src/main/java/com/android/tools/r8/dex/VDexFile.java

// Copyright (c) 2017, the R8 project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
package com.android.tools.r8.dex;

import static com.android.tools.r8.dex.Constants.VDEX_FILE_MAGIC_PREFIX;
import static com.android.tools.r8.dex.Constants.VDEX_FILE_VERSION;
import static com.android.tools.r8.dex.Constants.VDEX_NUMBER_OF_DEX_FILES_OFFSET;

import com.android.tools.r8.Resource;
import com.android.tools.r8.errors.CompilationError;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;

/**
 * See runtime/vdex_file.h and runtime/vdex_file.cc in the Art code for the vdex file format.
 */
public class VDexFile extends BaseFile {

  final String name;
  private final int version;

  public VDexFile(Resource resource) throws IOException {
    super(resource);
    this.name = "resource.vdex";
    version = parseMagic(buffer);
  }

  // Probes the file for vdex magic and version.
  static boolean couldBeVDexFile(Path file) {
    try (FileChannel channel = FileChannel.open(file, StandardOpenOption.READ)) {
      if (channel.size() < Constants.VDEX_HEADER_SIZE) {
        return false;
      }
      ByteBuffer buffer = ByteBuffer.allocate(Constants.VDEX_HEADER_SIZE);
      channel.read(buffer);
      return couldBeVDexFile(buffer);
    } catch (IOException | CompilationError e) {
      return false;
    }
  }

  static boolean couldBeVDexFile(ByteBuffer buffer) {
    return parseMagic(buffer) == Constants.ANDROID_O_VDEX_VERSION;
  }

  // Parse the magic header and determine the dex file version.
  private static int parseMagic(ByteBuffer buffer) {
    int index = 0;
    for (byte prefixByte : VDEX_FILE_MAGIC_PREFIX) {
      if (buffer.get(index++) != prefixByte) {
        throw new CompilationError("VDex file has invalid header");
      }
    }
    for (byte prefixByte : VDEX_FILE_VERSION) {
      if (buffer.get(index++) != prefixByte) {
        throw new CompilationError("VDex file has invalid version number");
      }
    }
    return Constants.ANDROID_O_VDEX_VERSION;
  }

  public static int firstDexOffset(int numberOfDexFiles) {
    return Constants.VDEX_CHECKSUM_SECTION_OFFSET
        + numberOfDexFiles * Constants.VDEX_DEX_CHECKSUM_SIZE;
  }

  void setByteOrder() {
    // Make sure we set the right endian for reading.
    buffer.order(ByteOrder.LITTLE_ENDIAN);
    int dexFiles = buffer.getInt(VDEX_NUMBER_OF_DEX_FILES_OFFSET);
    // Reading a strange number of dex files indicate reading with the wrong endian.
    if (dexFiles < 0 || dexFiles > 1000) {
      buffer.order(ByteOrder.BIG_ENDIAN);
      dexFiles = buffer.getInt(VDEX_NUMBER_OF_DEX_FILES_OFFSET);
      assert dexFiles < 0 || dexFiles > 1000;
    }

    // Make sure we did set the right endian for reading.
    int endian = buffer.getInt(firstDexOffset(dexFiles) + Constants.ENDIAN_TAG_OFFSET);
    if (endian != Constants.ENDIAN_CONSTANT) {
      throw new CompilationError("Unable to determine endianess for reading vdex file.");
    }
  }

  int getVDexVersion() {
    return version;
  }
}