[libfuzzer] prune_corpus option for disabling pruning during the load.

[aizatsky]

Summary

The option is very useful for testing, plus I intend to measure
its effect on fuzzer effectiveness.

[aizatsky]

always running fuzzer on corpus items.

[kcc]

LGTM,
but if you want this option to be user-visible (i.e. not experimental/internal/hidden/etc) please also document it.

lib/Fuzzer/FuzzerDriver.cpp

if (!ParseDictionaryFile(FileToString(Flags.dict), &Dictionary))
       return 1;
   if (Flags.verbosity > 0 && !Dictionary.empty())
     Printf("Dictionary: %zd entries\n", Dictionary.size());
   bool DoPlainRun = AllInputsAreFiles();
   Options.SaveArtifacts = !DoPlainRun;
   Options.PrintNewCovPcs = Flags.print_new_cov_pcs;
   Options.PrintFinalStats = Flags.print_final_stats;
   Options.TruncateUnits = Flags.truncate_units;
+  Options.PruneCorpus = Flags.prune_corpus;
 
   unsigned Seed = Flags.seed;
   // Initialize Seed.
   if (Seed == 0)
     Seed = (std::chrono::system_clock::now().time_since_epoch().count() << 10) +
            getpid();
   if (Flags.verbosity)
     Printf("INFO: Seed: %u\n", Seed);

lib/Fuzzer/FuzzerFlags.def

 FUZZER_FLAG_INT(close_fd_mask, 0, "If 1, close stdout at startup; "
     "if 2, close stderr; if 3, close both. "
     "Be careful, this will also close e.g. asan's stderr/stdout.")
 FUZZER_FLAG_INT(detect_leaks, 1, "If 1, and if LeakSanitizer is enabled "
     "try to detect memory leaks during fuzzing (i.e. not only at shut down).")
 FUZZER_FLAG_INT(rss_limit_mb, 2048, "If non-zero, the fuzzer will exit upon"
     "reaching this limit of RSS memory usage.")
 FUZZER_FLAG_INT(truncate_units, 0, "Try truncated units when loading corpus.")
+FUZZER_FLAG_INT(prune_corpus, 1, "Prune corpus items without new coverage when "
+                                 "loading corpus.")
 
 FUZZER_DEPRECATED_FLAG(exit_on_first)
 FUZZER_DEPRECATED_FLAG(save_minimized_corpus)
 FUZZER_DEPRECATED_FLAG(sync_command)
 FUZZER_DEPRECATED_FLAG(sync_timeout)
 FUZZER_DEPRECATED_FLAG(test_single_input)

lib/Fuzzer/FuzzerInternal.h

struct FuzzingOptions {
     std::string ExactArtifactPath;
     bool SaveArtifacts = true;
     bool PrintNEW = true; // Print a status line when new units are found;
     bool OutputCSV = false;
     bool PrintNewCovPcs = false;
     bool PrintFinalStats = false;
     bool DetectLeaks = true;
     bool TruncateUnits = false;
+    bool PruneCorpus = true;
   };
 
   // Aggregates all available coverage measurements.
   struct Coverage {
     Coverage() { Reset(); }
 
     void Reset() {
       BlockCoverage = 0;

lib/Fuzzer/FuzzerLoop.cpp

void Fuzzer::ShuffleAndMinimize() {
 
   if (Options.TruncateUnits) {
     ResetCoverage();
     TruncateUnits(&NewCorpus);
     ResetCoverage();
   }
 
   for (const auto &U : Corpus) {
-    if (RunOne(U)) {
+    if (!Options.PruneCorpus || RunOne(U)) {
       NewCorpus.push_back(U);
       if (Options.Verbosity >= 2)
         Printf("NEW0: %zd L %zd\n", MaxCoverage.BlockCoverage, U.size());
     }
     TryDetectingAMemoryLeak(U.data(), U.size(),
                             /*DuringInitialCorpusExecution*/ true);
   }
   Corpus = NewCorpus;

lib/Fuzzer/test/fuzzer-prunecorpus.test

+RUN: rm -rf %t/PruneCorpus
+RUN: mkdir -p %t/PruneCorpus
+RUN: echo a > %t/PruneCorpus/a
+RUN: echo b > %t/PruneCorpus/b
+RUN: LLVMFuzzer-EmptyTest %t/PruneCorpus -prune_corpus=1 -runs=0 2>&1 | FileCheck %s --check-prefix=PRUNE
+RUN: LLVMFuzzer-EmptyTest %t/PruneCorpus -prune_corpus=0 -runs=0 2>&1 | FileCheck %s --check-prefix=NOPRUNE
+RUN: rm -rf %t/PruneCorpus
+
+PRUNE: READ units: 2
+PRUNE: INITED{{.*}}units: 1
+NOPRUNE: READ units: 2
+NOPRUNE: INITED{{.*}}units: 2
+