当前程序架构
    getHostInfo(mainExecutableMH, mainExecutableSlide);
    //-------------第一步结束-------------
    
    // load shared cache
    // 第二步，加载共享缓存
    // 检查共享缓存是否开启，iOS必须开启
    checkSharedRegionDisable((mach_header*)mainExecutableMH);
    if ( gLinkContext.sharedRegionMode != ImageLoader::kDontUseSharedRegion ) {
        mapSharedCache();
    }
    ...
    try {
        // add dyld itself to UUID list
        addDyldImageToUUIDList();
        // instantiate ImageLoader for main executable
        // 第三步 实例化主程序
        sMainExecutable = instantiateFromLoadedImage(mainExecutableMH, mainExecutableSlide, sExecPath);
        gLinkContext.mainExecutable = sMainExecutable;
        gLinkContext.mainExecutableCodeSigned = hasCodeSignatureLoadCommand(mainExecutableMH);
        // Now that shared cache is loaded, setup an versioned dylib overrides
    #if SUPPORT_VERSIONED_PATHS
        checkVersionedPaths();
    #endif
        // dyld_all_image_infos image list does not contain dyld
        // add it as dyldPath field in dyld_all_image_infos
        // for simulator, dyld_sim is in image list, need host dyld added
#if TARGET_IPHONE_SIMULATOR
        // get path of host dyld from table of syscall vectors in host dyld
        void* addressInDyld = gSyscallHelpers;
#else
        // get path of dyld itself
        void*  addressInDyld = (void*)&__dso_handle;
#endif
        char dyldPathBuffer[MAXPATHLEN+1];
        int len = proc_regionfilename(getpid(), (uint64_t)(long)addressInDyld, dyldPathBuffer, MAXPATHLEN);
        if ( len > 0 ) {
            dyldPathBuffer[len] = '\0'; // proc_regionfilename() does not zero terminate returned string
            if ( strcmp(dyldPathBuffer, gProcessInfo->dyldPath) != 0 )
                gProcessInfo->dyldPath = strdup(dyldPathBuffer);
        }
        // load any inserted libraries
        // 第四步 加载插入的动态库
        if  ( sEnv.DYLD_INSERT_LIBRARIES != NULL ) {
            for (const char* const* lib = sEnv.DYLD_INSERT_LIBRARIES; *lib != NULL; ++lib)
                loadInsertedDylib(*lib);
        }
        // record count of inserted libraries so that a flat search will look at 
        // inserted libraries, then main, then others.
        // 记录插入的动态库数量
        sInsertedDylibCount = sAllImages.size()-1;
        // link main executable
        // 第五步 链接主程序
        gLinkContext.linkingMainExecutable = true;
#if SUPPORT_ACCELERATE_TABLES
        if ( mainExcutableAlreadyRebased ) {
            // previous link() on main executable has already adjusted its internal pointers for ASLR
            // work around that by rebasing by inverse amount
            sMainExecutable->rebase(gLinkContext, -mainExecutableSlide);
        }
#endif
        link(sMainExecutable, sEnv.DYLD_BIND_AT_LAUNCH, true, ImageLoader::RPathChain(NULL, NULL), -1);
        sMainExecutable->setNeverUnloadRecursive();
        if ( sMainExecutable->forceFlat() ) {
            gLinkContext.bindFlat = true;
            gLinkContext.prebindUsage = ImageLoader::kUseNoPrebinding;
        }
        // link any inserted libraries
        // do this after linking main executable so that any dylibs pulled in by inserted 
        // dylibs (e.g. libSystem) will not be in front of dylibs the program uses
        // 第六步 链接插入的动态库
        if ( sInsertedDylibCount > 0 ) {
            for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
                ImageLoader* image = sAllImages[i+1];
                link(image, sEnv.DYLD_BIND_AT_LAUNCH, true, ImageLoader::RPathChain(NULL, NULL), -1);
                image->setNeverUnloadRecursive();
            }
            // only INSERTED libraries can interpose
            // register interposing info after all inserted libraries are bound so chaining works
            for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
                ImageLoader* image = sAllImages[i+1];
                image->registerInterposing();
            }
        }
        // <rdar://problem/19315404> dyld should support interposition even without DYLD_INSERT_LIBRARIES
        for (long i=sInsertedDylibCount+1; i < sAllImages.size(); ++i) {
            ImageLoader* image = sAllImages[i];
            if ( image->inSharedCache() )
                continue;
            image->registerInterposing();
        }
        ...
        // apply interposing to initial set of images
        for(int i=0; i < sImageRoots.size(); ++i) {
            sImageRoots[i]->applyInterposing(gLinkContext);
        }
        gLinkContext.linkingMainExecutable = false;
        
