Android逆向之dex2oat的实现解析

目录

• 简介
• dex2oat介绍
• 为什么要使用dex2oat进行转换
• dex2oat代码
• 1.dex2oat类定义
• 2.OpenDexFiles函数定义
• 3.dex2oat入口函数定义
• 总结

简介

在Android系统5.0及以上系统开始逐渐丢弃Dalvik虚拟机，由于ART虚拟机对内存分配和回收都做了算法优化，降低了内存碎片化程度，回收时间也得以缩短，所有android系统5.0及以上都在主推ART虚拟机。在ART虚拟机中ART则会将Dex通过dex2oat工具编译得到一个ELF文件，它是一个可执行的文件。所以下面我们就针对ART的dex2oat实现进行做分析。

dex2oat介绍

Dex2oat的全称是：dalvik excutable file to optimized art file，它是一个对 android系统下的dex文件，进行编译优化的程序。通过dex2oat的编译优化，可以大大的提高android系统的启动的速度和使用手机过程的的流畅度。
dex2oat在安卓手机环境下的存放位置为/system/bin/dex2oat

dex2oat在开源系统中的路径为\art\dex2oat\dex2oat.cc。

为什么要使用dex2oat进行转换

在android系统中，Android 虚拟机可以识别到的是dex文件，App应用在使用过程中如果每次将dex文件加载进行内存，解释性执行字节码，效率就会变得非常低， 从而影响到用户在使用安卓手机的体验。通过利用dex2oat进行优化处理， 那么可以在android系统运行之前，利用合适的时机将dex文件字节码，提前转化为虚拟机上可以执行运行的机器码，后续直接从效率更高的机器码中运行，则运行阶段更加流畅，优化用户体验。

dex2oat代码

1.dex2oat类定义

class Dex2Oat {
 public:

 //创建函数，返回值为bool，
  static bool Create(Dex2Oat** p_dex2oat,
                     const RuntimeOptions& runtime_options,
                     const CompilerOptions& compiler_options,
                     Compiler::Kind compiler_kind,
                     InstructionSet instruction_set,
                     InstructionSetFeatures instruction_set_features,
                     VerificationResults* verification_results,
                     DexFileToMethodInlinerMap* method_inliner_map,
                     size_t thread_count)
      SHARED_TRYLOCK_FUNCTION(true, Locks::mutator_lock_) {
      //判断参数传递进来的释放为空
    CHECK(verification_results != nullptr);
    CHECK(method_inliner_map != nullptr);
    //用智能指针方式进行去实例化dex2oat
    std::unique_ptr<Dex2Oat> dex2oat(new Dex2Oat(&compiler_options,
                                           compiler_kind,
                                           instruction_set,
                                           instruction_set_features,
                                           verification_results,
                                           method_inliner_map,
                                           thread_count));
    if (!dex2oat->CreateRuntime(runtime_options, instruction_set)) {
      *p_dex2oat = nullptr;
      return false;
    }
    *p_dex2oat = dex2oat.release();
    return true;
  }
    //dex2oat的虚构函数，用于释放操作。
  ~Dex2Oat() {
    delete runtime_;
    LogCompletionTime();
  }

  void LogCompletionTime() {
    LOG(INFO) << "dex2oat took " << PrettyDuration(NanoTime() - start_ns_)
              << " (threads: " << thread_count_ << ")";
  }

  //从文件上获取到类名称
  std::set<std::string>* ReadImageClassesFromFile(const char* image_classes_filename) {
    std::unique_ptr<std::ifstream> image_classes_file(new std::ifstream(image_classes_filename,
                                                                  std::ifstream::in));
    if (image_classes_file.get() == nullptr) {
      LOG(ERROR) << "Failed to open image classes file " << image_classes_filename;
      return nullptr;
    }
    std::unique_ptr<std::set<std::string>> result(ReadImageClasses(*image_classes_file));
    image_classes_file->close();
    return result.release();
  }

 //读取imageclasses
  std::set<std::string>* ReadImageClasses(std::istream& image_classes_stream) {
    std::unique_ptr<std::set<std::string>> image_classes(new std::set<std::string>);
    while (image_classes_stream.good()) {
      std::string dot;
      std::getline(image_classes_stream, dot);
      if (StartsWith(dot, "#") || dot.empty()) {
        continue;
      }
      std::string descriptor(DotToDescriptor(dot.c_str()));
      image_classes->insert(descriptor);
    }
    return image_classes.release();
  }

  // Reads the class names (java.lang.Object) and returns a set of descriptors (Ljava/lang/Object;)
  //从zip文件（apk其实就是个zip文件)读取类名称，读取到返回一个描述
  std::set<std::string>* ReadImageClassesFromZip(const char* zip_filename,
                                                         const char* image_classes_filename,
                                                         std::string* error_msg) {
         //通过智能指针进行打开zip压缩包，也就是apk包
    std::unique_ptr<ZipArchive> zip_archive(ZipArchive::Open(zip_filename, error_msg));
    //判断打开是否失败
    if (zip_archive.get() == nullptr) {
      return nullptr;
    }
    //进行遍历zip包获取zip包里面的文件信息
    std::unique_ptr<ZipEntry> zip_entry(zip_archive->Find(image_classes_filename, error_msg));
    if (zip_entry.get() == nullptr) {
      *error_msg = StringPrintf("Failed to find '%s' within '%s': %s", image_classes_filename,
                                zip_filename, error_msg->c_str());
      return nullptr;
    }
    std::unique_ptr<MemMap> image_classes_file(zip_entry->ExtractToMemMap(zip_filename,
                                                                          image_classes_filename,
                                                                          error_msg));
    if (image_classes_file.get() == nullptr) {
      *error_msg = StringPrintf("Failed to extract '%s' from '%s': %s", image_classes_filename,
                                zip_filename, error_msg->c_str());
      return nullptr;
    }
    const std::string image_classes_string(reinterpret_cast<char*>(image_classes_file->Begin()),
                                           image_classes_file->Size());
    std::istringstream image_classes_stream(image_classes_string);
    return ReadImageClasses(image_classes_stream);
  }

