VeriFuzz: Program Aware Fuzzing
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VeriFuzz is a program aware fuzz testing tool, which combines the power of feedback-driven evolutionary fuzz testing with static analysis. VeriFuzz deploys lightweight static analysis to extract meaningful information about program behavior that can aid fuzzing based test-input generation to achieve coverage goals quickly. We use constraint-solver to generate an initial population of test-inputs. VeriFuzz could generate the maximum number of counterexamples for reachsafety category benchmarks in SV-COMP 2019 and in Test-Comp 2019 . (All the terms in typewriter font are competition specific. See .)
VeriFuzz is a coverage driven automated test-input generation tool based on grey-box fuzzing . The idea of grey-box fuzzing is to use lightweight instrumentation to observe behaviors exhibited during a test run. This information is used while fuzzing for new test-inputs that might exhibit new behaviors. For VeriFuzz, the behavior of interest is code coverage. VeriFuzz relies on evolutionary algorithms to generate newer test-inputs from an initial population of test-inputs. Central to an evolutionary algorithm is the selection of best-fit candidates from a population and generate offspring by applying crossover and mutating operations on them. The newer offspring are checked for their fitness against a goal. The population evolves by adding the fit offspring to the existing population. In an automated testing, a candidate test-input plays the role of an individual in a population. The new test-inputs are generated from a selected test-input by repeatedly applying mutation operations, for example, by flipping byte at a random position. The fitness of a generated test-input is determined by the code coverage during its run [11, 18].
State-of-the-art grey-box fuzzers such as afl-fuzz , though simple to use, have several key shortcomings. (a) The fuzzer is aware of neither the program structure nor the input structure. This leads to the generation of a large set of redundant test-inputs with respect to code coverage. (b) For programs that does complex validations on their input, the fuzzer finds it hard to generate a test-input that satisfies such validation conditions . Finding a suitable initial population of test-inputs for such programs requires the analysis of validation conditions. (c) For programs with unbounded loops, the fuzzer may get stuck forever without generating any new test-inputs. There are several approaches proposed in the literature to address some of these shortcomings [1, 7, 12, 13]. However, all these approaches address each concern separately.
2 Our Approach
In order to alleviate the problems described in Sect. 1, our approach analyses and transforms the subject program. The analysis information is then passed to the enhanced mutation engine of afl-fuzz for fuzzing the transformed program. The following are the key steps of our approach.
Efficient Instrumentation: To measure the coverage due to a run on a test-input, the subject program is instrumented. However, instrumentation adds a significant overhead to the program execution, impacting the fuzzer’s execution speed. We have optimized the instrumentation overhead by placing the probes either true or false branches of each conditional statement in the program. Our scheme is efficient to implement and preserves the coverage measure though it is sub-optimal than instrumentation schemes proposed in literature .
Loop Bounding: Certain class of programs, for example, reactive programs, during their execution, either does not terminate or crash upon reaching the error location1. In order to handle such non-terminating programs, our approach transforms the program loops by replacing the condition in their loop heads with a known bound. This bound is increased dynamically during the fuzzer run till it finds an input that can take program execution to an error location or the budgeted time elapses.
Novel Initial Test Population Generation: Grey-box fuzzers find it hard to generate test-inputs that can take program execution to cover the program blocks that are guarded by complex checks [10, 13]. If the initial test-input population can take program execution through some of the complex checks, fuzzing such inputs is likely to generate test-inputs that can pass through other complex checks . In order to create such initial test-input population, our approach first flattens the program by unrolling the loops up to a certain bound. Then, a program path is chosen that contains such complex checks and path constraints are generated along that path. The constraints are solved to create an initial test-input population.
Program Analysis to Assist Mutation: For programs that read input only within restricted range values, it is possible to fine-tune the fuzzer’s mutation operators to choose values within this restricted ranges. In order to determine the ranges of input values that can reach the error locations, our approach statically determines the input value ranges of the program using k-path interval analysis . For a given program, this analysis determines the conservative over approximate ranges of input values that may reach any given program point. We have enhanced the mutation engine of the fuzzer such that it accepts the input value ranges at the error location in the program and generates inputs that have values within the given ranges.