        // <rdar://problem/12186933> do weak binding only after all inserted images linked
        // 第七步 执行弱符号绑定
        sMainExecutable->weakBind(gLinkContext);
        // If cache has branch island dylibs, tell debugger about them
        if ( (sSharedCacheLoadInfo.loadAddress != NULL) && (sSharedCacheLoadInfo.loadAddress->header.mappingOffset >= 0x78) && (sSharedCacheLoadInfo.loadAddress->header.branchPoolsOffset != 0) ) {
            uint32_t count = sSharedCacheLoadInfo.loadAddress->header.branchPoolsCount;
            dyld_image_info info[count];
            const uint64_t* poolAddress = (uint64_t*)((char*)sSharedCacheLoadInfo.loadAddress + sSharedCacheLoadInfo.loadAddress->header.branchPoolsOffset);
            // <rdar://problem/20799203> empty branch pools can be in development cache
            if ( ((mach_header*)poolAddress)->magic == sMainExecutableMachHeader->magic ) {
                for (int poolIndex=0; poolIndex < count; ++poolIndex) {
                    uint64_t poolAddr = poolAddress[poolIndex] + sSharedCacheLoadInfo.slide;
                    info[poolIndex].imageLoadAddress = (mach_header*)(long)poolAddr;
                    info[poolIndex].imageFilePath = "dyld_shared_cache_branch_islands";
                    info[poolIndex].imageFileModDate = 0;
                }
                // add to all_images list
                addImagesToAllImages(count, info);
                // tell gdb about new branch island images
                gProcessInfo->notification(dyld_image_adding, count, info);
            }
        }
        CRSetCrashLogMessage("dyld: launch, running initializers");
        ...
        // run all initializers
        // 第八步 执行初始化方法
        initializeMainExecutable(); 
        // notify any montoring proccesses that this process is about to enter main()
        dyld3::kdebug_trace_dyld_signpost(DBG_DYLD_SIGNPOST_START_MAIN_DYLD2, 0, 0);
        notifyMonitoringDyldMain();
        // find entry point for main executable
        // 第九步 查找入口点并返回
        result = (uintptr_t)sMainExecutable->getThreadPC();
        if ( result != 0 ) {
            // main executable uses LC_MAIN, needs to return to glue in libdyld.dylib
            if ( (gLibSystemHelpers != NULL) && (gLibSystemHelpers->version >= 9) )
                *startGlue = (uintptr_t)gLibSystemHelpers->startGlueToCallExit;
            else
                halt("libdyld.dylib support not present for LC_MAIN");
        }
        else {
            // main executable uses LC_UNIXTHREAD, dyld needs to let "start" in program set up for main()
            result = (uintptr_t)sMainExecutable->getMain();
            *startGlue = 0;
        }
    }
    catch(const char* message) {
        syncAllImages();
        halt(message);
    }
    catch(...) {
        dyld::log("dyld: launch failed\n");
    }
    ...
    
    return result;
}

dyld::_main()函数主要处理步骤大致可分为以下九步：

• 第一步：设置运行环境 -- 主要是设置加载，运行参数、环境变量等。

  • 将入参mainExecutableMH赋值给了sMainExecutableMachHeader，这是主程序的Mach-O头部信息macho_header，加载器依据macho_header就可以解析整个Mach-O文件信息。
  • 将_start函数中计算得到的appSlider的值mainExecutableSlide传入sMainExecutableSlide，即主程序的ASLR的值；
  • 接着调用setContext()设置上下文信息，包括一些回调函数、参数、标志信息等。设置的回调函数都是dyld模块自身实现的，如loadLibrary()函数实际调用的是libraryLocator()，负责加载动态库。代码片断如下：

static void setContext(const macho_header* mainExecutableMH, int argc, const char* argv[], const char* envp[], const char* apple[])
{
   gLinkContext.loadLibrary         = &libraryLocator;
   gLinkContext.terminationRecorder = &terminationRecorder;
   ...
}

