Fuzzers love assertions

Fuzzers make things go wrong.
Assertions make sure we find out.

Assertions can improve code quality in many ways, but they truly shine when combined with fuzzing. Fuzzing is normally limited to finding obvious symptoms like crashes, because it's rare to be able to tell correct behavior from incorrect behavior when the input is generated randomly. Assertions expand the scope of fuzzing to include everything they check.

Assertions can even help find crash bugs: some bugs are relatively easy for fuzzers to trigger, but only lead to crashes when additional conditions are met. A well-placed assertion can let us know every time we trigger the bug.

Fuzzing JS and DOM has found about 4000 assertion bugs, including about 300 security bugs.

Asserting safe use of generic data structures

Assertions in widely-used data structures can find bugs in many callers.

  • Array indices must be within bounds. This simple precondition assert in nsTArray has caught about 90 bugs.
  • Hash tables must not be modified during enumeration. If the modification happened to resize the hash table, it would leave stack pointers dangling. This PLDHashTable assertion has caught over 50 bugs.
  • Cached values should not be out of date. When a cache's get method takes a key and a closure for computing values in the case of a cache miss, debug builds can check whether the cached values are still correct. This is effectively a form of differential testing that notices bugs in cache-invalidation logic.

Asserting module invariants

When an entire module must maintain an invariant, a single assertion can catch dozens of bugs.

Making the frame arena safer

Gecko's CSS box objects, called "frames", are created and destroyed manually. They are allocated within an arena to reduce malloc overhead and fragmentation. The arena also made it possible to reduce the risk associated with manual memory management. A combination of assertions (in debug builds) and runtime mitigations (in all builds) mitigates dangling pointer bugs that involve frames.

Requests for Gecko developers

Please add assertions, especially when:

  • A bug would be a security hole
  • Crashing is not guaranteed
  • Many callers must fulfill a precondition
  • Complex, extensive code must maintain an invariant

Also consider:

One Response to “Fuzzers love assertions”

  1. New testmachine release | David R. MacIver Says:

    […] benefits of generating tests with testmachine is that it can find ways to trigger those assertions (fuzzers love assertions), but if the process crashes before they can usefully reduce the test case the results are… […]

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