[libFuzzer] Enable entropic strategy. (#1822)

* [libFuzzer] Enable entropic strategy.

Entropic showed signficant improvement over default libFuzzer
in FuzzBench reports (example:
https://www.fuzzbench.com/reports/2020-05-24/index.html).

Enable the strategy for 50% of runs.

* Update strategy schema

src/python/bot/fuzzers/libFuzzer/constants.py

@@ -24,6 +24,8 @@
 
 DATA_FLOW_TRACE_FLAG = '-data_flow_trace='
 
+ENTROPIC_ARGUMENT = '-entropic=1'
+
 DICT_FLAG = '-dict='
 
 FOCUS_FUNCTION_FLAG = '-focus_function='

src/python/bot/fuzzers/libfuzzer.py

@@ -1772,6 +1772,10 @@ def pick_strategies(strategy_pool,
                                  fuzzer_path)):
     fuzzing_strategies.append(strategy.RECOMMENDED_DICTIONARY_STRATEGY.name)
 
+  if strategy_pool.do_strategy(strategy.ENTROPIC_STRATEGY):
+    arguments.append(constants.ENTROPIC_ARGUMENT)
+    fuzzing_strategies.append(strategy.ENTROPIC_STRATEGY.name)
+
   if strategy_pool.do_strategy(strategy.VALUE_PROFILE_STRATEGY):
     arguments.append(constants.VALUE_PROFILE_ARGUMENT)
     fuzzing_strategies.append(strategy.VALUE_PROFILE_STRATEGY.name)

src/python/fuzzing/strategy.py

@@ -34,6 +34,8 @@
     name='corpus_mutations_ml_rnn', probability=0.50, manually_enable=False)
 DATAFLOW_TRACING_STRATEGY = Strategy(
     name='dataflow_tracing', probability=0.50, manually_enable=True)
+ENTROPIC_STRATEGY = Strategy(
+    name='entropic', probability=0.50, manually_enable=False)
 CORPUS_SUBSET_STRATEGY = Strategy(
     name='corpus_subset', probability=0.50, manually_enable=True)
 FORK_STRATEGY = Strategy(name='fork', probability=0.50, manually_enable=False)

src/python/metrics/fuzzer_stats_schema.py

@@ -319,6 +319,10 @@
     'mode': 'NULLABLE',
     'name': 'strategy_corpus_mutations_ml_rnn',
     'type': 'INTEGER'
+}, {
+    'mode': 'NULLABLE',
+    'name': 'strategy_entropic',
+    'type': 'INTEGER'
 }, {
     'mode': 'NULLABLE',
     'name': 'slow_units_count',