• • configureProcessRestrictions()用来配置进程是否受限，代码逻辑比较简单，sEnvMode默认等于envNone（即受限模式），如果设置了get_task_allow权限或者是内核开发时会设置成envAll，如果设置了uid和gid则立即变成受限模式，代码片段如下：

static void configureProcessRestrictions(const macho_header* mainExecutableMH)
{
    sEnvMode = envNone; // 受限模式
    gLinkContext.requireCodeSignature = true; // 需要代码签名
    uint32_t flags;
    if ( csops(0, CS_OPS_STATUS, &flags, sizeof(flags)) != -1 ) {
        // 启用代码签名
        if ( flags & CS_ENFORCEMENT ) {
            // get_task_allow
            if ( flags & CS_GET_TASK_ALLOW ) {
                // Xcode built app for Debug allowed to use DYLD_* variables
                // Xcode调试时允许使用DYLD_*环境变量
                sEnvMode = envAll; // 非受限模式
            }
            else {
                // Development kernel can use DYLD_PRINT_* variables on any FairPlay encrypted app
                uint32_t secureValue = 0;
                size_t   secureValueSize = sizeof(secureValue);
                if ( (sysctlbyname("kern.secure_kernel", &secureValue, &secureValueSize, NULL, 0) == 0) && (secureValue == 0) && isFairPlayEncrypted(mainExecutableMH) ) {
                    sEnvMode = envPrintOnly;
                }
            }
        }
        else {
            // Development kernel can run unsigned code
            // 内核开发运行运行非签名代码
            sEnvMode = envAll; // 非受限模式
            gLinkContext.requireCodeSignature = false; // 无需代码签名
        }
    }
    // 如果设置了uid、gid则变成受限模式
    if ( issetugid() ) {
        sEnvMode = envNone;
    }
}

• • checkEnvironmentVariables()检测环境变量，如果sEnvMode为envNone就直接返回，否则调用processDyldEnvironmentVariable()处理并设置环境变量，代码如下：

static void checkEnvironmentVariables(const char* envp[])
{
   if ( sEnvMode == envNone )
      return;
   const char** p;
   for(p = envp; *p != NULL; p++) {
      const char* keyEqualsValue = *p;
       if ( strncmp(keyEqualsValue, "DYLD_", 5) == 0 ) {
         const char* equals = strchr(keyEqualsValue, '=');
         if ( equals != NULL ) {
            strlcat(sLoadingCrashMessage, "\n", sizeof(sLoadingCrashMessage));
            strlcat(sLoadingCrashMessage, keyEqualsValue, sizeof(sLoadingCrashMessage));
            const char* value = &equals[1];
            const size_t keyLen = equals-keyEqualsValue;
            char key[keyLen+1];
            strncpy(key, keyEqualsValue, keyLen);
            key[keyLen] = '\0';
            if ( (sEnvMode == envPrintOnly) && (strncmp(key, "DYLD_PRINT_", 11) != 0) )
               continue;
            // 处理并设置环境变量
            processDyldEnvironmentVariable(key, value, NULL);
         }
      }
      else if ( strncmp(keyEqualsValue, "LD_LIBRARY_PATH=", 16) == 0 ) {
         const char* path = &keyEqualsValue[16];
         sEnv.LD_LIBRARY_PATH = parseColonList(path, NULL);
      }
   }
   ...
}

• • 最后是调用getHostInfo()获取当前程序架构，至此，第一步的准备工作就完成了。 整个过程中有一些DYLD_*开头的环境变量，Edit Scheme -> Arguments -> Environment Variables 中配置变量,如图：

    

