1 files changed, 311 insertions, 149 deletions

diff --git a/lib/Fuzzer/FuzzerLoop.cpp b/lib/Fuzzer/FuzzerLoop.cpp
index 9ef47583cbb9..7ea82f4f15dd 100644
--- a/lib/Fuzzer/FuzzerLoop.cpp
+++ b/lib/Fuzzer/FuzzerLoop.cpp
@@ -10,10 +10,43 @@
 //===----------------------------------------------------------------------===//
 
 #include "FuzzerInternal.h"
-#include <sanitizer/coverage_interface.h>
 #include <algorithm>
 
+#if defined(__has_include)
+# if __has_include(<sanitizer/coverage_interface.h>)
+#  include <sanitizer/coverage_interface.h>
+# endif
+#endif
+
+extern "C" {
+// Re-declare some of the sanitizer functions as "weak" so that
+// libFuzzer can be linked w/o the sanitizers and sanitizer-coverage
+// (in which case it will complain at start-up time).
+__attribute__((weak)) void __sanitizer_print_stack_trace();
+__attribute__((weak)) void __sanitizer_reset_coverage();
+__attribute__((weak)) size_t __sanitizer_get_total_unique_caller_callee_pairs();
+__attribute__((weak)) size_t __sanitizer_get_total_unique_coverage();
+__attribute__((weak))
+void __sanitizer_set_death_callback(void (*callback)(void));
+__attribute__((weak)) size_t __sanitizer_get_number_of_counters();
+__attribute__((weak))
+uintptr_t __sanitizer_update_counter_bitset_and_clear_counters(uint8_t *bitset);
+}
+
 namespace fuzzer {
+static const size_t kMaxUnitSizeToPrint = 256;
+
+static void MissingWeakApiFunction(const char *FnName) {
+  Printf("ERROR: %s is not defined. Exiting.\n"
+         "Did you use -fsanitize-coverage=... to build your code?\n", FnName);
+  exit(1);
+}
+
+#define CHECK_WEAK_API_FUNCTION(fn)                                            \
+  do {                                                                         \
+    if (!fn)                                                                   \
+      MissingWeakApiFunction(#fn);                                             \
+  } while (false)
 
 // Only one Fuzzer per process.
 static Fuzzer *F;
@@ -27,17 +60,12 @@ Fuzzer::Fuzzer(UserSuppliedFuzzer &USF, FuzzingOptions Options)
 }
 
 void Fuzzer::SetDeathCallback() {
+  CHECK_WEAK_API_FUNCTION(__sanitizer_set_death_callback);
   __sanitizer_set_death_callback(StaticDeathCallback);
 }
 
-void Fuzzer::PrintUnitInASCIIOrTokens(const Unit &U, const char *PrintAfter) {
-  if (Options.Tokens.empty()) {
-    PrintASCII(U, PrintAfter);
-  } else {
-    auto T = SubstituteTokens(U);
-    T.push_back(0);
-    Printf("%s%s", T.data(), PrintAfter);
-  }
+void Fuzzer::PrintUnitInASCII(const Unit &U, const char *PrintAfter) {
+  PrintASCII(U, PrintAfter);
 }
 
 void Fuzzer::StaticDeathCallback() {
@@ -47,9 +75,11 @@ void Fuzzer::StaticDeathCallback() {
 
 void Fuzzer::DeathCallback() {
   Printf("DEATH:\n");
-  Print(CurrentUnit, "\n");
-  PrintUnitInASCIIOrTokens(CurrentUnit, "\n");
-  WriteToCrash(CurrentUnit, "crash-");
+  if (CurrentUnit.size() <= kMaxUnitSizeToPrint) {
+    Print(CurrentUnit, "\n");
+    PrintUnitInASCII(CurrentUnit, "\n");
+  }
+  WriteUnitToFileWithPrefix(CurrentUnit, "crash-");
 }
 
 void Fuzzer::StaticAlarmCallback() {
@@ -68,19 +98,49 @@ void Fuzzer::AlarmCallback() {
     Printf("ALARM: working on the last Unit for %zd seconds\n", Seconds);
     Printf("       and the timeout value is %d (use -timeout=N to change)\n",
            Options.UnitTimeoutSec);
-    Print(CurrentUnit, "\n");
-    PrintUnitInASCIIOrTokens(CurrentUnit, "\n");
-    WriteToCrash(CurrentUnit, "timeout-");
+    if (CurrentUnit.size() <= kMaxUnitSizeToPrint) {
+      Print(CurrentUnit, "\n");
+      PrintUnitInASCII(CurrentUnit, "\n");
+    }
+    WriteUnitToFileWithPrefix(CurrentUnit, "timeout-");
+    Printf("==%d== ERROR: libFuzzer: timeout after %d seconds\n", GetPid(),
+           Seconds);
+    if (__sanitizer_print_stack_trace)
+      __sanitizer_print_stack_trace();
+    Printf("SUMMARY: libFuzzer: timeout\n");
     exit(1);
   }
 }
 