  bool PatchOatCode(const CompilerDriver* compiler_driver, File* oat_file,
                    const std::string& oat_location, std::string* error_msg) {
    // We asked to include patch information but we are not making an image. We need to fix
    // everything up manually.
    std::unique_ptr<ElfFile> elf_file(ElfFile::Open(oat_file, PROT_READ|PROT_WRITE,
                                                    MAP_SHARED, error_msg));
    if (elf_file.get() == NULL) {
      LOG(ERROR) << error_msg;
      return false;
    }
    {
      ReaderMutexLock mu(Thread::Current(), *Locks::mutator_lock_);
      return ElfPatcher::Patch(compiler_driver, elf_file.get(), oat_location, error_msg);
    }
  }
    //创建一个oat文件，返回一个常量指针
  const CompilerDriver* CreateOatFile(const std::string& boot_image_option,
                                      const std::string& android_root,
                                      bool is_host,
                                      const std::vector<const DexFile*>& dex_files,
                                      File* oat_file,
                                      const std::string& oat_location,
                                      const std::string& bitcode_filename,
                                      bool image,
                                      std::unique_ptr<std::set<std::string>>& image_classes,
                                      bool dump_stats,
                                      bool dump_passes,
                                      TimingLogger& timings,
                                      CumulativeLogger& compiler_phases_timings,
                                      std::string profile_file,
                                      SafeMap<std::string, std::string>* key_value_store) {
    CHECK(key_value_store != nullptr);

    // Handle and ClassLoader creation needs to come after Runtime::Create
    jobject class_loader = nullptr;
    //获取自身进程
    Thread* self = Thread::Current();
    //如果boot_image_option不为空的话，执行下面的代码
    if (!boot_image_option.empty()) {

      ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
      std::vector<const DexFile*> class_path_files(dex_files);
      OpenClassPathFiles(runtime_->GetClassPathString(), class_path_files);
      ScopedObjectAccess soa(self);
      //循环遍历并类文件大小，并进行dex文件进行注册
      for (size_t i = 0; i < class_path_files.size(); i++) {
        class_linker->RegisterDexFile(*class_path_files[i]);
      }
      soa.Env()->AllocObject(WellKnownClasses::dalvik_system_PathClassLoader);
      ScopedLocalRef<jobject> class_loader_local(soa.Env(),
          soa.Env()->AllocObject(WellKnownClasses::dalvik_system_PathClassLoader));
      class_loader = soa.Env()->NewGlobalRef(class_loader_local.get());

      Runtime::Current()->SetCompileTimeClassPath(class_loader, class_path_files);
    }

    std::unique_ptr<CompilerDriver> driver(new CompilerDriver(compiler_options_,
                                                              verification_results_,
                                                              method_inliner_map_,
                                                              compiler_kind_,
                                                              instruction_set_,
                                                              instruction_set_features_,
                                                              image,
                                                              image_classes.release(),
                                                              thread_count_,
                                                              dump_stats,
                                                              dump_passes,
                                                              &compiler_phases_timings,
                                                              profile_file));

    driver->GetCompiler()->SetBitcodeFileName(*driver.get(), bitcode_filename);

    driver->CompileAll(class_loader, dex_files, &timings);

    TimingLogger::ScopedTiming t2("dex2oat OatWriter", &timings);
    std::string image_file_location;
    uint32_t image_file_location_oat_checksum = 0;
    uintptr_t image_file_location_oat_data_begin = 0;
    int32_t image_patch_delta = 0;
    if (!driver->IsImage()) {
      TimingLogger::ScopedTiming t3("Loading image checksum", &timings);
      gc::space::ImageSpace* image_space = Runtime::Current()->GetHeap()->GetImageSpace();
      image_file_location_oat_checksum = image_space->GetImageHeader().GetOatChecksum();
      image_file_location_oat_data_begin =
          reinterpret_cast<uintptr_t>(image_space->GetImageHeader().GetOatDataBegin());
      image_file_location = image_space->GetImageFilename();
      image_patch_delta = image_space->GetImageHeader().GetPatchDelta();
    }

    if (!image_file_location.empty()) {
      key_value_store->Put(OatHeader::kImageLocationKey, image_file_location);
    }

    //oat写入操作
    OatWriter oat_writer(dex_files, image_file_location_oat_checksum,
                         image_file_location_oat_data_begin,
                         image_patch_delta,
                         driver.get(),
                         &timings,
                         key_value_store);

    t2.NewTiming("Writing ELF");
    if (!driver->WriteElf(android_root, is_host, dex_files, &oat_writer, oat_file)) {
      LOG(ERROR) << "Failed to write ELF file " << oat_file->GetPath();
      return nullptr;
    }

    // Flush result to disk. Patching code will re-open the file (mmap), so ensure that our view
    // of the file already made it there and won't be re-ordered with writes from PatchOat or
    // image patching.
    oat_file->Flush();

    if (!driver->IsImage() && driver->GetCompilerOptions().GetIncludePatchInformation()) {
      t2.NewTiming("Patching ELF");
      std::string error_msg;
      if (!PatchOatCode(driver.get(), oat_file, oat_location, &error_msg)) {
        LOG(ERROR) << "Failed to fixup ELF file " << oat_file->GetPath() << ": " << error_msg;
        return nullptr;
      }
    }

    return driver.release();
  }
   //创建一个映射文件，成功返回true，失败返回false
  bool CreateImageFile(const std::string& image_filename,
                       uintptr_t image_base,
                       const std::string& oat_filename,
                       const std::string& oat_location,
                       const CompilerDriver& compiler)
      LOCKS_EXCLUDED(Locks::mutator_lock_) {
    uintptr_t oat_data_begin;
    {
      // ImageWriter is scoped so it can free memory before doing FixupElf
      ImageWriter image_writer(compiler);
      if (!image_writer.Write(image_filename, image_base, oat_filename, oat_location)) {
        LOG(ERROR) << "Failed to create image file " << image_filename;
        return false;
      }
      oat_data_begin = image_writer.GetOatDataBegin();
    }