// 如果设置了DYLD_PRINT_OPTS则调用printOptions()打印参数 主程序MachO的地址
    if ( sEnv.DYLD_PRINT_OPTS )
        printOptions(argv);
    // 如果设置了DYLD_PRINT_ENV则调用printEnvironmentVariables()打印环境变量
    if ( sEnv.DYLD_PRINT_ENV ) 
        printEnvironmentVariables(envp);
// 以下为打印结果，会在所有类加载前打印：
opt[0] = "/var/mobile/Containers/Bundle/Application/B64DB1D3-AD29-4F47-9C77-97B40079E276/Launch.app/Launch"
TMPDIR=/private/var/mobile/Containers/Data/Application/F5D83A13-1C1B-48B7-9764-356C1E1A6764/tmp
__CF_USER_TEXT_ENCODING=0x1F5:0:0
SHELL=/bin/sh
SQLITE_ENABLE_THREAD_ASSERTIONS=1
OS_ACTIVITY_DT_MODE=YES
HOME=/private/var/mobile/Containers/Data/Application/F5D83A13-1C1B-48B7-9764-356C1E1A6764
CFFIXED_USER_HOME=/private/var/mobile/Containers/Data/Application/F5D83A13-1C1B-48B7-9764-356C1E1A6764
FBSClientLogging=0
NSUnbufferedIO=YES
PATH=/usr/bin:/bin:/usr/sbin:/sbin
LOGNAME=mobile
XPC_SERVICE_NAME=UIKitApplication:com.chinapyg.Launch1[0x6e2d]
DYLD_INSERT_LIBRARIES=/Library/MobileSubstrate/MobileSubstrate.dylib:/Developer/usr/lib/libBacktraceRecording.dylib:/Developer/Library/PrivateFrameworks/DTDDISupport.framework/libViewDebuggerSupport.dylib
CLASSIC=0
CFLOG_FORCE_STDERR=YES
DYLD_PRINT_OPTS=1
DYLD_PRINT_ENV=1
USER=mobile
_MSSafeMode=0
XPC_FLAGS=0x1
DYLD_LIBRARY_PATH=/usr/lib/system/introspection

• 第二步：加载共享缓存

  • 先调用checkSharedRegionDisable()检查共享缓存库是否禁用。该函数的iOS实现部分仅有一句注释，从注释可知iOS共享缓存不能被禁用：

//检查共享缓存库的禁用状态，iOS不能被禁用
static void checkSharedRegionDisable(const dyld3::MachOLoaded* mainExecutableMH, uintptr_t mainExecutableSlide)
{
#if __MAC_OS_X_VERSION_MIN_REQUIRED
	// if main executable has segments that overlap the shared region,
	// then disable using the shared region
	if ( mainExecutableMH->intersectsRange(SHARED_REGION_BASE, SHARED_REGION_SIZE) ) {
		gLinkContext.sharedRegionMode = ImageLoader::kDontUseSharedRegion;
		if ( gLinkContext.verboseMapping )
			dyld::warn("disabling shared region because main executable overlaps\n");
	}
#if __i386__ // 模拟器架构
	if ( !gLinkContext.allowEnvVarsPath ) {
		// <rdar://problem/15280847> use private or no shared region for suid processes
		gLinkContext.sharedRegionMode = ImageLoader::kUsePrivateSharedRegion;
	}
#endif
#endif
	// iOS cannot run without shared region 
}

• • 调用mapSharedCache()，加载共享缓存库；
• • 在 mapSharedCache()方法中，调用loadDyldCache()去进行共享缓存加载的判断
    • 仅加载到当前进程，调用mapCachePrivate()。
    • 共享缓存已加载，不做任何处理。
    • 最后若是首次加载才调用mapCacheSystemWide()去加载共享缓存；

static void mapSharedCache()
{
	dyld3::SharedCacheOptions opts;
	opts.cacheDirOverride	= sSharedCacheOverrideDir;
	opts.forcePrivate		= (gLinkContext.sharedRegionMode == ImageLoader::kUsePrivateSharedRegion);


#if __x86_64__ && !TARGET_IPHONE_SIMULATOR  
	opts.useHaswell			= sHaswell;
#else
	opts.useHaswell			= false;
#endif
	opts.verbose			= gLinkContext.verboseMapping;
	loadDyldCache(opts, &sSharedCacheLoadInfo);

	// update global state
	if ( sSharedCacheLoadInfo.loadAddress != nullptr ) {
		gLinkContext.dyldCache 								= sSharedCacheLoadInfo.loadAddress;
		dyld::gProcessInfo->processDetachedFromSharedRegion = opts.forcePrivate;
		dyld::gProcessInfo->sharedCacheSlide                = sSharedCacheLoadInfo.slide;
		dyld::gProcessInfo->sharedCacheBaseAddress          = (unsigned long)sSharedCacheLoadInfo.loadAddress;
		sSharedCacheLoadInfo.loadAddress->getUUID(dyld::gProcessInfo->sharedCacheUUID);
		dyld3::kdebug_trace_dyld_image(DBG_DYLD_UUID_SHARED_CACHE_A, (const uuid_t *)&dyld::gProcessInfo->sharedCacheUUID[0], {0,0}, {{ 0, 0 }}, (const mach_header *)sSharedCacheLoadInfo.loadAddress);
	}