-void Fuzzer::PrintStats(const char *Where, size_t Cov, const char *End) {
-  if (!Options.Verbosity) return;
+void Fuzzer::PrintStats(const char *Where, const char *End) {
   size_t Seconds = secondsSinceProcessStartUp();
   size_t ExecPerSec = (Seconds ? TotalNumberOfRuns / Seconds : 0);
-  Printf("#%zd\t%s cov %zd bits %zd units %zd exec/s %zd %s", TotalNumberOfRuns,
-         Where, Cov, TotalBits(), Corpus.size(), ExecPerSec, End);
+
+  if (Options.OutputCSV) {
+    static bool csvHeaderPrinted = false;
+    if (!csvHeaderPrinted) {
+      csvHeaderPrinted = true;
+      Printf("runs,block_cov,bits,cc_cov,corpus,execs_per_sec,tbms,reason\n");
+    }
+    Printf("%zd,%zd,%zd,%zd,%zd,%zd,%zd,%s\n", TotalNumberOfRuns,
+           LastRecordedBlockCoverage, TotalBits(),
+           LastRecordedCallerCalleeCoverage, Corpus.size(), ExecPerSec,
+           TotalNumberOfExecutedTraceBasedMutations, Where);
+  }
+
+  if (!Options.Verbosity)
+    return;
+  Printf("#%zd\t%s", TotalNumberOfRuns, Where);
+  if (LastRecordedBlockCoverage)
+    Printf(" cov: %zd", LastRecordedBlockCoverage);
+  if (auto TB = TotalBits())
+    Printf(" bits: %zd", TB);
+  if (LastRecordedCallerCalleeCoverage)
+    Printf(" indir: %zd", LastRecordedCallerCalleeCoverage);
+  Printf(" units: %zd exec/s: %zd", Corpus.size(), ExecPerSec);
+  if (TotalNumberOfExecutedTraceBasedMutations)
+    Printf(" tbm: %zd", TotalNumberOfExecutedTraceBasedMutations);
+  Printf("%s", End);
 }
 
 void Fuzzer::RereadOutputCorpus() {
@@ -101,143 +161,127 @@ void Fuzzer::RereadOutputCorpus() {
     if (UnitHashesAddedToCorpus.insert(Hash(X)).second) {
       CurrentUnit.clear();
       CurrentUnit.insert(CurrentUnit.begin(), X.begin(), X.end());
-      size_t NewCoverage = RunOne(CurrentUnit);
-      if (NewCoverage) {
+      if (RunOne(CurrentUnit)) {
         Corpus.push_back(X);
-        if (Options.Verbosity >= 1)
-          PrintStats("RELOAD", NewCoverage);
+        PrintStats("RELOAD");
       }
     }
   }
 }
 
 void Fuzzer::ShuffleAndMinimize() {
-  size_t MaxCov = 0;
-  bool PreferSmall =
-      (Options.PreferSmallDuringInitialShuffle == 1 ||
-       (Options.PreferSmallDuringInitialShuffle == -1 && rand() % 2));
+  bool PreferSmall = (Options.PreferSmallDuringInitialShuffle == 1 ||
+                      (Options.PreferSmallDuringInitialShuffle == -1 &&
+                       USF.GetRand().RandBool()));
   if (Options.Verbosity)
     Printf("PreferSmall: %d\n", PreferSmall);
-  PrintStats("READ  ", 0);
+  PrintStats("READ  ");
   std::vector<Unit> NewCorpus;
-  std::random_shuffle(Corpus.begin(), Corpus.end());
-  if (PreferSmall)
-    std::stable_sort(
-        Corpus.begin(), Corpus.end(),
-        [](const Unit &A, const Unit &B) { return A.size() < B.size(); });
+  if (Options.ShuffleAtStartUp) {
+    std::random_shuffle(Corpus.begin(), Corpus.end(), USF.GetRand());
+    if (PreferSmall)
+      std::stable_sort(
+          Corpus.begin(), Corpus.end(),
+          [](const Unit &A, const Unit &B) { return A.size() < B.size(); });
+  }
   Unit &U = CurrentUnit;
   for (const auto &C : Corpus) {
     for (size_t First = 0; First < 1; First++) {
       U.clear();
       size_t Last = std::min(First + Options.MaxLen, C.size());
       U.insert(U.begin(), C.begin() + First, C.begin() + Last);
-      size_t NewCoverage = RunOne(U);
-      if (NewCoverage) {
-        MaxCov = NewCoverage;
+      if (Options.OnlyASCII)
+        ToASCII(U);
+      if (RunOne(U)) {
         NewCorpus.push_back(U);
         if (Options.Verbosity >= 2)
-          Printf("NEW0: %zd L %zd\n", NewCoverage, U.size());
+          Printf("NEW0: %zd L %zd\n", LastRecordedBlockCoverage, U.size());
       }
     }
   }
   Corpus = NewCorpus;
   for (auto &X : Corpus)
     UnitHashesAddedToCorpus.insert(Hash(X));
-  PrintStats("INITED", MaxCov);
+  PrintStats("INITED");
 }
 
