Coverage-guided fuzzing is one of the most successful approaches for discovering software bugs and security vulnerabilities. Of its three main components: (1) test case generation, (2) code coverage tracing, and (3) crash triage, code coverage tracing is a dominant source of overhead. Coverage-guided fuzzers trace every test case's code coverage through either static or dynamic binary instrumentation, or more recently, using hardware support. Unfortunately, tracing all test cases incurs significant performance penalties--even when the overwhelming majority of test cases and their coverage information are discarded because they do not increase code coverage. To eliminate needless tracing by coverage-guided fuzzers, we introduce the notion of coverage-guided tracing. Coverage-guided tracing leverages two observations: (1) only a fraction of generated test cases increase coverage, and thus require tracing; and (2) coverage-increasing test cases become less frequent over time. Coverage-guided tracing encodes the current frontier of coverage in the target binary so that it self-reports when a test case produces new coverage--without tracing. This acts as a filter for tracing; restricting the expense of tracing to only coverage-increasing test cases. Thus, coverage-guided tracing trades increased time handling coverage-increasing test cases for decreased time handling non-coverage-increasing test cases. To show the potential of coverage-guided tracing, we create an implementation based on the static binary instrumentor Dyninst called UnTracer. We evaluate UnTracer using eight real-world binaries commonly used by the fuzzing community. Experiments show that after only an hour of fuzzing, UnTracer's average overhead is below 1%, and after 24-hours of fuzzing, UnTracer approaches 0% overhead, while tracing every test case with popular white- and black-box-binary tracers AFL-Clang, AFL-QEMU, and AFL-Dyninst incurs overheads of 36%, 612%, and 518%, respectively. We further integrate UnTracer with the state-of-the-art hybrid fuzzer QSYM and show that in 24-hours of fuzzing, QSYM-UnTracer executes 79% and 616% more test cases than QSYM-Clang and QSYM-QEMU, respectively.
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