//#if __IPHONE_OS_VERSION_MIN_REQUIRED && !TARGET_IPHONE_SIMULATOR
// RAM disk booting does not have shared cache yet
// Don't make lack of a shared cache fatal in that case
//	if ( sSharedCacheLoadInfo.loadAddress == nullptr ) {
//		if ( sSharedCacheLoadInfo.errorMessage != nullptr )
//			halt(sSharedCacheLoadInfo.errorMessage);
//		else
//			halt("error loading dyld shared cache");
//	}
//#endif
}

bool loadDyldCache(const SharedCacheOptions& options, SharedCacheLoadInfo* results)
{
    results->loadAddress        = 0;
    results->slide              = 0;
    results->errorMessage       = nullptr;

#if TARGET_IPHONE_SIMULATOR  //模拟器
    // simulator only supports mmap()ing cache privately into process
    return mapCachePrivate(options, results);
#else
    if ( options.forcePrivate ) {// 仅加载到当前进程
        // mmap cache into this process only
        return mapCachePrivate(options, results);
    }
    else {// 加载到共享缓存的
        // fast path: when cache is already mapped into shared region
        bool hasError = false;
        if ( reuseExistingCache(options, results) ) {// 共享缓存已加载 
            hasError = (results->errorMessage != nullptr);
        } else {// 首次加载
            // slow path: this is first process to load cache
            hasError = mapCacheSystemWide(options, results);//首次加载调用
        }
        return hasError;
    }
#endif
}

• 第三步：实例化主程序

  • 这一步将主程序的Mach-O加载进内存，并实例化一个ImageLoader。instantiateFromLoadedImage()首先调用isCompatibleMachO()检测Mach-O头部的magic、cputype、cpusubtype等相关属性，判断Mach-O文件的兼容性，如果兼容性满足，则调用ImageLoaderMachO::instantiateMainExecutable()实例化主程序的ImageLoader，代码如下：

static ImageLoaderMachO* instantiateFromLoadedImage(const macho_header* mh, uintptr_t slide, const char* path)
{
  // try mach-o loader
  // 尝试加载MachO
  if ( isCompatibleMachO((const uint8_t*)mh, path) ) {// 判断Mach-O文件的兼容性
    ImageLoader* image = ImageLoaderMachO::instantiateMainExecutable(mh, slide, path, gLinkContext);
    addImage(image);
    return (ImageLoaderMachO*)image;
  }
  
  throw "main executable not a known format";
}

• • ImageLoaderMachO::instantiateMainExecutable()函数里面首先会调用sniffLoadCommands()函数来获取一些数据，包括：
    • compressed：若Mach-O存在LC_DYLD_INFO和LC_DYLD_INFO_ONLY加载命令，则说明是压缩类型的Mach-O，代码片段如下：

switch (cmd->cmd) {
case LC_DYLD_INFO:
case LC_DYLD_INFO_ONLY:
    if ( cmd->cmdsize != sizeof(dyld_info_command) )
        throw "malformed mach-o image: LC_DYLD_INFO size wrong";
    dyldInfoCmd = (struct dyld_info_command*)cmd;
    // 存在LC_DYLD_INFO或者LC_DYLD_INFO_ONLY则表示是压缩类型的Mach-O
    *compressed = true;
    break;
    ...
}

• • • segCount：根据 LC_SEGMENT_COMMAND 加载命令来统计段数量，这里抛出的错误日志也说明了段的数量是不能超过255个，代码片段如下：

case LC_SEGMENT_COMMAND:
    segCmd = (struct macho_segment_command*)cmd;
...
    if ( segCmd->vmsize != 0 )
        *segCount += 1;
if ( *segCount > 255 )
    dyld::throwf("malformed mach-o image: more than 255 segments in %s", path);

