ApproxiFuzzer: Fuzzing towards Deep Code Snippets in Java Programs

A real-world, complex software system can contain a number of code snippets. Many snippets are deep, surrounded by complicated triggering conditions and/or hidden in functions less frequently invoked. Fuzzing and symbolic execution are two mainstreams for exploring input spaces and increasing code coverage of complicated software systems. Meanwhile, it remains a challenge to determine whether a deep code snippet is reachable, and if it is reachable, which test(s) can reach it.This paper presents ApproxiFuzzer, an effective, demand-driven approach to fuzzing towards deep code snippets in Java programs. Given a program P, a target deep code snippet tcs, and a set of seeding test inputs, the key idea behind ApproxiFuzzer is to selectively mutate the test inputs and collect their execution traces such that the execution traces gradually approximate tcs; several measures are designed for measuring the distances between execution traces and the code snippet and directing the fuzzing process towards generating test inputs reaching tcs.We have implemented ApproxiFuzzer and evaluated it against Kelinci (an AFL-based fuzzer) and JDart (a concolic execution tool) on a set of real-world benchmarks. The evaluation clearly demonstrates the strengths of ApproxiFuzzer—ApproxiFuzzer outperforms Kelinci by 36× in efficiently generating test inputs, obtaining up to 18.2% higher code coverage; ApproxiFuzzer also outperforms JDart by 46.2∼96.2% in hitting deep code snippets.