A spin on libFuzzer so as to favor inputs incurring a slowdown. The key modifications consist of changing the fitness function, to favor inputs that excercise more basic block edges, as well as introducing probabilities in the selection of mutations to be performed, so as to preserve "locality" of the created inputs.

In particular, there are 4 available modes for mutations:

• Random: the mutation is selected at random.

• Mutation priority: Prioritize mutations that were successful in previous runs

• Offset priority: Prioritize mutating certain offsets (buckets of the input). The number of buckets to split the input into is controlled via the score_buckets parameter.

• Hybrid mode: Keep a score for the different mutations for each input bucket

The respective mutation mode to be used is controlled via the -death_mode option (run the compiled binary with -help=1 for all available options)

Finally, this implementation adds some functionality with respect to using dictionary files. In particular, the options -only_dict=1 limits libfuzzer to only use characters provided in the given dictionary (one character per line) and to not introduce any other characters.

For more information see the SlowFuzz paper.

To see how SlowFuzz generates inputs causing a slowdown for a given implementation, let's pick a simple example of an insertion sort implementation. To this end, run:

cd apps/isort
make fuzzer
make
make test

A directory out/ is created, containing all the units that caused a slowdown.