• • • libCount：根据 LC_LOAD_DYLIB、LC_LOAD_WEAK_DYLIB、LC_REEXPORT_DYLIB、LC_LOAD_UPWARD_DYLIB 这几个加载命令来统计库的数量，库的数量不能超过4095个。代码片段如下：

case LC_LOAD_DYLIB:
case LC_LOAD_WEAK_DYLIB:
case LC_REEXPORT_DYLIB:
case LC_LOAD_UPWARD_DYLIB:
*libCount += 1;
if ( *libCount > 4095 )
    dyld::throwf("malformed mach-o image: more than 4095 dependent libraries in %s", path);

• • • codeSigCmd：通过解析LC_CODE_SIGNATURE来获取代码签名加载命令，代码片段如下：

case LC_CODE_SIGNATURE:
*codeSigCmd = (struct linkedit_data_command*)cmd;
break;

• • • encryptCmd：通过LC_ENCRYPTION_INFO和LC_ENCRYPTION_INFO_64来获取段的加密信息，代码片段如下：

case LC_ENCRYPTION_INFO:
...
*encryptCmd = (encryption_info_command*)cmd;
break;
case LC_ENCRYPTION_INFO_64:
...
*encryptCmd = (encryption_info_command*)cmd;
break;

• • ImageLoader是抽象类，其子类负责把Mach-O文件实例化为image，当sniffLoadCommands()解析完以后，根据compressed的值来决定调用哪个子类进行实例化，代码如下：

ImageLoader* ImageLoaderMachO::instantiateMainExecutable(const macho_header* mh, uintptr_t slide, const char* path, const LinkContext& context)
{
  bool compressed;
  unsigned int segCount;
  unsigned int libCount;
  const linkedit_data_command* codeSigCmd;
  const encryption_info_command* encryptCmd;
  sniffLoadCommands(mh, path, false, &compressed, &segCount, &libCount, context, &codeSigCmd, &encryptCmd);
  // instantiate concrete class based on content of load commands
  if ( compressed ) 
    return ImageLoaderMachOCompressed::instantiateMainExecutable(mh, slide, path, segCount, libCount, context);
  else
#if SUPPORT_CLASSIC_MACHO
    return ImageLoaderMachOClassic::instantiateMainExecutable(mh, slide, path, segCount, libCount, context);
#else
    throw "missing LC_DYLD_INFO load command";
#endif
}

• • 这里总结为4步：
    • ImageLoaderMachOCompressed::instantiateStart()创建ImageLoaderMachOCompressed对象。
    • image->disableCoverageCheck()禁用段覆盖检测。
    • image->instantiateFinish()首先调用parseLoadCmds()解析加载命令，然后调用this->setDyldInfo()设置动态库链接信息，最后调用this->setSymbolTableInfo() 设置符号表相关信息，代码片段如下：
    • image->setMapped()函数注册通知回调、计算执行时间等等。
• • 在调用完ImageLoaderMachO::instantiateMainExecutable()后继续调用addImage()，将image加入到sAllImages全局镜像列表，并将image映射到申请的内存中。

• 第四步：加载插入的动态库

  • 这一步是加载环境变量DYLD_INSERT_LIBRARIES中配置的动态库，先判断环境变量DYLD_INSERT_LIBRARIES中是否存在要加载的动态库，如果存在则调用loadInsertedDylib()依次加载，代码如下：

if  ( sEnv.DYLD_INSERT_LIBRARIES != NULL ) {
  for (const char* const* lib = sEnv.DYLD_INSERT_LIBRARIES; *lib != NULL; ++lib)
    loadInsertedDylib(*lib);
}

• • loadInsertedDylib()内部设置了一个LoadContext参数后，调用了load()函数， load()函数的实现为一系列的loadPhase*()函数，loadPhase0()~loadPhase1()函数会调用不同的函数来继续处理。
• • 当内部调用到loadPhase5load()函数的时候，会先在共享缓存中搜寻，如果存在则使用ImageLoaderMachO::instantiateFromCache()来实例化ImageLoader，否则通过loadPhase5open()打开文件并读取数据到内存后，再调用loadPhase6()，通过ImageLoaderMachO::instantiateFromFile()实例化ImageLoader，最后调用checkandAddImage()验证镜像并将其加入到全局镜像列表中。 load()函数代码如下：

ImageLoader* load(const char* path, const LoadContext& context, unsigned& cacheIndex)
{
    ...
   
