iOS 逆向07 -- Dyld动态加载器

2021-01-18  本文已影响0人  YanZi_33

Dyld

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共享缓存技术
动态链接的优势
Dyld源码分析
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#if __arm64__ && !TARGET_OS_SIMULATOR
    .text
    .align 2
    .globl __dyld_start
__dyld_start:
    mov     x28, sp
    and     sp, x28, #~15       // force 16-byte alignment of stack
    mov x0, #0
    mov x1, #0
    stp x1, x0, [sp, #-16]! // make aligned terminating frame
    mov fp, sp          // set up fp to point to terminating frame
    sub sp, sp, #16             // make room for local variables
#if __LP64__
    ldr     x0, [x28]               // get app's mh into x0
    ldr     x1, [x28, #8]
    add     x2, x28, #16            // get argv into x2
#else
    ldr     w0, [x28]               // get app's mh into x0
    ldr     w1, [x28, #4]
    add     w2, w28, #8             // get argv into x2
#endif
    adrp    x3,___dso_handle@page
    add     x3,x3,___dso_handle@pageoff // get dyld's mh in to x4
    mov x4,sp                   // x5 has &startGlue
    //call dyldbootstrap::start(app_mh, argc, argv, dyld_mh, &startGlue)
    bl  __ZN13dyldbootstrap5startEPKN5dyld311MachOLoadedEiPPKcS3_Pm
    mov x16,x0                  // save entry point address in x16
#if __LP64__
    ldr     x1, [sp]
#else
    ldr     w1, [sp]
#endif
    cmp x1, #0
    b.ne    Lnew

uintptr_t start(const dyld3::MachOLoaded* appsMachHeader, int argc, const char* argv[],
                const dyld3::MachOLoaded* dyldsMachHeader, uintptr_t* startGlue)
{
    // Emit kdebug tracepoint to indicate dyld bootstrap has started <rdar://46878536>
    dyld3::kdebug_trace_dyld_marker(DBG_DYLD_TIMING_BOOTSTRAP_START, 0, 0, 0, 0);

    //1.Dyld的重定位
    rebaseDyld(dyldsMachHeader);

    //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;

    //2.栈溢出保护
    __guard_setup(apple);

#if DYLD_INITIALIZER_SUPPORT
    //3.初始化Dyld
    runDyldInitializers(argc, argv, envp, apple);
#endif

    _subsystem_init(apple);

    //4.进入dyld::main() 主函数 (核心)
    uintptr_t appsSlide = appsMachHeader->getSlide();
    return dyld::_main((macho_header*)appsMachHeader, appsSlide, argc, argv, envp, apple, startGlue);
}
为什么要执行Rebase重定位
uintptr_t
_main(const macho_header* mainExecutableMH, uintptr_t mainExecutableSlide, 
        int argc, const char* argv[], const char* envp[], const char* apple[], 
        uintptr_t* startGlue){
    
    if (dyld3::kdebug_trace_dyld_enabled(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE)) {
        launchTraceID = dyld3::kdebug_trace_dyld_duration_start(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE, (uint64_t)mainExecutableMH, 0, 0);
    }

    //Check and see if there are any kernel flags
    dyld3::BootArgs::setFlags(hexToUInt64(_simple_getenv(apple, "dyld_flags"), nullptr));

#if __has_feature(ptrauth_calls)
    if ( const char* disableStr = _simple_getenv(apple, "ptrauth_disabled") ) {
        if ( strcmp(disableStr, "1") == 0 )
            sKeysDisabled = true;
    }else {
        // needed until kernel passes ptrauth_disabled for arm64 main executables
        if ( (mainExecutableMH->cpusubtype == CPU_SUBTYPE_ARM64_V8) || (mainExecutableMH->cpusubtype == CPU_SUBTYPE_ARM64_ALL) )
            sKeysDisabled = true;
    }
#endif

    //第一步：环境变量的配置
    uint8_t mainExecutableCDHashBuffer[20];
    const uint8_t* mainExecutableCDHash = nullptr;
    if ( const char* mainExeCdHashStr = _simple_getenv(apple, "executable_cdhash")) {
        unsigned bufferLenUsed;
        if ( hexStringToBytes(mainExeCdHashStr, mainExecutableCDHashBuffer, sizeof(mainExecutableCDHashBuffer), bufferLenUsed))
            //获取主程序的hash
            mainExecutableCDHash = mainExecutableCDHashBuffer;
    }

    //获取主机信息
    getHostInfo(mainExecutableMH, mainExecutableSlide);
#if !TARGET_OS_SIMULATOR
    // Trace dyld's load
    notifyKernelAboutImage((macho_header*)&__dso_handle, _simple_getenv(apple, "dyld_file"));
    // 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
    sMainExecutableMachHeader = mainExecutableMH;
    //获取主程序的slide值
    sMainExecutableSlide = mainExecutableSlide;

    CRSetCrashLogMessage("dyld: launch started");

