X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=blobdiff_plain;f=lib%2FFuzzer%2FFuzzerLoop.cpp;h=5237682ff24d58001e1d9159133dc792050874c5;hp=33a09d60829ccd4eeaafc54a793d23ca26df0266;hb=43a24b5d9351abd97a0e2378cd167bb9ee6d9a8d;hpb=29c6bd6e4ff84956837f464785416bf74f7a8e05

diff --git a/lib/Fuzzer/FuzzerLoop.cpp b/lib/Fuzzer/FuzzerLoop.cpp
index 33a09d60829..5237682ff24 100644
--- a/lib/Fuzzer/FuzzerLoop.cpp
+++ b/lib/Fuzzer/FuzzerLoop.cpp
@@ -10,158 +10,294 @@
 //===----------------------------------------------------------------------===//
 
 #include "FuzzerInternal.h"
-#include <sanitizer/coverage_interface.h>
 #include <algorithm>
-#include <iostream>
+
+#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);
+__attribute__((weak)) uintptr_t
+__sanitizer_get_coverage_pc_buffer(uintptr_t **data);
+}
 
 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);
+}
 
-// static
-Unit Fuzzer::CurrentUnit;
-system_clock::time_point Fuzzer::UnitStartTime;
+#define CHECK_WEAK_API_FUNCTION(fn)                                            \
+  do {                                                                         \
+    if (!fn)                                                                   \
+      MissingWeakApiFunction(#fn);                                             \
+  } while (false)
+
+// Only one Fuzzer per process.
+static Fuzzer *F;
+
+Fuzzer::Fuzzer(UserSuppliedFuzzer &USF, FuzzingOptions Options)
+    : USF(USF), Options(Options) {
+  SetDeathCallback();
+  InitializeTraceState();
+  assert(!F);
+  F = this;
+}
 
 void Fuzzer::SetDeathCallback() {
-  __sanitizer_set_death_callback(DeathCallback);
+  CHECK_WEAK_API_FUNCTION(__sanitizer_set_death_callback);
+  __sanitizer_set_death_callback(StaticDeathCallback);
+}
+
+void Fuzzer::PrintUnitInASCII(const Unit &U, const char *PrintAfter) {
+  PrintASCII(U, PrintAfter);
+}
+
+void Fuzzer::StaticDeathCallback() {
+  assert(F);
+  F->DeathCallback();
 }
 
 void Fuzzer::DeathCallback() {
-  std::cerr << "DEATH: " <<  std::endl;
-  Print(CurrentUnit, "\n");
-  PrintASCII(CurrentUnit, "\n");
-  WriteToCrash(CurrentUnit, "crash-");
+  Printf("DEATH:\n");
+  if (CurrentUnit.size() <= kMaxUnitSizeToPrint) {
+    Print(CurrentUnit, "\n");
+    PrintUnitInASCII(CurrentUnit, "\n");
+  }
+  WriteUnitToFileWithPrefix(CurrentUnit, "crash-");
+}
+
+void Fuzzer::StaticAlarmCallback() {
+  assert(F);
+  F->AlarmCallback();
 }
 
 void Fuzzer::AlarmCallback() {
+  assert(Options.UnitTimeoutSec > 0);
   size_t Seconds =
       duration_cast<seconds>(system_clock::now() - UnitStartTime).count();
-  std::cerr << "ALARM: working on the last Unit for " << Seconds << " seconds"
-            << std::endl;
-  if (Seconds >= 3) {
-    Print(CurrentUnit, "\n");
-    PrintASCII(CurrentUnit, "\n");
-    WriteToCrash(CurrentUnit, "timeout-");
+  if (Seconds == 0) return;
+  if (Options.Verbosity >= 2)
+    Printf("AlarmCallback %zd\n", Seconds);
+  if (Seconds >= (size_t)Options.UnitTimeoutSec) {
+    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);
+    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, const char *End) {
+  size_t Seconds = secondsSinceProcessStartUp();
+  size_t ExecPerSec = (Seconds ? TotalNumberOfRuns / Seconds : 0);
+
+  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() {
+  if (Options.OutputCorpus.empty()) return;
+  std::vector<Unit> AdditionalCorpus;
+  ReadDirToVectorOfUnits(Options.OutputCorpus.c_str(), &AdditionalCorpus,
+                         &EpochOfLastReadOfOutputCorpus);
+  if (Corpus.empty()) {
+    Corpus = AdditionalCorpus;
+    return;
+  }
+  if (!Options.Reload) return;
+  if (Options.Verbosity >= 2)
+    Printf("Reload: read %zd new units.\n",  AdditionalCorpus.size());
+  for (auto &X : AdditionalCorpus) {
+    if (X.size() > (size_t)Options.MaxLen)
+      X.resize(Options.MaxLen);
+    if (UnitHashesAddedToCorpus.insert(Hash(X)).second) {
+      CurrentUnit.clear();
+      CurrentUnit.insert(CurrentUnit.begin(), X.begin(), X.end());
+      if (RunOne(CurrentUnit)) {
+        Corpus.push_back(X);
+        PrintStats("RELOAD");
+      }
+    }
   }
-  exit(1);
 }
 