    std::unique_ptr<File> oat_file(OS::OpenFileReadWrite(oat_filename.c_str()));
    if (oat_file.get() == nullptr) {
      PLOG(ERROR) << "Failed to open ELF file: " << oat_filename;
      return false;
    }
    if (!ElfFixup::Fixup(oat_file.get(), oat_data_begin)) {
      LOG(ERROR) << "Failed to fixup ELF file " << oat_file->GetPath();
      return false;
    }
    return true;
  }

 private:
 //定义一个显示的dex2oat构造函数
  explicit Dex2Oat(const CompilerOptions* compiler_options,
                   Compiler::Kind compiler_kind,
                   InstructionSet instruction_set,
                   InstructionSetFeatures instruction_set_features,
                   VerificationResults* verification_results,
                   DexFileToMethodInlinerMap* method_inliner_map,
                   size_t thread_count)
      : compiler_options_(compiler_options),
        compiler_kind_(compiler_kind),
        instruction_set_(instruction_set),
        instruction_set_features_(instruction_set_features),
        verification_results_(verification_results),
        method_inliner_map_(method_inliner_map),
        runtime_(nullptr),
        thread_count_(thread_count),
        start_ns_(NanoTime()) {
    CHECK(compiler_options != nullptr);
    CHECK(verification_results != nullptr);
    CHECK(method_inliner_map != nullptr);
  }

  bool CreateRuntime(const RuntimeOptions& runtime_options, InstructionSet instruction_set)
      SHARED_TRYLOCK_FUNCTION(true, Locks::mutator_lock_) {
    if (!Runtime::Create(runtime_options, false)) {
      LOG(ERROR) << "Failed to create runtime";
      return false;
    }
    Runtime* runtime = Runtime::Current();
    runtime->SetInstructionSet(instruction_set);
    for (int i = 0; i < Runtime::kLastCalleeSaveType; i++) {
      Runtime::CalleeSaveType type = Runtime::CalleeSaveType(i);
      if (!runtime->HasCalleeSaveMethod(type)) {
        runtime->SetCalleeSaveMethod(runtime->CreateCalleeSaveMethod(type), type);
      }
    }
    runtime->GetClassLinker()->FixupDexCaches(runtime->GetResolutionMethod());
    runtime->GetClassLinker()->RunRootClinits();
    runtime_ = runtime;
    return true;
  }

  // Appends to dex_files any elements of class_path that it doesn't already
  // contain. This will open those dex files as necessary.
  static void OpenClassPathFiles(const std::string& class_path,
                                 std::vector<const DexFile*>& dex_files) {
   //通过定义l的vector向量的字符串
   std::vector<std::string> parsed;
    Split(class_path, ':', parsed);
    // Take Locks::mutator_lock_ so that lock ordering on the ClassLinker::dex_lock_ is maintained.
    ScopedObjectAccess soa(Thread::Current());
    for (size_t i = 0; i < parsed.size(); ++i) {
        //判断是否包含dex文件
      if (DexFilesContains(dex_files, parsed[i])) {
        continue;
      }
      std::string error_msg;
      //判断是否可以打得开dex文件
      if (!DexFile::Open(parsed[i].c_str(), parsed[i].c_str(), &error_msg, &dex_files)) {
        LOG(WARNING) << "Failed to open dex file '" << parsed[i] << "': " << error_msg;
      }
    }
  }

  //如果dex文件有指定位置的话，那么就返回为true
  static bool DexFilesContains(const std::vector<const DexFile*>& dex_files,
                               const std::string& location) {
    //循环变量dex文件的大小，并进行判断location是否相等。
   for (size_t i = 0; i < dex_files.size(); ++i) {
      if (dex_files[i]->GetLocation() == location) {
        return true;
      }
    }
    return false;
  }

   //定义了个四个常量
  const CompilerOptions* const compiler_options_;
  const Compiler::Kind compiler_kind_;

  const InstructionSet instruction_set_;
  const InstructionSetFeatures instruction_set_features_;

  VerificationResults* const verification_results_;
  DexFileToMethodInlinerMap* const method_inliner_map_;
  Runtime* runtime_;
  size_t thread_count_;
  uint64_t start_ns_;

  DISALLOW_IMPLICIT_CONSTRUCTORS(Dex2Oat);
};

2.OpenDexFiles函数定义

//OpenDexFiles打开dex文件，成功返回dex文件的大小
static size_t OpenDexFiles(const std::vector<const char*>& dex_filenames,
                           const std::vector<const char*>& dex_locations,
                           std::vector<const DexFile*>& dex_files) {
  size_t failure_count = 0;
  //循环遍历dex文件的大小。
  for (size_t i = 0; i < dex_filenames.size(); i++) {
    const char* dex_filename = dex_filenames[i];
    const char* dex_location = dex_locations[i];
    ATRACE_BEGIN(StringPrintf("Opening dex file '%s'", dex_filenames[i]).c_str());
    std::string error_msg;
    //判断文件是否存在，
    if (!OS::FileExists(dex_filename)) {
      LOG(WARNING) << "Skipping non-existent dex file '" << dex_filename << "'";
      continue;
    }
    //真正的打开操作还是调用底层的open函数实现的。
    if (!DexFile::Open(dex_filename, dex_location, &error_msg, &dex_files)) {
      LOG(WARNING) << "Failed to open .dex from file '" << dex_filename << "': " << error_msg;
      ++failure_count;
    }
    ATRACE_END();
  }
  return failure_count;
}

3.dex2oat入口函数定义

下面dex2oat函数的整个流程

做一个arm上的workaround。
构造Dex2oat对象
处理命令行参数
判断对于文件是否有写的权限
打印命令行参数
判断dex2oat的setup是否完成
根据是否image分别调用CompileImage或CompileApp的处理