    //设置上下文信息
    setContext(mainExecutableMH, argc, argv, envp, apple);

    //获取主程序路径
    sExecPath = _simple_getenv(apple, "executable_path");

    //进程的名字
    sExecShortName = ::strrchr(sExecPath, '/');
    if ( sExecShortName != NULL )
        ++sExecShortName;
    else
        sExecShortName = sExecPath;

    //进程的头环境配置
    configureProcessRestrictions(mainExecutableMH, envp);

#if TARGET_OS_OSX
    if ( !gLinkContext.allowEnvVarsPrint && !gLinkContext.allowEnvVarsPath && !gLinkContext.allowEnvVarsSharedCache ) {
        pruneEnvironmentVariables(envp, &apple);
        setContext(mainExecutableMH, argc, argv, envp, apple);
    }else
#endif
    {
        //检测环境变量
        checkEnvironmentVariables(envp);
        defaultUninitializedFallbackPaths(envp);
    }

    //第二步:加载共享缓存
    //检查缓存共享区域是否开启，iOS必须开启
    checkSharedRegionDisable((dyld3::MachOLoaded*)mainExecutableMH, mainExecutableSlide);
    if ( gLinkContext.sharedRegionMode != ImageLoader::kDontUseSharedRegion ) {
#if TARGET_OS_SIMULATOR
        if ( sSharedCacheOverrideDir)
            mapSharedCache(mainExecutableSlide);
#else
        //共享缓存加载
        mapSharedCache(mainExecutableSlide);
#endif
    }

    //第三步：实例化主程序
    sMainExecutable = instantiateFromLoadedImage(mainExecutableMH, mainExecutableSlide, sExecPath);
    gLinkContext.mainExecutable = sMainExecutable;
    gLinkContext.mainExecutableCodeSigned = hasCodeSignatureLoadCommand(mainExecutableMH);

    //load any inserted libraries
    //第四步：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);
    }
        
    //记录插入的动态库的数量
    sInsertedDylibCount = sAllImages.size()-1;

    //link main executable
    gLinkContext.linkingMainExecutable = true;

    //第五步：链接主程序
    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;
    }

    //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();
        }
        if ( gLinkContext.allowInterposing ) {
            for(unsigned int i=0; i < sInsertedDylibCount; ++i) {
                ImageLoader* image = sAllImages[i+1];
                image->registerInterposing(gLinkContext);
            }
        }
    }

    //第七步：弱引用绑定主程序
    sMainExecutable->weakBind(gLinkContext);
    gLinkContext.linkingMainExecutable = false;

    sMainExecutable->recursiveMakeDataReadOnly(gLinkContext);

    CRSetCrashLogMessage("dyld: launch, running initializers");
#if SUPPORT_OLD_CRT_INITIALIZATION
    // Old way is to run initializers via a callback from crt1.o
    if ( ! gRunInitializersOldWay )
        initializeMainExecutable();
#else
    
    //第八步：执行所有初始化(动态库+主程序)
    initializeMainExecutable();
#endif

    //第九步：查找程序入口函数 main并返回
    notifyMonitoringDyldMain();
        
    catch(const char* message) {
        syncAllImages();
        halt(message);
    }
    catch(...) {
        dyld::log("dyld: launch failed\n");
    }

    CRSetCrashLogMessage("dyld2 mode");
#if !TARGET_OS_SIMULATOR
    if (sLogClosureFailure) {
        // We failed to launch in dyld3, but dyld2 can handle it. synthesize a crash report for analytics
        dyld3::syntheticBacktrace("Could not generate launchClosure, falling back to dyld2", true);
    }
#endif

    if (sSkipMain) {
        notifyMonitoringDyldMain();
        if (dyld3::kdebug_trace_dyld_enabled(DBG_DYLD_TIMING_LAUNCH_EXECUTABLE)) {
            dyld3::kdebug_trace_dyld_duration_end(launchTraceID, DBG_DYLD_TIMING_LAUNCH_EXECUTABLE, 0, 0, 2);
        }
        ARIADNEDBG_CODE(220, 1);
        result = (uintptr_t)&fake_main;
        *startGlue = (uintptr_t)gLibSystemHelpers->startGlueToCallExit;
    }
    return result;
}
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下面重点分析第八步执行所有初始化，即调用initializeMainExecutable()函数
image.png 
void ImageLoader::runInitializers(const LinkContext& context, InitializerTimingList& timingInfo)
{
    uint64_t t1 = mach_absolute_time();
    mach_port_t thisThread = mach_thread_self();
    ImageLoader::UninitedUpwards up;
    up.count = 1;
    up.imagesAndPaths[0] = { this, this->getPath() };
    processInitializers(context, thisThread, timingInfo, up);
    context.notifyBatch(dyld_image_state_initialized, false);
    mach_port_deallocate(mach_task_self(), thisThread);
    uint64_t t2 = mach_absolute_time();
    fgTotalInitTime += (t2 - t1);
}