 void Fuzzer::ShuffleAndMinimize() {
-  bool PreferSmall =
-      (Options.PreferSmallDuringInitialShuffle == 1 ||
-       (Options.PreferSmallDuringInitialShuffle == -1 && rand() % 2));
+  bool PreferSmall = (Options.PreferSmallDuringInitialShuffle == 1 ||
+                      (Options.PreferSmallDuringInitialShuffle == -1 &&
+                       USF.GetRand().RandBool()));
   if (Options.Verbosity)
-    std::cerr << "Shuffle: Size: " << Corpus.size()
-              << " prefer small: " << PreferSmall
-              << "\n";
+    Printf("PreferSmall: %d\n", PreferSmall);
+  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(); });
-  size_t MaxCov = 0;
+  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)
-          std::cerr << "NEW0: " << NewCoverage
-                    << " L " << U.size()
-                    << "\n";
+          Printf("NEW0: %zd L %zd\n", LastRecordedBlockCoverage, U.size());
       }
     }
   }
   Corpus = NewCorpus;
-  if (Options.Verbosity)
-    std::cerr << "Shuffle done: " << Corpus.size() << " IC: " << MaxCov << "\n";
+  for (auto &X : Corpus)
+    UnitHashesAddedToCorpus.insert(Hash(X));
+  PrintStats("INITED");
 }
 
-size_t Fuzzer::RunOne(const Unit &U) {
+bool Fuzzer::RunOne(const Unit &U) {
   UnitStartTime = system_clock::now();
   TotalNumberOfRuns++;
-  if (Options.UseFullCoverageSet)
-    return RunOneMaximizeFullCoverageSet(U);
-  if (Options.UseCoveragePairs)
-    return RunOneMaximizeCoveragePairs(U);
-  return RunOneMaximizeTotalCoverage(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;
+  PrepareCoverageBeforeRun();
+  ExecuteCallback(U);
+  bool Res = CheckCoverageAfterRun();
+
+  auto UnitStopTime = system_clock::now();
+  auto TimeOfUnit =
+      duration_cast<seconds>(UnitStopTime - UnitStartTime).count();
+  if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)) &&
+      secondsSinceProcessStartUp() >= 2)
+    PrintStats("pulse ");
+  if (TimeOfUnit > TimeOfLongestUnitInSeconds &&
+      TimeOfUnit >= Options.ReportSlowUnits) {
+    TimeOfLongestUnitInSeconds = TimeOfUnit;
+    Printf("Slowest unit: %zd s:\n", TimeOfLongestUnitInSeconds);
+    WriteUnitToFileWithPrefix(U, "slow-unit-");
   }
   return Res;
 }
 
-// Experimental. Does not yet scale.
-// Fuly reset the current coverage state, run a single unit,
-// collect all coverage pairs and return non-zero if a new pair is observed.
-size_t Fuzzer::RunOneMaximizeCoveragePairs(const Unit &U) {
-  __sanitizer_reset_coverage();
-  Callback(U.data(), U.size());
-  uintptr_t *PCs;
-  uintptr_t NumPCs = __sanitizer_get_coverage_guards(&PCs);
-  bool HasNewPairs = false;
-  for (uintptr_t i = 0; i < NumPCs; i++) {
-    if (!PCs[i]) continue;
-    for (uintptr_t j = 0; j < NumPCs; j++) {
-      if (!PCs[j]) continue;
-      uint64_t Pair = (i << 32) | j;
-      HasNewPairs |= CoveragePairs.insert(Pair).second;
-    }
+void Fuzzer::RunOneAndUpdateCorpus(Unit &U) {
+  if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
+    return;
+  if (Options.OnlyASCII)
+    ToASCII(U);
+  if (RunOne(U))
+    ReportNewCoverage(U);
+}
+
+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);
+}
+
+size_t Fuzzer::RecordBlockCoverage() {
+  CHECK_WEAK_API_FUNCTION(__sanitizer_get_total_unique_coverage);
+  uintptr_t PrevCoverage = LastRecordedBlockCoverage;
+  LastRecordedBlockCoverage = __sanitizer_get_total_unique_coverage();
+
+  if (PrevCoverage == LastRecordedBlockCoverage || !Options.PrintNewCovPcs)
+    return LastRecordedBlockCoverage;
+
+  uintptr_t PrevBufferLen = LastCoveragePcBufferLen;
+  uintptr_t *CoverageBuf;
+  LastCoveragePcBufferLen = __sanitizer_get_coverage_pc_buffer(&CoverageBuf);
+  assert(CoverageBuf);
+  for (size_t i = PrevBufferLen; i < LastCoveragePcBufferLen; ++i) {
+    Printf("0x%x\n", CoverageBuf[i]);
   }
-  if (HasNewPairs)
-    return CoveragePairs.size();
-  return 0;
+
+  return LastRecordedBlockCoverage;
 }
 
-// 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();
-  Callback(U.data(), U.size());
-  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::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();
 }
 