//dex2oat两次参数通过控制窗口方式进行输入确
static int dex2oat(int argc, char** argv) {
#if defined(__linux__) && defined(__arm__)
  //定义变量
  int major, minor;
  //定义获取主机信息结构体
  struct utsname uts;
  //调用uname判断是否可以显示系统信息
  if (uname(&uts) != -1 &&
      sscanf(uts.release, "%d.%d", &major, &minor) == 2 &&
      ((major < 3) || ((major == 3) && (minor < 4)))) {
    // Kernels before 3.4 don't handle the ASLR well and we can run out of address
    // space (http://b/13564922). Work around the issue by inhibiting further mmap() randomization.
    int old_personality = personality(0xffffffff);
    if ((old_personality & ADDR_NO_RANDOMIZE) == 0) {
      int new_personality = personality(old_personality | ADDR_NO_RANDOMIZE);
      if (new_personality == -1) {
        LOG(WARNING) << "personality(. | ADDR_NO_RANDOMIZE) failed.";
      }
    }
  }
#endif
    //参数传递赋值到全局变量
  original_argc = argc;
  original_argv = argv;
   //打印程序执行时间
  TimingLogger timings("compiler", false, false);
  CumulativeLogger compiler_phases_timings("compilation times");

  InitLogging(argv);

  // Skip over argv[0].
  argv++;
  argc--;

  if (argc == 0) {
    Usage("No arguments specified");
  }
  //到这里为止前面都是进行初始化及环境操作，真正的dex2oat功能在后面代码实现。

//定义一系列的向量，字符串，常量为后面代码使用
  std::vector<const char*> dex_filenames;
  std::vector<const char*> dex_locations;
  int zip_fd = -1;
  std::string zip_location;
  std::string oat_filename;
  std::string oat_symbols;
  std::string oat_location;
  int oat_fd = -1;
  std::string bitcode_filename;
  const char* image_classes_zip_filename = nullptr;
  const char* image_classes_filename = nullptr;
  std::string image_filename;
  std::string boot_image_filename;
  uintptr_t image_base = 0;
  std::string android_root;
  std::vector<const char*> runtime_args;
  int thread_count = sysconf(_SC_NPROCESSORS_CONF);
  Compiler::Kind compiler_kind = kUsePortableCompiler
      ? Compiler::kPortable
      : Compiler::kQuick;
  const char* compiler_filter_string = nullptr;
  int huge_method_threshold = CompilerOptions::kDefaultHugeMethodThreshold;
  int large_method_threshold = CompilerOptions::kDefaultLargeMethodThreshold;
  int small_method_threshold = CompilerOptions::kDefaultSmallMethodThreshold;
  int tiny_method_threshold = CompilerOptions::kDefaultTinyMethodThreshold;
  int num_dex_methods_threshold = CompilerOptions::kDefaultNumDexMethodsThreshold;

  //从构建中获取默认的指令功能集。
  InstructionSetFeatures instruction_set_features =
      ParseFeatureList(Runtime::GetDefaultInstructionSetFeatures());

  InstructionSet instruction_set = kRuntimeISA;

  // 配置文件的定义使用
  std::string profile_file;
  double top_k_profile_threshold = CompilerOptions::kDefaultTopKProfileThreshold;

  bool is_host = false;
  bool dump_stats = false;
  bool dump_timing = false;
  bool dump_passes = false;
  bool include_patch_information = CompilerOptions::kDefaultIncludePatchInformation;
  bool include_debug_symbols = kIsDebugBuild;
  bool dump_slow_timing = kIsDebugBuild;
  bool watch_dog_enabled = true;
  bool generate_gdb_information = kIsDebugBuild;

  // Checks are all explicit until we know the architecture.
  bool implicit_null_checks = false;
  bool implicit_so_checks = false;
  bool implicit_suspend_checks = false;

 //下面主要代码通过一系列进行执行打印命令行操作。
//统计用户输入的参数总和
  for (int i = 0; i < argc; i++) {
    const StringPiece option(argv[i]);
    const bool log_options = false;
    if (log_options) {
      LOG(INFO) << "dex2oat: option[" << i << "]=" << argv[i];
    }
    //判断字符串是否包含
    if (option.starts_with("--dex-file=")) {
       //将dex文件名称数据传入vector里面
      dex_filenames.push_back(option.substr(strlen("--dex-file=")).data());
    } else if (option.starts_with("--dex-location=")) {
      dex_locations.push_back(option.substr(strlen("--dex-location=")).data());