Algorithmic Selection of Strategies: All the aforementioned techniques are generic enough to use across the programs. However, in order to optimize the given resource budgets in the competition, we have selectively applied a subset of techniques to a specific class of programs. For example, loop bounding technique is applied to programs where syntactic unbounded loop structures are detected. In order to identify and map the best performing set of techniques to all benchmark programs, we have grouped them into a finite set of classes and formulated it as multi-label classification problem. The classifier model is developed using a non-parametric supervised learning based approach . The model has been trained using nine syntactic structures of a C program and a subset of techniques as classification labels. The benchmark programs from SV-COMP 2018  were used as training and validation set. We have used the decision tree classifier for this multi-label classification .
3 Tool Architecture and Flow
The input to the tool is a program P and a safety property \(\phi \). In the first step, the syntactic features are extracted and the class of the program is determined using a program classification module. This class information is used in subsequent steps. In the second step, the program \(P_i\) is generated using an instrumentation and transformation module. This step also emits the transformed programs for witness generation (\(P_w\)) and initial test-input generation modules. In the next step, the program is analysed to determine input ranges. These input ranges are used to formulate the fuzzing engine parameters \(F_i\). Subsequently, initial test-input population \(T_i\) is generated using the initial input generation module. The fuzz engine is then invoked with \(P_i\), \(F_i\), and \(T_i\) as inputs.
As a first step of the fuzzing engine, the program \(P_i\) and a harness program that implemented __VERIFIER_* functions are compiled together using gcc to generate the executable program \(P_e\). The core fuzzer begins with \(T_i\) as its initial population, executes \(P_e\), and measures the coverage. A test-input from the population is selected and mutated several times to generate newer test-inputs \(T_g\). The program \(P_e\) is repeatedly executed with each test-input \(t_g \in T_g\) and coverage is measured. The fitness check step compares the code coverage due run on each \(t_g\) with the historical code coverage and determines whether \(t_g\) should be added to the population or not. This process is repeated until the core fuzzer finds a crashing test-input \(t_c\) that causes the program run to reach the error location or the time budget is elapsed. The \(t_c\) and \(P_w\) are passed to a witness generation program to generate error-witness.
4 Strengths and Weaknesses
The core strength of VeriFuzz is its ability to find a test-input that can cause the program execution to reach the error locations quickly. The tool participated both in SV-COMP and Test-Comp . In SV-COMP, VeriFuzz could identify 1264 out of 1458 reachsafety FALSE benchmarks with 67 s as mean time per verification task. Whereas in Test-Comp, it could identify bugs in 592 out of 636 benchmarks. VeriFuzz could generate test-inputs that can, on an average, cover 70% branches in 1720 benchmarks.
VeriFuzz explores the concrete program paths randomly and redundantly due to its evolutionary approach. Therefore it may not always discover a test-input that cause the execution to reach an error location.
5 Tool Configuration and Setup
The VeriFuzz tool for testing SV-COMP benchmarks is available at the URL https://gitlab.com/sosy-lab/sv-comp/archives-2019/blob/master/2019/verifuzz.zip. Its Test-Comp 2019 variant is available at the URL https://gitlab.com/sosy-lab/test-comp/archives-2019/blob/master/2019/verifuzz.zip. The benchexec tool-info module is verifuzz.py and the benchmark description file is verifuzz.xml. To install and run the tool, follow the instructions provided in README.txt with the tool. A sample run command is as follows:
./scripts/verifuzz.py --propertyFile unreach-call.prp example.c
6 Software Project and Contributors
VeriFuzz is developed by the authors at TCS Research. We would like to thank B. Chimdyalwar and S. Kumar from VeriAbs  team for the help in the understanding of k-path interval analysis.
Program statement where error function __VERIFIER_error() is called.
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