    // try all path permutations and check against existing loaded images
    ImageLoader* image = loadPhase0(path, orgPath, context, cacheIndex, NULL);
    if ( image != NULL ) {
        CRSetCrashLogMessage2(NULL);
        return image;
    }
    // try all path permutations and try open() until first success
    std::vector<const char*> exceptions;
    image = loadPhase0(path, orgPath, context, cacheIndex, &exceptions);
#if !TARGET_IPHONE_SIMULATOR
    // <rdar://problem/16704628> support symlinks on disk to a path in dyld shared cache
    if ( image == NULL)
        image = loadPhase2cache(path, orgPath, context, cacheIndex, &exceptions);
#endif
    ...
}

• 第五步：链接主程序

这一步调用link()函数将实例化后的主程序进行动态修正，让二进制变为可正常执行的状态。link()函数内部调用了ImageLoader::link()函数，从源代码可以看到，这一步主要做了以下几个事情： - recursiveLoadLibraries() 根据LC_LOAD_DYLIB加载命令把所有依赖库加载进内存。 - recursiveUpdateDepth() 递归刷新依赖库的层级。 - recursiveRebase() 由于ASLR的存在，必须递归对主程序以及依赖库进行重定位操作。 - recursiveBind() 把主程序二进制和依赖进来的动态库全部执行符号表绑定。 - weakBind() 如果链接的不是主程序二进制的话，会在此时执行弱符号绑定，主程序二进制则在link()完后再执行弱符号绑定，后面会进行分析。 - recursiveGetDOFSections()、context.registerDOFs() 注册DOF（DTrace Object Format）节。 ImageLoader::link()源代码如下：

void ImageLoader::link(const LinkContext& context, bool forceLazysBound, bool preflightOnly, bool neverUnload, const RPathChain& loaderRPaths, const char* imagePath)
{
...
uint64_t t0 = mach_absolute_time();
// 递归加载加载主程序所需依赖库
this->recursiveLoadLibraries(context, preflightOnly, loaderRPaths, imagePath);
...
uint64_t t1 = mach_absolute_time();
context.clearAllDepths();
// 递归刷新依赖库的层级
this->recursiveUpdateDepth(context.imageCount());
uint64_t t2 = mach_absolute_time();
// 递归进行rebase
this->recursiveRebase(context);
uint64_t t3 = mach_absolute_time();
// 递归绑定符号表
this->recursiveBind(context, forceLazysBound, neverUnload);
uint64_t t4 = mach_absolute_time();
if ( !context.linkingMainExecutable )
    // 弱符号绑定
    this->weakBind(context);
uint64_t t5 = mach_absolute_time(); 
context.notifyBatch(dyld_image_state_bound, false);
uint64_t t6 = mach_absolute_time(); 
std::vector<DOFInfo> dofs;
// 注册DOF节
this->recursiveGetDOFSections(context, dofs);
context.registerDOFs(dofs);
uint64_t t7 = mach_absolute_time(); 
...
}

• 第六步：链接插入的动态库

  • 这一步与链接主程序一样，将前面调用addImage()函数保存在sAllImages中的动态库列表循环取出并调用link()进行链接，需要注意的是，sAllImages中保存的第一项是主程序的镜像，所以要从i+1的位置开始，取到的才是动态库的ImageLoader：ImageLoader* image = sAllImages[i+1];
  • 接下来循环调用每个镜像的registerInterposing()函数，该函数会遍历Mach-O的LC_SEGMENT_COMMAND加载命令，读取__DATA,__interpose，并将读取到的信息保存到fgInterposingTuples中，接着调用applyInterposing()函数，内部经由doInterpose()虚函数进行替换操作，以ImageLoaderMachOCompressed::doInterpose()函数的实现为例：该函数内部调用了eachBind()与eachLazyBind()，具体处理函数是interposeAt()，该函数调用interposedAddress()在fgInterposingTuples中查找需要替换的符号地址，进行最终的符号地址替换，代码如下：

void ImageLoaderMachOCompressed::doInterpose(const LinkContext& context)
{
    // update prebound symbols
    eachBind(context, &ImageLoaderMachOCompressed::interposeAt);
    eachLazyBind(context, &ImageLoaderMachOCompressed::interposeAt);
}
uintptr_t ImageLoaderMachOCompressed::interposeAt(const LinkContext& context, uintptr_t addr, uint8_t type, const char*, 
                                                uint8_t, intptr_t, long, const char*, LastLookup*, bool runResolver)
{
    if ( type == BIND_TYPE_POINTER ) {
        uintptr_t* fixupLocation = (uintptr_t*)addr;
        uintptr_t curValue = *fixupLocation;
        uintptr_t newValue = interposedAddress(context, curValue, this);
        if ( newValue != curValue) {
            *fixupLocation = newValue;
        }
    }
    return 0;
}