    }
    //判断是否是zip文件，并对zip文件操作，并对字符串信息进行截取
    else if (option.starts_with("--zip-fd=")) {
      const char* zip_fd_str = option.substr(strlen("--zip-fd=")).data();
      if (!ParseInt(zip_fd_str, &zip_fd)) {
        Usage("Failed to parse --zip-fd argument '%s' as an integer", zip_fd_str);
      }
      if (zip_fd < 0) {
        Usage("--zip-fd passed a negative value %d", zip_fd);
      }
    } else if (option.starts_with("--zip-location=")) {
      zip_location = option.substr(strlen("--zip-location=")).data();
    } else if (option.starts_with("--oat-file=")) {
      oat_filename = option.substr(strlen("--oat-file=")).data();
    } else if (option.starts_with("--oat-symbols=")) {
      oat_symbols = option.substr(strlen("--oat-symbols=")).data();
    } else if (option.starts_with("--oat-fd=")) {
      const char* oat_fd_str = option.substr(strlen("--oat-fd=")).data();
      if (!ParseInt(oat_fd_str, &oat_fd)) {
        Usage("Failed to parse --oat-fd argument '%s' as an integer", oat_fd_str);
      }
      if (oat_fd < 0) {
        Usage("--oat-fd passed a negative value %d", oat_fd);
      }
    } else if (option == "--watch-dog") {
      watch_dog_enabled = true;
    } else if (option == "--no-watch-dog") {
      watch_dog_enabled = false;
    } else if (option == "--gen-gdb-info") {
      generate_gdb_information = true;
      // Debug symbols are needed for gdb information.
      include_debug_symbols = true;
    } else if (option == "--no-gen-gdb-info") {
      generate_gdb_information = false;
    } else if (option.starts_with("-j")) {
      const char* thread_count_str = option.substr(strlen("-j")).data();
      if (!ParseInt(thread_count_str, &thread_count)) {
        Usage("Failed to parse -j argument '%s' as an integer", thread_count_str);
      }
    } else if (option.starts_with("--oat-location=")) {
      oat_location = option.substr(strlen("--oat-location=")).data();
    } else if (option.starts_with("--bitcode=")) {
      bitcode_filename = option.substr(strlen("--bitcode=")).data();
    } else if (option.starts_with("--image=")) {
      image_filename = option.substr(strlen("--image=")).data();
    } else if (option.starts_with("--image-classes=")) {
      image_classes_filename = option.substr(strlen("--image-classes=")).data();
    } else if (option.starts_with("--image-classes-zip=")) {
      image_classes_zip_filename = option.substr(strlen("--image-classes-zip=")).data();
    } else if (option.starts_with("--base=")) {
      const char* image_base_str = option.substr(strlen("--base=")).data();
      char* end;
      image_base = strtoul(image_base_str, &end, 16);
      if (end == image_base_str || *end != '\0') {
        Usage("Failed to parse hexadecimal value for option %s", option.data());
      }
    } else if (option.starts_with("--boot-image=")) {
      boot_image_filename = option.substr(strlen("--boot-image=")).data();
    } else if (option.starts_with("--android-root=")) {
      android_root = option.substr(strlen("--android-root=")).data();
    }
    else if (option.starts_with("--instruction-set=")) {
      StringPiece instruction_set_str = option.substr(strlen("--instruction-set=")).data();
      if (instruction_set_str == "arm") {
        instruction_set = kThumb2;
      } else if (instruction_set_str == "arm64") {
        instruction_set = kArm64;
      } else if (instruction_set_str == "mips") {
        instruction_set = kMips;
      } else if (instruction_set_str == "x86") {
        instruction_set = kX86;
      } else if (instruction_set_str == "x86_64") {
        instruction_set = kX86_64;
      }
    } else if (option.starts_with("--instruction-set-features=")) {
      StringPiece str = option.substr(strlen("--instruction-set-features=")).data();
      instruction_set_features = ParseFeatureList(str.as_string());
    } else if (option.starts_with("--compiler-backend=")) {
      StringPiece backend_str = option.substr(strlen("--compiler-backend=")).data();
      if (backend_str == "Quick") {
        compiler_kind = Compiler::kQuick;
      } else if (backend_str == "Optimizing") {
        compiler_kind = Compiler::kOptimizing;
      } else if (backend_str == "Portable") {
        compiler_kind = Compiler::kPortable;
      }
    } else if (option.starts_with("--compiler-filter=")) {
      compiler_filter_string = option.substr(strlen("--compiler-filter=")).data();
    } else if (option.starts_with("--huge-method-max=")) {
      const char* threshold = option.substr(strlen("--huge-method-max=")).data();
      if (!ParseInt(threshold, &huge_method_threshold)) {
        Usage("Failed to parse --huge-method-max '%s' as an integer", threshold);
      }
      if (huge_method_threshold < 0) {
        Usage("--huge-method-max passed a negative value %s", huge_method_threshold);
      }
    } else if (option.starts_with("--large-method-max=")) {
      const char* threshold = option.substr(strlen("--large-method-max=")).data();
      if (!ParseInt(threshold, &large_method_threshold)) {
        Usage("Failed to parse --large-method-max '%s' as an integer", threshold);
      }
      if (large_method_threshold < 0) {
        Usage("--large-method-max passed a negative value %s", large_method_threshold);
      }
    } else if (option.starts_with("--small-method-max=")) {
      const char* threshold = option.substr(strlen("--small-method-max=")).data();
      if (!ParseInt(threshold, &small_method_threshold)) {
        Usage("Failed to parse --small-method-max '%s' as an integer", threshold);
      }
      if (small_method_threshold < 0) {
        Usage("--small-method-max passed a negative value %s", small_method_threshold);
      }
    } else if (option.starts_with("--tiny-method-max=")) {
      const char* threshold = option.substr(strlen("--tiny-method-max=")).data();
      if (!ParseInt(threshold, &tiny_method_threshold)) {
        Usage("Failed to parse --tiny-method-max '%s' as an integer", threshold);
      }
      if (tiny_method_threshold < 0) {
        Usage("--tiny-method-max passed a negative value %s", tiny_method_threshold);
      }
    } else if (option.starts_with("--num-dex-methods=")) {
      const char* threshold = option.substr(strlen("--num-dex-methods=")).data();
      if (!ParseInt(threshold, &num_dex_methods_threshold)) {
        Usage("Failed to parse --num-dex-methods '%s' as an integer", threshold);
      }
      if (num_dex_methods_threshold < 0) {
        Usage("--num-dex-methods passed a negative value %s", num_dex_methods_threshold);
      }
    } else if (option == "--host") {
      is_host = true;
    } else if (option == "--runtime-arg") {
      if (++i >= argc) {
        Usage("Missing required argument for --runtime-arg");
      }
      if (log_options) {
        LOG(INFO) << "dex2oat: option[" << i << "]=" << argv[i];
      }
      runtime_args.push_back(argv[i]);
    } else if (option == "--dump-timing") {
      dump_timing = true;
    } else if (option == "--dump-passes") {
      dump_passes = true;
    } else if (option == "--dump-stats") {
      dump_stats = true;
    } else if (option == "--include-debug-symbols" || option == "--no-strip-symbols") {
      include_debug_symbols = true;
    } else if (option == "--no-include-debug-symbols" || option == "--strip-symbols") {
      include_debug_symbols = false;
      generate_gdb_information = false;  // Depends on debug symbols, see above.
    } else if (option.starts_with("--profile-file=")) {
      profile_file = option.substr(strlen("--profile-file=")).data();
      VLOG(compiler) << "dex2oat: profile file is " << profile_file;
    } else if (option == "--no-profile-file") {
      // No profile
    } else if (option.starts_with("--top-k-profile-threshold=")) {
      ParseDouble(option.data(), '=', 0.0, 100.0, &top_k_profile_threshold);
    } else if (option == "--print-pass-names") {
      PassDriverMEOpts::PrintPassNames();
    } else if (option.starts_with("--disable-passes=")) {
      std::string disable_passes = option.substr(strlen("--disable-passes=")).data();
      PassDriverMEOpts::CreateDefaultPassList(disable_passes);
    } else if (option.starts_with("--print-passes=")) {
      std::string print_passes = option.substr(strlen("--print-passes=")).data();
      PassDriverMEOpts::SetPrintPassList(print_passes);
    } else if (option == "--print-all-passes") {
      PassDriverMEOpts::SetPrintAllPasses();
    } else if (option.starts_with("--dump-cfg-passes=")) {
      std::string dump_passes = option.substr(strlen("--dump-cfg-passes=")).data();
      PassDriverMEOpts::SetDumpPassList(dump_passes);
    } else if (option == "--include-patch-information") {
      include_patch_information = true;
    } else if (option == "--no-include-patch-information") {
      include_patch_information = false;
    } else {
      Usage("Unknown argument %s", option.data());
    }
  }
   //判断oat文件是否存在
  if (oat_filename.empty() && oat_fd == -1) {
    Usage("Output must be supplied with either --oat-file or --oat-fd");
  }