-size_t Fuzzer::RunOne(const Unit &U) {
+bool Fuzzer::RunOne(const Unit &U) {
   UnitStartTime = system_clock::now();
   TotalNumberOfRuns++;
-  size_t Res = 0;
-  if (Options.UseFullCoverageSet)
-    Res = RunOneMaximizeFullCoverageSet(U);
-  else
-    Res = RunOneMaximizeTotalCoverage(U);
+
+  PrepareCoverageBeforeRun();
+  ExecuteCallback(U);
+  bool Res = CheckCoverageAfterRun();
+
   auto UnitStopTime = system_clock::now();
   auto TimeOfUnit =
       duration_cast<seconds>(UnitStopTime - UnitStartTime).count();
-  if (TimeOfUnit > TimeOfLongestUnitInSeconds) {
+  if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)) &&
+      secondsSinceProcessStartUp() >= 2)
+    PrintStats("pulse ");
+  if (TimeOfUnit > TimeOfLongestUnitInSeconds &&
+      TimeOfUnit >= Options.ReportSlowUnits) {
     TimeOfLongestUnitInSeconds = TimeOfUnit;
-    Printf("Longest unit: %zd s:\n", TimeOfLongestUnitInSeconds);
-    Print(U, "\n");
+    Printf("Slowest unit: %zd s:\n", TimeOfLongestUnitInSeconds);
+    WriteUnitToFileWithPrefix(U, "slow-unit-");
   }
   return Res;
 }
 
-void Fuzzer::RunOneAndUpdateCorpus(const Unit &U) {
+void Fuzzer::RunOneAndUpdateCorpus(Unit &U) {
   if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
     return;
-  ReportNewCoverage(RunOne(U), U);
+  if (Options.OnlyASCII)
+    ToASCII(U);
+  if (RunOne(U))
+    ReportNewCoverage(U);
 }
 
-static uintptr_t HashOfArrayOfPCs(uintptr_t *PCs, uintptr_t NumPCs) {
-  uintptr_t Res = 0;
-  for (uintptr_t i = 0; i < NumPCs; i++) {
-    Res = (Res + PCs[i]) * 7;
-  }
-  return Res;
+void Fuzzer::ExecuteCallback(const Unit &U) {
+  const uint8_t *Data = U.data();
+  uint8_t EmptyData;
+  if (!Data) 
+    Data = &EmptyData;
+  int Res = USF.TargetFunction(Data, U.size());
+  (void)Res;
+  assert(Res == 0);
 }
 
-Unit Fuzzer::SubstituteTokens(const Unit &U) const {
-  Unit Res;
-  for (auto Idx : U) {
-    if (Idx < Options.Tokens.size()) {
-      std::string Token = Options.Tokens[Idx];
-      Res.insert(Res.end(), Token.begin(), Token.end());
-    } else {
-      Res.push_back(' ');
-    }
-  }
-  // FIXME: Apply DFSan labels.
-  return Res;
+size_t Fuzzer::RecordBlockCoverage() {
+  CHECK_WEAK_API_FUNCTION(__sanitizer_get_total_unique_coverage);
+  return LastRecordedBlockCoverage = __sanitizer_get_total_unique_coverage();
 }
 