• 第七步：执行弱符号绑定

  • weakBind()首先通过getCoalescedImages()合并所有动态库的弱符号到一个列表里，然后调用initializeCoalIterator()对需要绑定的弱符号进行排序，接着调用incrementCoalIterator()读取dyld_info_command结构的weak_bind_off和weak_bind_size字段，确定弱符号的数据偏移与大小，最终进行弱符号绑定，代码如下：

bool ImageLoaderMachOCompressed::incrementCoalIterator(CoalIterator& it)
{
    if ( it.done )
        return false;
    
    if ( this->fDyldInfo->weak_bind_size == 0 ) {
        /// hmmm, ld set MH_WEAK_DEFINES or MH_BINDS_TO_WEAK, but there is no weak binding info
        it.done = true;
        it.symbolName = "~~~";
        return true;
    }
    const uint8_t* start = fLinkEditBase + fDyldInfo->weak_bind_off;
    const uint8_t* p = start + it.curIndex;
    const uint8_t* end = fLinkEditBase + fDyldInfo->weak_bind_off + this->fDyldInfo->weak_bind_size;
    uintptr_t count;
    uintptr_t skip;
    uintptr_t segOffset;
    while ( p < end ) {
        uint8_t immediate = *p & BIND_IMMEDIATE_MASK;
        uint8_t opcode = *p & BIND_OPCODE_MASK;
        ++p;
        switch (opcode) {
            case BIND_OPCODE_DONE:
                it.done = true;
                it.curIndex = p - start;
                it.symbolName = "~~~"; // sorts to end
                return true;
        }
        break;
        ...
    }
    ...
    return true;
}

• 第八步：执行初始化方法

  • 这一步由initializeMainExecutable()完成。dyld会优先初始化动态库，然后初始化主程序。该函数首先执行runInitializers()，内部再依次调用processInitializers()、recursiveInitialization()。我们在recursiveInitialization()函数里找到了notifySingle()函数：

context.notifySingle(dyld_image_state_dependents_initialized, this, &timingInfo);

- 接着跟进notifySingle函数，看到下面处理代码：

if ( (state == dyld_image_state_dependents_initialized) && (sNotifyObjCInit != NULL) && image->notifyObjC() ) {
    uint64_t t0 = mach_absolute_time();
    (*sNotifyObjCInit)(image->getRealPath(), image->machHeader());
    uint64_t t1 = mach_absolute_time();
    uint64_t t2 = mach_absolute_time();
    uint64_t timeInObjC = t1-t0;
    uint64_t emptyTime = (t2-t1)*100;
    if ( (timeInObjC > emptyTime) && (timingInfo != NULL) ) {
        timingInfo->addTime(image->getShortName(), timeInObjC);
    }
}

我们关心的只有sNotifyObjCInit这个回调，继续寻找赋值的地方：

void registerObjCNotifiers(_dyld_objc_notify_mapped mapped, _dyld_objc_notify_init init, _dyld_objc_notify_unmapped unmapped)
{
    // record functions to call
    sNotifyObjCMapped   = mapped;
    sNotifyObjCInit     = init;
    sNotifyObjCUnmapped = unmapped;
    ...

再接着找registerObjCNotifiers函数调用，最终找到这里：

void _dyld_objc_notify_register(_dyld_objc_notify_mapped    mapped,
                                _dyld_objc_notify_init      init,
                                _dyld_objc_notify_unmapped  unmapped)
{
    dyld::registerObjCNotifiers(mapped, init, unmapped);
}

• 第九步：查找入口点并返回

  • 这一步调用主程序镜像的getThreadPC()，从加载命令读取LC_MAIN入口，如果没有LC_MAIN就调用getMain()读取LC_UNIXTHREAD，找到后就跳到入口点指定的地址并返回。 至此，整个dyld的加载过程就分析完成了。