  if (!oat_filename.empty() && oat_fd != -1) {
    Usage("--oat-file should not be used with --oat-fd");
  }
   //判断oat符号表是否为空
  if (!oat_symbols.empty() && oat_fd != -1) {
    Usage("--oat-symbols should not be used with --oat-fd");
  }

  if (!oat_symbols.empty() && is_host) {
    Usage("--oat-symbols should not be used with --host");
  }

  if (oat_fd != -1 && !image_filename.empty()) {
    Usage("--oat-fd should not be used with --image");
  }
   //判断android_root是否为空
  if (android_root.empty()) {
    const char* android_root_env_var = getenv("ANDROID_ROOT");
    if (android_root_env_var == nullptr) {
      Usage("--android-root unspecified and ANDROID_ROOT not set");
    }
    android_root += android_root_env_var;
  }

  bool image = (!image_filename.empty());
  if (!image && boot_image_filename.empty()) {
    boot_image_filename += android_root;
    boot_image_filename += "/framework/boot.art";
  }
  std::string boot_image_option;
  if (!boot_image_filename.empty()) {
    boot_image_option += "-Ximage:";
    boot_image_option += boot_image_filename;
  }

  if (image_classes_filename != nullptr && !image) {
    Usage("--image-classes should only be used with --image");
  }

  if (image_classes_filename != nullptr && !boot_image_option.empty()) {
    Usage("--image-classes should not be used with --boot-image");
  }

  if (image_classes_zip_filename != nullptr && image_classes_filename == nullptr) {
    Usage("--image-classes-zip should be used with --image-classes");
  }

  if (dex_filenames.empty() && zip_fd == -1) {
    Usage("Input must be supplied with either --dex-file or --zip-fd");
  }

  if (!dex_filenames.empty() && zip_fd != -1) {
    Usage("--dex-file should not be used with --zip-fd");
  }

  if (!dex_filenames.empty() && !zip_location.empty()) {
    Usage("--dex-file should not be used with --zip-location");
  }

  if (dex_locations.empty()) {
    for (size_t i = 0; i < dex_filenames.size(); i++) {
      dex_locations.push_back(dex_filenames[i]);
    }
  } else if (dex_locations.size() != dex_filenames.size()) {
    Usage("--dex-location arguments do not match --dex-file arguments");
  }

  if (zip_fd != -1 && zip_location.empty()) {
    Usage("--zip-location should be supplied with --zip-fd");
  }

  if (boot_image_option.empty()) {
    if (image_base == 0) {
      Usage("Non-zero --base not specified");
    }
  }

  std::string oat_stripped(oat_filename);
  std::string oat_unstripped;
  if (!oat_symbols.empty()) {
    oat_unstripped += oat_symbols;
  } else {
    oat_unstripped += oat_filename;
  }

  if (compiler_filter_string == nullptr) {
    if (instruction_set == kMips64) {
      // TODO: fix compiler for Mips64.
      compiler_filter_string = "interpret-only";
    } else if (image) {
      compiler_filter_string = "speed";
    } else {
#if ART_SMALL_MODE
      compiler_filter_string = "interpret-only";
#else
      compiler_filter_string = "speed";
#endif
    }
  }
  CHECK(compiler_filter_string != nullptr);
  CompilerOptions::CompilerFilter compiler_filter = CompilerOptions::kDefaultCompilerFilter;
  if (strcmp(compiler_filter_string, "verify-none") == 0) {
    compiler_filter = CompilerOptions::kVerifyNone;
  } else if (strcmp(compiler_filter_string, "interpret-only") == 0) {
    compiler_filter = CompilerOptions::kInterpretOnly;
  } else if (strcmp(compiler_filter_string, "space") == 0) {
    compiler_filter = CompilerOptions::kSpace;
  } else if (strcmp(compiler_filter_string, "balanced") == 0) {
    compiler_filter = CompilerOptions::kBalanced;
  } else if (strcmp(compiler_filter_string, "speed") == 0) {
    compiler_filter = CompilerOptions::kSpeed;
  } else if (strcmp(compiler_filter_string, "everything") == 0) {
    compiler_filter = CompilerOptions::kEverything;
  } else {
    Usage("Unknown --compiler-filter value %s", compiler_filter_string);
  }

  // Set the compilation target's implicit checks options.
  switch (instruction_set) {
    case kArm:
    case kThumb2:
    case kArm64:
    case kX86:
    case kX86_64:
      implicit_null_checks = true;
      implicit_so_checks = true;
      break;

    default:
      // Defaults are correct.
      break;
  }

  std::unique_ptr<CompilerOptions> compiler_options(new CompilerOptions(compiler_filter,
                                                                        huge_method_threshold,
                                                                        large_method_threshold,
                                                                        small_method_threshold,
                                                                        tiny_method_threshold,
                                                                        num_dex_methods_threshold,
                                                                        generate_gdb_information,
                                                                        include_patch_information,
                                                                        top_k_profile_threshold,
                                                                        include_debug_symbols,
                                                                        implicit_null_checks,
                                                                        implicit_so_checks,
                                                                        implicit_suspend_checks
#ifdef ART_SEA_IR_MODE
                                                                        , compiler_options.sea_ir_ =
                                                                              true;
#endif
  ));  // NOLINT(whitespace/parens)