-void Fuzzer::ExecuteCallback(const Unit &U) {
-  if (Options.Tokens.empty()) {
-    USF.TargetFunction(U.data(), U.size());
-  } else {
-    auto T = SubstituteTokens(U);
-    USF.TargetFunction(T.data(), T.size());
-  }
+size_t Fuzzer::RecordCallerCalleeCoverage() {
+  if (!Options.UseIndirCalls)
+    return 0;
+  if (!__sanitizer_get_total_unique_caller_callee_pairs)
+    return 0;
+  return LastRecordedCallerCalleeCoverage =
+             __sanitizer_get_total_unique_caller_callee_pairs();
 }
 
-// Experimental.
-// Fuly reset the current coverage state, run a single unit,
-// compute a hash function from the full coverage set,
-// return non-zero if the hash value is new.
-// This produces tons of new units and as is it's only suitable for small tests,
-// e.g. test/FullCoverageSetTest.cpp. FIXME: make it scale.
-size_t Fuzzer::RunOneMaximizeFullCoverageSet(const Unit &U) {
-  __sanitizer_reset_coverage();
-  ExecuteCallback(U);
-  uintptr_t *PCs;
-  uintptr_t NumPCs =__sanitizer_get_coverage_guards(&PCs);
-  if (FullCoverageSets.insert(HashOfArrayOfPCs(PCs, NumPCs)).second)
-    return FullCoverageSets.size();
-  return 0;
-}
-
-size_t Fuzzer::RunOneMaximizeTotalCoverage(const Unit &U) {
-  size_t NumCounters = __sanitizer_get_number_of_counters();
+void Fuzzer::PrepareCoverageBeforeRun() {
   if (Options.UseCounters) {
+    size_t NumCounters = __sanitizer_get_number_of_counters();
     CounterBitmap.resize(NumCounters);
     __sanitizer_update_counter_bitset_and_clear_counters(0);
   }
-  size_t OldCoverage = __sanitizer_get_total_unique_coverage();
-  ExecuteCallback(U);
-  size_t NewCoverage = __sanitizer_get_total_unique_coverage();
+  RecordBlockCoverage();
+  RecordCallerCalleeCoverage();
+}
+
+bool Fuzzer::CheckCoverageAfterRun() {
+  size_t OldCoverage = LastRecordedBlockCoverage;
+  size_t NewCoverage = RecordBlockCoverage();
+  size_t OldCallerCalleeCoverage = LastRecordedCallerCalleeCoverage;
+  size_t NewCallerCalleeCoverage = RecordCallerCalleeCoverage();
   size_t NumNewBits = 0;
   if (Options.UseCounters)
     NumNewBits = __sanitizer_update_counter_bitset_and_clear_counters(
         CounterBitmap.data());
-
-  if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)) && Options.Verbosity)
-    PrintStats("pulse ", NewCoverage);
-
-  if (NewCoverage > OldCoverage || NumNewBits)
-    return NewCoverage;
-  return 0;
+  return NewCoverage > OldCoverage ||
+         NewCallerCalleeCoverage > OldCallerCalleeCoverage || NumNewBits;
 }
 
 void Fuzzer::WriteToOutputCorpus(const Unit &U) {
@@ -246,13 +290,20 @@ void Fuzzer::WriteToOutputCorpus(const Unit &U) {
   WriteToFile(U, Path);
   if (Options.Verbosity >= 2)
     Printf("Written to %s\n", Path.c_str());
+  assert(!Options.OnlyASCII || IsASCII(U));
 }
 
-void Fuzzer::WriteToCrash(const Unit &U, const char *Prefix) {
-  std::string Path = Prefix + Hash(U);
+void Fuzzer::WriteUnitToFileWithPrefix(const Unit &U, const char *Prefix) {
+  if (!Options.SaveArtifacts)
+    return;
+  std::string Path = Options.ArtifactPrefix + Prefix + Hash(U);
+  if (!Options.ExactArtifactPath.empty())
+    Path = Options.ExactArtifactPath;  // Overrides ArtifactPrefix.
   WriteToFile(U, Path);
-  Printf("CRASHED; file written to %s\nBase64: ", Path.c_str());
-  PrintFileAsBase64(Path);
+  Printf("artifact_prefix='%s'; Test unit written to %s\n",
+         Options.ArtifactPrefix.c_str(), Path.c_str());
+  if (U.size() <= kMaxUnitSizeToPrint)
+    Printf("Base64: %s\n", Base64(U).c_str());
 }
 
 void Fuzzer::SaveCorpus() {
@@ -264,70 +315,181 @@ void Fuzzer::SaveCorpus() {
            Options.OutputCorpus.c_str());
 }
 
