Fine-Grained Coverage-Based Fuzzing

Abstract

Fuzzing is an effective software testing method that discovers bugs by feeding target applications with (usually a massive amount of) automatically generated inputs. Many stateof-art fuzzers use branch coverage as a feedback metric to guide the fuzzing process. The fuzzer retains inputs for further mutation only if branch coverage is increased. However, branch coverage only provides a shallow sampling of program behaviours and hence may discard inputs that might be interesting to mutate. This work aims at taking advantage of the large body of research over defining finer-grained code coverage metrics (such as mutation coverage) and use these metrics as better proxies to select interesting inputs for mutation. We propose to make coveragebased fuzzers support most fine-grained coverage metrics out of the box (i.e., without changing fuzzer internals). We achieve this by making the test objectives defined by these metrics (such as mutants to kill) explicit as new branches in the target program. Fuzzing such a modified target is then equivalent to fuzzing the original target, but the fuzzer will also retain inputs covering the additional metrics objectives for mutation. We propose a preliminary evaluation of this novel idea using two state-of-art fuzzers, namely AFL++(3.14c) and QSYM with AFL(2.52b), on the four standard LAVA-M benchmarks. Significantly positive results are obtained on one benchmark and marginally negative ones on the three others. We discuss directions towards a strong and complete evaluation of the proposed approach and call for early feedback from the fuzzing community.