  // Done with usage checks, enable watchdog if requested
  WatchDog watch_dog(watch_dog_enabled);

  // Check early that the result of compilation can be written
  std::unique_ptr<File> oat_file;
  bool create_file = !oat_unstripped.empty();  // as opposed to using open file descriptor
  if (create_file) {
    oat_file.reset(OS::CreateEmptyFile(oat_unstripped.c_str()));
    if (oat_location.empty()) {
      oat_location = oat_filename;
    }
  } else {
    oat_file.reset(new File(oat_fd, oat_location));
    oat_file->DisableAutoClose();
    oat_file->SetLength(0);
  }
  if (oat_file.get() == nullptr) {
    PLOG(ERROR) << "Failed to create oat file: " << oat_location;
    return EXIT_FAILURE;
  }
  if (create_file && fchmod(oat_file->Fd(), 0644) != 0) {
    PLOG(ERROR) << "Failed to make oat file world readable: " << oat_location;
    return EXIT_FAILURE;
  }
//开始真正的执行dex2oat工作了
  timings.StartTiming("dex2oat Setup");
  LOG(INFO) << CommandLine();

  RuntimeOptions runtime_options;
  std::vector<const DexFile*> boot_class_path;
  art::MemMap::Init();  // For ZipEntry::ExtractToMemMap.
  if (boot_image_option.empty()) {
  //打开zip文件中的dex文件
    size_t failure_count = OpenDexFiles(dex_filenames, dex_locations, boot_class_path);
    if (failure_count > 0) {
      LOG(ERROR) << "Failed to open some dex files: " << failure_count;
      return EXIT_FAILURE;
    }
    runtime_options.push_back(std::make_pair("bootclasspath", &boot_class_path));
  } else {
    runtime_options.push_back(std::make_pair(boot_image_option.c_str(), nullptr));
  }
  for (size_t i = 0; i < runtime_args.size(); i++) {
    runtime_options.push_back(std::make_pair(runtime_args[i], nullptr));
  }

  std::unique_ptr<VerificationResults> verification_results(new VerificationResults(
                                                            compiler_options.get()));
  DexFileToMethodInlinerMap method_inliner_map;
  QuickCompilerCallbacks callbacks(verification_results.get(), &method_inliner_map);
  runtime_options.push_back(std::make_pair("compilercallbacks", &callbacks));
  runtime_options.push_back(
      std::make_pair("imageinstructionset",
                     reinterpret_cast<const void*>(GetInstructionSetString(instruction_set))));

  Dex2Oat* p_dex2oat;
  //创建一个dex2oat
  if (!Dex2Oat::Create(&p_dex2oat,
                       runtime_options,
                       *compiler_options,
                       compiler_kind,
                       instruction_set,
                       instruction_set_features,
                       verification_results.get(),
                       &method_inliner_map,
                       thread_count)) {
    LOG(ERROR) << "Failed to create dex2oat";
    return EXIT_FAILURE;
  }
  std::unique_ptr<Dex2Oat> dex2oat(p_dex2oat);

  Thread* self = Thread::Current();
  self->TransitionFromRunnableToSuspended(kNative);

  WellKnownClasses::Init(self->GetJniEnv());

  // If --image-classes was specified, calculate the full list of classes to include in the image
  std::unique_ptr<std::set<std::string>> image_classes(nullptr);
  if (image_classes_filename != nullptr) {
    std::string error_msg;
    if (image_classes_zip_filename != nullptr) {
      image_classes.reset(dex2oat->ReadImageClassesFromZip(image_classes_zip_filename,
                                                           image_classes_filename,
                                                           &error_msg));
    } else {
      image_classes.reset(dex2oat->ReadImageClassesFromFile(image_classes_filename));
    }
    if (image_classes.get() == nullptr) {
      LOG(ERROR) << "Failed to create list of image classes from '" << image_classes_filename <<
          "': " << error_msg;
      return EXIT_FAILURE;
    }
  } else if (image) {
    image_classes.reset(new std::set<std::string>);
  }

  std::vector<const DexFile*> dex_files;
  if (boot_image_option.empty()) {
    dex_files = Runtime::Current()->GetClassLinker()->GetBootClassPath();
  } else {
    if (dex_filenames.empty()) {
      ATRACE_BEGIN("Opening zip archive from file descriptor");
      std::string error_msg;
      std::unique_ptr<ZipArchive> zip_archive(ZipArchive::OpenFromFd(zip_fd, zip_location.c_str(),
                                                               &error_msg));
      if (zip_archive.get() == nullptr) {
        LOG(ERROR) << "Failed to open zip from file descriptor for '" << zip_location << "': "
            << error_msg;
        return EXIT_FAILURE;
      }
      if (!DexFile::OpenFromZip(*zip_archive.get(), zip_location, &error_msg, &dex_files)) {
        LOG(ERROR) << "Failed to open dex from file descriptor for zip file '" << zip_location
            << "': " << error_msg;
        return EXIT_FAILURE;
      }
      ATRACE_END();
    } else {
      size_t failure_count = OpenDexFiles(dex_filenames, dex_locations, dex_files);
      if (failure_count > 0) {
        LOG(ERROR) << "Failed to open some dex files: " << failure_count;
        return EXIT_FAILURE;
      }
    }

    const bool kSaveDexInput = false;
    if (kSaveDexInput) {
      for (size_t i = 0; i < dex_files.size(); ++i) {
        const DexFile* dex_file = dex_files[i];
        std::string tmp_file_name(StringPrintf("/data/local/tmp/dex2oat.%d.%zd.dex", getpid(), i));
        std::unique_ptr<File> tmp_file(OS::CreateEmptyFile(tmp_file_name.c_str()));
        if (tmp_file.get() == nullptr) {
            PLOG(ERROR) << "Failed to open file " << tmp_file_name
                        << ". Try: adb shell chmod 777 /data/local/tmp";
            continue;
        }
        //进行对dex文件写入操作
        tmp_file->WriteFully(dex_file->Begin(), dex_file->Size());
        LOG(INFO) << "Wrote input to " << tmp_file_name;
      }
    }
  }
  // Ensure opened dex files are writable for dex-to-dex transformations.
  for (const auto& dex_file : dex_files) {
    if (!dex_file->EnableWrite()) {
      PLOG(ERROR) << "Failed to make .dex file writeable '" << dex_file->GetLocation() << "'\n";
    }
  }