-void Fuzzer::ReportNewCoverage(size_t NewCoverage, const Unit &U) {
-  if (!NewCoverage) return;
-  Corpus.push_back(U);
-  UnitHashesAddedToCorpus.insert(Hash(U));
-  PrintStats("NEW   ", NewCoverage, "");
+void Fuzzer::PrintStatusForNewUnit(const Unit &U) {
+  if (!Options.PrintNEW)
+    return;
+  PrintStats("NEW   ", "");
   if (Options.Verbosity) {
-    Printf(" L: %zd", U.size());
-    if (U.size() < 30) {
-      Printf(" ");
-      PrintUnitInASCIIOrTokens(U, "\t");
-      Print(U);
-    }
+    Printf(" L: %zd ", U.size());
+    USF.PrintMutationSequence();
     Printf("\n");
   }
+}
+
+void Fuzzer::ReportNewCoverage(const Unit &U) {
+  Corpus.push_back(U);
+  UnitHashesAddedToCorpus.insert(Hash(U));
+  PrintStatusForNewUnit(U);
   WriteToOutputCorpus(U);
   if (Options.ExitOnFirst)
     exit(0);
 }
 
-void Fuzzer::MutateAndTestOne(Unit *U) {
+void Fuzzer::Merge(const std::vector<std::string> &Corpora) {
+  if (Corpora.size() <= 1) {
+    Printf("Merge requires two or more corpus dirs\n");
+    return;
+  }
+  auto InitialCorpusDir = Corpora[0];
+  ReadDir(InitialCorpusDir, nullptr);
+  Printf("Merge: running the initial corpus '%s' of %d units\n",
+         InitialCorpusDir.c_str(), Corpus.size());
+  for (auto &U : Corpus)
+    RunOne(U);
+
+  std::vector<std::string> ExtraCorpora(Corpora.begin() + 1, Corpora.end());
+
+  size_t NumTried = 0;
+  size_t NumMerged = 0;
+  for (auto &C : ExtraCorpora) {
+    Corpus.clear();
+    ReadDir(C, nullptr);
+    Printf("Merge: merging the extra corpus '%s' of %zd units\n", C.c_str(),
+           Corpus.size());
+    for (auto &U : Corpus) {
+      NumTried++;
+      if (RunOne(U)) {
+        WriteToOutputCorpus(U);
+        NumMerged++;
+      }
+    }
+  }
+  Printf("Merge: written %zd out of %zd units\n", NumMerged, NumTried);
+}
+
+void Fuzzer::MutateAndTestOne() {
+  auto &U = CurrentUnit;
+  USF.StartMutationSequence();
+
+  U = ChooseUnitToMutate();
+
   for (int i = 0; i < Options.MutateDepth; i++) {
     StartTraceRecording();
-    size_t Size = U->size();
-    U->resize(Options.MaxLen);
-    size_t NewSize = USF.Mutate(U->data(), Size, U->size());
+    size_t Size = U.size();
+    U.resize(Options.MaxLen);
+    size_t NewSize = USF.Mutate(U.data(), Size, U.size());
     assert(NewSize > 0 && "Mutator returned empty unit");
     assert(NewSize <= (size_t)Options.MaxLen &&
            "Mutator return overisized unit");
-    U->resize(NewSize);
-    RunOneAndUpdateCorpus(*U);
+    U.resize(NewSize);
+    RunOneAndUpdateCorpus(U);
     size_t NumTraceBasedMutations = StopTraceRecording();
-    for (size_t j = 0; j < NumTraceBasedMutations; j++) {
-      ApplyTraceBasedMutation(j, U);
-      RunOneAndUpdateCorpus(*U);
+    size_t TBMWidth =
+        std::min((size_t)Options.TBMWidth, NumTraceBasedMutations);
+    size_t TBMDepth =
+        std::min((size_t)Options.TBMDepth, NumTraceBasedMutations);
+    Unit BackUp = U;
+    for (size_t w = 0; w < TBMWidth; w++) {
+      U = BackUp;
+      for (size_t d = 0; d < TBMDepth; d++) {
+        TotalNumberOfExecutedTraceBasedMutations++;
+        ApplyTraceBasedMutation(USF.GetRand()(NumTraceBasedMutations), &U);
+        RunOneAndUpdateCorpus(U);
+      }
+    }
+  }
+}
+
+// Returns an index of random unit from the corpus to mutate.
+// Hypothesis: units added to the corpus last are more likely to be interesting.
+// This function gives more wieght to the more recent units.
+size_t Fuzzer::ChooseUnitIdxToMutate() {
+    size_t N = Corpus.size();
+    size_t Total = (N + 1) * N / 2;
+    size_t R = USF.GetRand()(Total);
+    size_t IdxBeg = 0, IdxEnd = N;
+    // Binary search.
+    while (IdxEnd - IdxBeg >= 2) {
+      size_t Idx = IdxBeg + (IdxEnd - IdxBeg) / 2;
+      if (R > (Idx + 1) * Idx / 2)
+        IdxBeg = Idx;
+      else
+        IdxEnd = Idx;
+    }
+    assert(IdxBeg < N);
+    return IdxBeg;
+}
+
+// Experimental search heuristic: drilling.
+// - Read, shuffle, execute and minimize the corpus.
+// - Choose one random unit.
+// - Reset the coverage.
+// - Start fuzzing as if the chosen unit was the only element of the corpus.
+// - When done, reset the coverage again.
+// - Merge the newly created corpus into the original one.
+void Fuzzer::Drill() {
+  // The corpus is already read, shuffled, and minimized.
+  assert(!Corpus.empty());
+  Options.PrintNEW = false;  // Don't print NEW status lines when drilling.
+
+  Unit U = ChooseUnitToMutate();
+
+  CHECK_WEAK_API_FUNCTION(__sanitizer_reset_coverage);
+  __sanitizer_reset_coverage();
+
+  std::vector<Unit> SavedCorpus;
+  SavedCorpus.swap(Corpus);
+  Corpus.push_back(U);
+  assert(Corpus.size() == 1);
+  RunOne(U);
+  PrintStats("DRILL ");
+  std::string SavedOutputCorpusPath; // Don't write new units while drilling.
+  SavedOutputCorpusPath.swap(Options.OutputCorpus);
+  Loop();
+
+  __sanitizer_reset_coverage();
+
+  PrintStats("REINIT");
+  SavedOutputCorpusPath.swap(Options.OutputCorpus);
+  for (auto &U : SavedCorpus)
+    RunOne(U);
+  PrintStats("MERGE ");
+  Options.PrintNEW = true;
+  size_t NumMerged = 0;
+  for (auto &U : Corpus) {
+    if (RunOne(U)) {
+      PrintStatusForNewUnit(U);
+      NumMerged++;
+      WriteToOutputCorpus(U);
     }
   }
+  PrintStats("MERGED");
+  if (NumMerged && Options.Verbosity)
+    Printf("Drilling discovered %zd new units\n", NumMerged);
 }
 