-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();
-  Callback(U.data(), U.size());
-  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) {
-    size_t Seconds = secondsSinceProcessStartUp();
-    std::cerr
-        << "#" << TotalNumberOfRuns
-        << "\tcov: " << NewCoverage
-        << "\tbits: " << TotalBits()
-        << "\texec/s: " << (Seconds ? TotalNumberOfRuns / Seconds : 0) << "\n";
-  }
-  if (NewCoverage > OldCoverage || NumNewBits)
-    return NewCoverage;
-  return 0;
+  return NewCoverage > OldCoverage ||
+         NewCallerCalleeCoverage > OldCallerCalleeCoverage || NumNewBits;
 }
 
 void Fuzzer::WriteToOutputCorpus(const Unit &U) {
@@ -169,13 +305,21 @@ void Fuzzer::WriteToOutputCorpus(const Unit &U) {
   std::string Path = DirPlusFile(Options.OutputCorpus, Hash(U));
   WriteToFile(U, Path);
   if (Options.Verbosity >= 2)
-    std::cerr << "Written to " << Path << std::endl;
+    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);
-  std::cerr << "CRASHED; file written to " << Path << std::endl;
+  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() {
@@ -183,64 +327,186 @@ void Fuzzer::SaveCorpus() {
   for (const auto &U : Corpus)
     WriteToFile(U, DirPlusFile(Options.OutputCorpus, Hash(U)));
   if (Options.Verbosity)
-    std::cerr << "Written corpus of " << Corpus.size() << " files to "
-              << Options.OutputCorpus << "\n";
+    Printf("Written corpus of %zd files to %s\n", Corpus.size(),
+           Options.OutputCorpus.c_str());
 }
 
-size_t Fuzzer::MutateAndTestOne(Unit *U) {
-  size_t NewUnits = 0;
-  for (int i = 0; i < Options.MutateDepth; i++) {
-    if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
-      return NewUnits;
-    MutateWithDFSan(U);
-    Mutate(U, Options.MaxLen);
-    size_t NewCoverage = RunOne(*U);
-    if (NewCoverage) {
-      Corpus.push_back(*U);
-      NewUnits++;
-      if (Options.Verbosity) {
-        std::cerr << "#" << TotalNumberOfRuns
-                  << "\tNEW: " << NewCoverage
-                  << " B: " << TotalBits()
-                  << " L: " << U->size()
-                  << " S: " << Corpus.size()
-                  << " I: " << i
-                  << "\t";
-        if (U->size() < 30) {
-          PrintASCII(*U);
-          std::cerr << "\t";
-          Print(*U);
-        }
-        std::cerr << "\n";
+void Fuzzer::PrintStatusForNewUnit(const Unit &U) {
+  if (!Options.PrintNEW)
+    return;
+  PrintStats("NEW   ", "");
+  if (Options.Verbosity) {
+    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::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++;
       }
-      WriteToOutputCorpus(*U);
-      if (Options.ExitOnFirst)
-        exit(0);
     }
   }
-  return NewUnits;
-}
-
-size_t Fuzzer::Loop(size_t NumIterations) {
-  size_t NewUnits = 0;
-  for (size_t i = 1; i <= NumIterations; i++) {
-    for (size_t J1 = 0; J1 < Corpus.size(); J1++) {
-      if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
-        return NewUnits;
-      // First, simply mutate the unit w/o doing crosses.
-      CurrentUnit = Corpus[J1];
-      NewUnits += MutateAndTestOne(&CurrentUnit);
-      // Now, cross with others.
-      if (Options.DoCrossOver) {
-        for (size_t J2 = 0; J2 < Corpus.size(); J2++) {
-          CurrentUnit.clear();
-          CrossOver(Corpus[J1], Corpus[J2], &CurrentUnit, Options.MaxLen);
-          NewUnits += MutateAndTestOne(&CurrentUnit);
-        }
-      }
+  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++) {
+    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);
+    if (i == 0)
+      StartTraceRecording();
+    RunOneAndUpdateCorpus(U);
+    StopTraceRecording();
+  }
+}
+
+// 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) {
+    CurrentUnit = U;
+    RunOne(U);
+  }
+  PrintStats("MERGE ");
+  Options.PrintNEW = true;
+  size_t NumMerged = 0;
+  for (auto &U : Corpus) {
+    CurrentUnit = U;
+    if (RunOne(U)) {
+      PrintStatusForNewUnit(U);
+      NumMerged++;
+      WriteToOutputCorpus(U);
     }
   }
-  return NewUnits;
+  PrintStats("MERGED");
+  if (NumMerged && Options.Verbosity)
+    Printf("Drilling discovered %zd new units\n", NumMerged);
+}
+
+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();
+      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() {
+  if (Options.SyncCommand.empty() || Options.OutputCorpus.empty()) return;
+  auto Now = system_clock::now();
+  if (duration_cast<seconds>(Now - LastExternalSync).count() <
+      Options.SyncTimeout)
+    return;
+  LastExternalSync = Now;
+  ExecuteCommand(Options.SyncCommand + " " + Options.OutputCorpus);
 }
 
 }  // namespace fuzzer