Android內核，Android 8.0 VDEX機制簡介

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commit 7b49e6cade09bc65b3b5f22d45fc9d0a7184e4f2
Author: David Brazdil <dbrazdil@google.com>
Date:   Thu Sep 1 11:06:18 2016 +0100

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// art/dex2oat/dexoat.cc
int main(int argc, char** argv) {return static_cast<int>(art::Dex2oat(argc, argv));
}static dex2oat::ReturnCode Dex2oat(int argc, char** argv) {dex2oat->ParseArgs(argc, argv);dex2oat::ReturnCode setup_code = dex2oat->Setup();dex2oat::ReturnCode result = CompileApp(*dex2oat);return result;
}dex2oat::ReturnCode Setup() {// DoDexLayoutOptimizations()內部會校驗dex文件，所以當vdex存在或者DoDexLayoutOptimizations()也ok時// 后面就不用再次去檢查dex文件的合法性了const bool verify = !DoDexLayoutOptimizations() && (input_vdex_file_ == nullptr);if (!oat_writers_[i]->WriteAndOpenDexFiles(kIsVdexEnabled ? vdex_files_[i].get() : oat_files_[i].get(),rodata_.back(),instruction_set_,instruction_set_features_.get(),key_value_store_.get(),verify,update_input_vdex_,&opened_dex_files_map,&opened_dex_files)) {return dex2oat::ReturnCode::kOther;}
}

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// art/compiler/oat_writer.cc
bool OatWriter::WriteAndOpenDexFiles(File* vdex_file,OutputStream* oat_rodata,InstructionSet instruction_set,const InstructionSetFeatures* instruction_set_features,SafeMap<std::string, std::string>* key_value_store,bool verify,bool update_input_vdex,/*out*/ std::unique_ptr<MemMap>* opened_dex_files_map,/*out*/ std::vector<std::unique_ptr<const DexFile>>* opened_dex_files) {...if (kIsVdexEnabled) {std::unique_ptr<BufferedOutputStream> vdex_out(MakeUnique<BufferedOutputStream>(MakeUnique<FileOutputStream>(vdex_file)));// 啟用了vdex，把dex文件寫到vdex文件里，并map到內存里，用于后續compile流程if (!WriteDexFiles(vdex_out.get(), vdex_file, update_input_vdex) ||!OpenDexFiles(vdex_file, verify, &dex_files_map, &dex_files)) {return false;}} else {// 沒有啟用vdex，就把dex文件寫到oat(odex)文件里if (!WriteDexFiles(oat_rodata, vdex_file, update_input_vdex) ||!OpenDexFiles(vdex_file, verify, &dex_files_map, &dex_files)) {return false;}}
...
}

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// art/runtime/dex_file_verifier.cc
bool DexFileVerifier::Verify() {// Check the header.if (!CheckHeader()) {return false;}// Check the map section.if (!CheckMap()) {return false;}// Check structure within remaining sections.if (!CheckIntraSection()) {return false;}// Check references from one section to another.return CheckInterSection();
}

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struct MapList {uint32_t size_; // 數組list_實際的長度 MapItem list_[1];
};
struct MapItem {// item類型，e.g. kDexTypeStringIdItem, kDexTypeStringDataItem... // 詳細見 DexFile::MapItemType(位于art/runtime/dex_file.h)uint16_t type_;uint16_t unused_; // 暫時僅用作4字節對齊用uint32_t size_;   // 該類item的數量// mmap后，該類型的第一個item相對與dex文件起始地址的偏移(即處于dex文件的第$offset_個字節)uint32_t offset_;
};

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// Iterate and decode class_data_item
class ClassDataItemIterator {
private:// 為了壓縮dex文件大小，這部分數據是用的leb12b編碼，所以在運行時遍歷靜態/非靜態屬性和方法的時候，再依次decode出來struct ClassDataHeader {uint32_t static_fields_size_;  // the number of static fieldsuint32_t instance_fields_size_;  // the number of instance fieldsuint32_t direct_methods_size_;  // the number of direct methods，如私有方法，靜態方法，構造函數uint32_t virtual_methods_size_;  // the number of virtual methods} header_;// A decoded version of the field of a class_data_itemstruct ClassDataField {uint32_t field_idx_delta_;  // 表示在filed_ids鏈表中的索引uint32_t access_flags_;  // access flags for the field};ClassDataField field_;// A decoded version of the method of a class_data_itemstruct ClassDataMethod {uint32_t method_idx_delta_;  // 表示在method_ids鏈表中的索引uint32_t access_flags_;uint32_t code_off_;};ClassDataMethod method_;
};
struct FieldId {dex::TypeIndex class_idx_;   // index into type_ids_ array for defining classdex::TypeIndex type_idx_;    // index into type_ids_ array for field typedex::StringIndex name_idx_;  // index into string_ids_ array for field name
};
struct MethodId {dex::TypeIndex class_idx_;   // index into type_ids_ array for defining classuint16_t proto_idx_;         // index into proto_ids_ array for method prototypedex::StringIndex name_idx_;  // index into string_ids_ array for method name
};

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struct StringId {uint32_t string_data_off_;  // offset in bytes from the base address
};

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struct TypeId {dex::StringIndex descriptor_idx_;  // index into string_ids
};

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struct ProtoId {dex::StringIndex shorty_idx_;     // index into string_ids array for shorty descriptordex::TypeIndex return_type_idx_;  // index into type_ids array for return typeuint16_t pad_;                    // padding = 0uint32_t parameters_off_;         // file offset to type_list for parameter types
};

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// VDEX files contain extracted DEX files. The VdexFile class maps the file to
// memory and provides tools for accessing its individual sections.
//
// File format:
//   VdexFile::VerifierDepsHeader    fixed-length header
//      Dex file checksums
//
//   Optionally:
//      VdexFile::DexSectionHeader   fixed-length header
//
//      quicken_table_off[0]  offset into QuickeningInfo section for offset table for DEX[0].
//      DEX[0]                array of the input DEX files, the bytecode may have been quickened.
//      quicken_table_off[1]
//      DEX[1]
//      ...
//      DEX[D]
//
//   VerifierDeps
//      uint8[D][]                 verification dependencies
//
//   Optionally:
//      QuickeningInfo
//        uint8[D][]                  quickening data
//        uint32[D][]                 quickening data offset tables

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commit ca3c8c33501bf199d6fd0a5db30a27d8e010cb23
Author: David Brazdil <dbrazdil@google.com>
Date:   Tue Sep 6 14:04:48 2016 +0100Collect verifier dependenciesMethodVerifier tests whether a DEX method is valid w.r.t. the classesin class path. Since the APK does not change across OTA updates, itis not necessary to analyze the bytecode again with MethodVerifier,as long as its dependencies on the class path (which may have changed)are satisfied.This patch introduces VerifierDeps, a class path dependency collector,and adds hooks into MethodVerifier where classes/methods/fields areresolved and where assignability of types is tested.