-void Fuzzer::Loop(size_t NumIterations) {
-  for (size_t i = 1; i <= NumIterations; i++) {
-    for (size_t J1 = 0; J1 < Corpus.size(); J1++) {
-      SyncCorpus();
+void Fuzzer::Loop() {
+  system_clock::time_point LastCorpusReload = system_clock::now();
+  if (Options.DoCrossOver)
+    USF.SetCorpus(&Corpus);
+  while (true) {
+    SyncCorpus();
+    auto Now = system_clock::now();
+    if (duration_cast<seconds>(Now - LastCorpusReload).count()) {
       RereadOutputCorpus();
-      if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
-        return;
-      // First, simply mutate the unit w/o doing crosses.
-      CurrentUnit = Corpus[J1];
-      MutateAndTestOne(&CurrentUnit);
-      // Now, cross with others.
-      if (Options.DoCrossOver && !Corpus[J1].empty()) {
-        for (size_t J2 = 0; J2 < Corpus.size(); J2++) {
-          CurrentUnit.resize(Options.MaxLen);
-          size_t NewSize = USF.CrossOver(
-              Corpus[J1].data(), Corpus[J1].size(), Corpus[J2].data(),
-              Corpus[J2].size(), CurrentUnit.data(), CurrentUnit.size());
-          assert(NewSize > 0 && "CrossOver returned empty unit");
-          assert(NewSize <= (size_t)Options.MaxLen &&
-                 "CrossOver return overisized unit");
-          CurrentUnit.resize(NewSize);
-          MutateAndTestOne(&CurrentUnit);
-        }
-      }
+      LastCorpusReload = Now;
     }
+    if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
+      break;
+    if (Options.MaxTotalTimeSec > 0 &&
+        secondsSinceProcessStartUp() >
+        static_cast<size_t>(Options.MaxTotalTimeSec))
+      break;
+    // Perform several mutations and runs.
+    MutateAndTestOne();
   }
+
+  PrintStats("DONE  ", "\n");
 }
 
 void Fuzzer::SyncCorpus() {