examples/src/main/java/com/example/MazeFuzzer.java

// Copyright 2022 Code Intelligence GmbH
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.example;

import com.code_intelligence.jazzer.api.Consumer3;
import com.code_intelligence.jazzer.api.Jazzer;
import java.util.Arrays;
import java.util.Objects;
import java.util.stream.Collectors;

// A fuzz target that shows how manually informing the fuzzer about important state can make a fuzz
// target much more effective.
// This is a Java version of the famous "maze game" discussed in
// "IJON: Exploring Deep State Spaces via Fuzzing", available at:
// https://wcventure.github.io/FuzzingPaper/Paper/SP20_IJON.pdf
public final class MazeFuzzer {
  private static final String[] MAZE_STRING = new String[] {
      "  ███████████████████",
      "    █ █ █   █ █     █",
      "█ █ █ █ ███ █ █ █ ███",
      "█ █ █   █       █   █",
      "█ █████ ███ ███ █ ███",
      "█       █   █ █ █   █",
      "█ ███ ███████ █ ███ █",
      "█ █     █ █     █   █",
      "███████ █ █ █████ ███",
      "█   █       █     █ █",
      "█ ███████ █ ███ ███ █",
      "█   █     █ █ █   █ █",
      "███ ███ █ ███ █ ███ █",
      "█     █ █ █   █     █",
      "█ ███████ █ █ █ █ █ █",
      "█ █         █ █ █ █ █",
      "█ █ █████████ ███ ███",
      "█   █   █   █ █ █   █",
      "█ █ █ ███ █████ ███ █",
      "█ █         █        ",
      "███████████████████ #",
  };

  private static final char[][] MAZE = parseMaze();
  private static final char[][] REACHED_FIELDS = parseMaze();

  public static void fuzzerTestOneInput(byte[] commands) {
    executeCommands(commands, (x, y, won) -> {
      if (won) {
        throw new TreasureFoundException(commands);
      }
      // This is the key line that makes this fuzz target work: It instructs the fuzzer to track
      // every new combination of x and y as a new feature. Without it, the fuzzer would be
      // completely lost in the maze as guessing an escaping path by chance is close to impossible.
      Jazzer.exploreState((byte) Objects.hash(x, y), 0);
      if (REACHED_FIELDS[y][x] == ' ') {
        // Fuzzer reached a new field in the maze, print its progress.
        REACHED_FIELDS[y][x] = '.';
        System.out.println(renderMaze(REACHED_FIELDS));
      }
    });
  }

  private static class TreasureFoundException extends RuntimeException {
    TreasureFoundException(byte[] commands) {
      super(renderPath(commands));
    }
  }

  private static void executeCommands(byte[] commands, Consumer3<Byte, Byte, Boolean> callback) {
    byte x = 0;
    byte y = 0;
    callback.accept(x, y, false);

    for (byte command : commands) {
      byte nextX = x;
      byte nextY = y;
      switch (command) {
        case 'L':
          nextX--;
          break;
        case 'R':
          nextX++;
          break;
        case 'U':
          nextY--;
          break;
        case 'D':
          nextY++;
          break;
        default:
          return;
      }
      char nextFieldType;
      try {
        nextFieldType = MAZE[nextY][nextX];
      } catch (IndexOutOfBoundsException e) {
        // Fuzzer tried to walk through the exterior walls of the maze.
        continue;
      }
      if (nextFieldType != ' ' && nextFieldType != '#') {
        // Fuzzer tried to walk through the interior walls of the maze.
        continue;
      }
      // Fuzzer performed a valid move.
      x = nextX;
      y = nextY;
      callback.accept(x, y, nextFieldType == '#');
    }
  }

  private static char[][] parseMaze() {
    return Arrays.stream(MazeFuzzer.MAZE_STRING).map(String::toCharArray).toArray(char[][] ::new);
  }

  private static String renderMaze(char[][] maze) {
    return Arrays.stream(maze).map(String::new).collect(Collectors.joining("\n", "\n", "\n"));
  }

  private static String renderPath(byte[] commands) {
    char[][] mutableMaze = parseMaze();
    executeCommands(commands, (x, y, won) -> {
      if (!won) {
        mutableMaze[y][x] = '.';
      }
    });
    return renderMaze(mutableMaze);
  }
}