  if (!image && compiler_options->IsCompilationEnabled()) {
    size_t num_methods = 0;
    for (size_t i = 0; i != dex_files.size(); ++i) {
      const DexFile* dex_file = dex_files[i];
      CHECK(dex_file != nullptr);
      num_methods += dex_file->NumMethodIds();
    }
    if (num_methods <= compiler_options->GetNumDexMethodsThreshold()) {
      compiler_options->SetCompilerFilter(CompilerOptions::kSpeed);
      VLOG(compiler) << "Below method threshold, compiling anyways";
    }
  }

  // Fill some values into the key-value store for the oat header.
  std::unique_ptr<SafeMap<std::string, std::string> > key_value_store(
      new SafeMap<std::string, std::string>());

  // Insert some compiler things.
  std::ostringstream oss;
  for (int i = 0; i < argc; ++i) {
    if (i > 0) {
      oss << ' ';
    }
    oss << argv[i];
  }
  key_value_store->Put(OatHeader::kDex2OatCmdLineKey, oss.str());
  oss.str("");  // Reset.
  oss << kRuntimeISA;
  key_value_store->Put(OatHeader::kDex2OatHostKey, oss.str());

//编译dex文件功能，主要将dex文件转换我oat文件
  std::unique_ptr<const CompilerDriver> compiler(dex2oat->CreateOatFile(boot_image_option,
                                                                        android_root,
                                                                        is_host,
                                                                        dex_files,
                                                                        oat_file.get(),
                                                                        oat_location,
                                                                        bitcode_filename,
                                                                        image,
                                                                        image_classes,
                                                                        dump_stats,
                                                                        dump_passes,
                                                                        timings,
                                                                        compiler_phases_timings,
                                                                        profile_file,
                                                                        key_value_store.get()));
  if (compiler.get() == nullptr) {
    LOG(ERROR) << "Failed to create oat file: " << oat_location;
    return EXIT_FAILURE;
  }

  VLOG(compiler) << "Oat file written successfully (unstripped): " << oat_location;

  if (image) {
     //打印运行时间日志
    TimingLogger::ScopedTiming t("dex2oat ImageWriter", &timings);
    //创建一个oat映射文件
    bool image_creation_success = dex2oat->CreateImageFile(image_filename,
                                                           image_base,
                                                           oat_unstripped,
                                                           oat_location,
                                                           *compiler.get());
    if (!image_creation_success) {
      return EXIT_FAILURE;
    }
    VLOG(compiler) << "Image written successfully: " << image_filename;
  }

  if (is_host) {
    timings.EndTiming();
    if (dump_timing || (dump_slow_timing && timings.GetTotalNs() > MsToNs(1000))) {
      LOG(INFO) << Dumpable<TimingLogger>(timings);
    }
    if (dump_passes) {
      LOG(INFO) << Dumpable<CumulativeLogger>(*compiler.get()->GetTimingsLogger());
    }
    return EXIT_SUCCESS;
  }

  if (oat_unstripped != oat_stripped) {
     //记录程序执行时间
    TimingLogger::ScopedTiming t("dex2oat OatFile copy", &timings);
    oat_file.reset();
    //用智能指针方式进行打开读取文件
     std::unique_ptr<File> in(OS::OpenFileForReading(oat_unstripped.c_str()));
    std::unique_ptr<File> out(OS::CreateEmptyFile(oat_stripped.c_str()));
    size_t buffer_size = 8192;
    std::unique_ptr<uint8_t> buffer(new uint8_t[buffer_size]);
    while (true) {
      int bytes_read = TEMP_FAILURE_RETRY(read(in->Fd(), buffer.get(), buffer_size));
      if (bytes_read <= 0) {
        break;
      }
      bool write_ok = out->WriteFully(buffer.get(), bytes_read);
      CHECK(write_ok);
    }
    oat_file.reset(out.release());
    VLOG(compiler) << "Oat file copied successfully (stripped): " << oat_stripped;
  }

#if ART_USE_PORTABLE_COMPILER  // We currently only generate symbols on Portable
  if (!compiler_options.GetIncludeDebugSymbols()) {
    timings.NewSplit("dex2oat ElfStripper");
    // Strip unneeded sections for target
    off_t seek_actual = lseek(oat_file->Fd(), 0, SEEK_SET);
    CHECK_EQ(0, seek_actual);
    std::string error_msg;
    CHECK(ElfStripper::Strip(oat_file.get(), &error_msg)) << error_msg;

    // 成功的编译成oat文件
    VLOG(compiler) << "Oat file written successfully (stripped): " << oat_location;
  } else {
    VLOG(compiler) << "Oat file written successfully without stripping: " << oat_location;
  }
#endif  // ART_USE_PORTABLE_COMPILER

  timings.EndTiming();

  if (dump_timing || (dump_slow_timing && timings.GetTotalNs() > MsToNs(1000))) {
    LOG(INFO) << Dumpable<TimingLogger>(timings);
  }
  if (dump_passes) {
    LOG(INFO) << Dumpable<CumulativeLogger>(compiler_phases_timings);
  }

  if (!kIsDebugBuild && (RUNNING_ON_VALGRIND == 0)) {
    dex2oat->LogCompletionTime();
    exit(EXIT_SUCCESS);
  }

  return EXIT_SUCCESS;
}  // NOLINT(readability/fn_size)
}  // namespace art

总结

基于以上的分析，我们可以指定dex2oat在我们现在android系统运行过程中占据很重要的地位，因为app安装，手机屏幕滑动，系统启动等等都需要和dex2oat打交道，同时dex2oat在加壳和脱壳方面应用场景，在脱壳方面通过修改dex2oat代码可以进行更好的脱壳。