void ImageLoader::processInitializers(const LinkContext& context, mach_port_t thisThread,
                                     InitializerTimingList& timingInfo, ImageLoader::UninitedUpwards& images)
{
    uint32_t maxImageCount = context.imageCount()+2;
    ImageLoader::UninitedUpwards upsBuffer[maxImageCount];
    ImageLoader::UninitedUpwards& ups = upsBuffer[0];
    ups.count = 0;
    // Calling recursive init on all images in images list, building a new list of
    // uninitialized upward dependencies.
    for (uintptr_t i=0; i < images.count; ++i) {
        images.imagesAndPaths[i].first->recursiveInitialization(context, thisThread, images.imagesAndPaths[i].second, timingInfo, ups);
    }
    // If any upward dependencies remain, init them.
    if ( ups.count > 0 )
        processInitializers(context, thisThread, timingInfo, ups);
}

void ImageLoader::recursiveInitialization(const LinkContext& context, mach_port_t this_thread, const char* pathToInitialize,
                                          InitializerTimingList& timingInfo, UninitedUpwards& uninitUps)
{
    recursive_lock lock_info(this_thread);
    recursiveSpinLock(lock_info);

    if ( fState < dyld_image_state_dependents_initialized-1 ) {
        uint8_t oldState = fState;
        // break cycles
        fState = dyld_image_state_dependents_initialized-1;
        try {
            // initialize lower level libraries first
            for(unsigned int i=0; i < libraryCount(); ++i) {
                ImageLoader* dependentImage = libImage(i);
                if ( dependentImage != NULL ) {
                    // don't try to initialize stuff "above" me yet
                    if ( libIsUpward(i) ) {
                        uninitUps.imagesAndPaths[uninitUps.count] = { dependentImage, libPath(i) };
                        uninitUps.count++;
                    }
                    else if ( dependentImage->fDepth >= fDepth ) {
                        dependentImage->recursiveInitialization(context, this_thread, libPath(i), timingInfo, uninitUps);
                    }
                }
            }
            
            // record termination order
            if ( this->needsTermination() )
                context.terminationRecorder(this);

            // let objc know we are about to initialize this image
            uint64_t t1 = mach_absolute_time();
            fState = dyld_image_state_dependents_initialized;
            oldState = fState;
            //核心步骤 
            context.notifySingle(dyld_image_state_dependents_initialized, this, &timingInfo);
            // initialize this image
            bool hasInitializers = this->doInitialization(context);

            // let anyone know we finished initializing this image
            fState = dyld_image_state_initialized;
            oldState = fState;
            context.notifySingle(dyld_image_state_initialized, this, NULL);
            
            if ( hasInitializers ) {
                uint64_t t2 = mach_absolute_time();
                timingInfo.addTime(this->getShortName(), t2-t1);
            }
        }
        catch (const char* msg) {
            // this image is not initialized
            fState = oldState;
            recursiveSpinUnLock();
            throw;
        }
    }
    recursiveSpinUnLock();
}
notifySingle()函数的核心逻辑如下：
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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);
}

void _objc_init(void)
{
    static bool initialized = false;
    if (initialized) return;
    initialized = true;
    
    // fixme defer initialization until an objc-using image is found?
    environ_init();
    tls_init();
    static_init();
    runtime_init();
    exception_init();
#if __OBJC2__
    cache_t::init();
#endif
    _imp_implementationWithBlock_init();

    _dyld_objc_notify_register(&map_images, load_images, unmap_image);

#if __OBJC2__
    didCallDyldNotifyRegister = true;
#endif
}

load_images(const char *path __unused, const struct mach_header *mh)
{
    if (!didInitialAttachCategories && didCallDyldNotifyRegister) {
        didInitialAttachCategories = true;
        loadAllCategories();
    }

    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        mutex_locker_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);
    }

    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}
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总结：类的load方法调用堆栈：

_dyld_start --> dyldbootstrap::start --> dyld::_main --> dyld::initializeMainExecutable --> ImageLoader::runInitializers --> ImageLoader::processInitializers --> ImageLoader::recursiveInitialization --> dyld::notifySingle --> sNotifyObjCInit --> load_images(libobjc.A.dylib)

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bool ImageLoaderMachO::doInitialization(const LinkContext& context)
{
    CRSetCrashLogMessage2(this->getPath());

    // mach-o has -init and static initializers
    doImageInit(context);
    doModInitFunctions(context);
    
    CRSetCrashLogMessage2(NULL);
    
    return (fHasDashInit || fHasInitializers);
}

其内部也包含两个核心步骤：

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总结_objc_init的调用堆栈：

_dyld_start --> dyldbootstrap::start --> dyld::_main --> dyld::initializeMainExecutable --> ImageLoader::runInitializers --> ImageLoader::processInitializers --> ImageLoader::recursiveInitialization --> doInitialization -->libSystem_initializer（libSystem.B.dylib） --> _os_object_init（libdispatch.dylib） --> _objc_init(libobjc.A.dylib)

总结：
Dyld_Objc.png
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