lib/Fuzzer/FuzzerMutate.cpp

   1 //===- FuzzerMutate.cpp - Mutate a test input -----------------------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 // Mutate a test input.
  10 //===----------------------------------------------------------------------===//
  11
  12 #include "FuzzerMutate.h"
  13 #include "FuzzerCorpus.h"
  14 #include "FuzzerDefs.h"
  15 #include "FuzzerExtFunctions.h"
  16 #include "FuzzerIO.h"
  17 #include "FuzzerOptions.h"
  18
  19 namespace fuzzer {
  20
  21 const size_t Dictionary::kMaxDictSize;
  22
  23 static void PrintASCII(const Word &W, const char *PrintAfter) {
  24   PrintASCII(W.data(), W.size(), PrintAfter);
  25 }
  26
  27 MutationDispatcher::MutationDispatcher(Random &Rand,
  28                                        const FuzzingOptions &Options)
  29     : Rand(Rand), Options(Options) {
  30   DefaultMutators.insert(
  31       DefaultMutators.begin(),
  32       {
  33           {&MutationDispatcher::Mutate_EraseBytes, "EraseBytes"},
  34           {&MutationDispatcher::Mutate_InsertByte, "InsertByte"},
  35           {&MutationDispatcher::Mutate_InsertRepeatedBytes,
  36            "InsertRepeatedBytes"},
  37           {&MutationDispatcher::Mutate_ChangeByte, "ChangeByte"},
  38           {&MutationDispatcher::Mutate_ChangeBit, "ChangeBit"},
  39           {&MutationDispatcher::Mutate_ShuffleBytes, "ShuffleBytes"},
  40           {&MutationDispatcher::Mutate_ChangeASCIIInteger, "ChangeASCIIInt"},
  41           {&MutationDispatcher::Mutate_ChangeBinaryInteger, "ChangeBinInt"},
  42           {&MutationDispatcher::Mutate_CopyPart, "CopyPart"},
  43           {&MutationDispatcher::Mutate_CrossOver, "CrossOver"},
  44           {&MutationDispatcher::Mutate_AddWordFromManualDictionary,
  45            "ManualDict"},
  46           {&MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary,
  47            "PersAutoDict"},
  48       });
  49   if(Options.UseCmp)
  50     DefaultMutators.push_back(
  51         {&MutationDispatcher::Mutate_AddWordFromTORC, "CMP"});
  52
  53   if (EF->LLVMFuzzerCustomMutator)
  54     Mutators.push_back({&MutationDispatcher::Mutate_Custom, "Custom"});
  55   else
  56     Mutators = DefaultMutators;
  57
  58   if (EF->LLVMFuzzerCustomCrossOver)
  59     Mutators.push_back(
  60         {&MutationDispatcher::Mutate_CustomCrossOver, "CustomCrossOver"});
  61 }
  62
  63 static char RandCh(Random &Rand) {
  64   if (Rand.RandBool()) return Rand(256);
  65   const char *Special = "!*'();:@&=+$,/?%#[]012Az-`~.\xff\x00";
  66   return Special[Rand(sizeof(Special) - 1)];
  67 }
  68
  69 size_t MutationDispatcher::Mutate_Custom(uint8_t *Data, size_t Size,
  70                                          size_t MaxSize) {
  71   return EF->LLVMFuzzerCustomMutator(Data, Size, MaxSize, Rand.Rand());
  72 }
  73
  74 size_t MutationDispatcher::Mutate_CustomCrossOver(uint8_t *Data, size_t Size,
  75                                                   size_t MaxSize) {
  76   if (!Corpus || Corpus->size() < 2 || Size == 0)
  77     return 0;
  78   size_t Idx = Rand(Corpus->size());
  79   const Unit &Other = (*Corpus)[Idx];
  80   if (Other.empty())
  81     return 0;
  82   CustomCrossOverInPlaceHere.resize(MaxSize);
  83   auto &U = CustomCrossOverInPlaceHere;
  84   size_t NewSize = EF->LLVMFuzzerCustomCrossOver(
  85       Data, Size, Other.data(), Other.size(), U.data(), U.size(), Rand.Rand());
  86   if (!NewSize)
  87     return 0;
  88   assert(NewSize <= MaxSize && "CustomCrossOver returned overisized unit");
  89   memcpy(Data, U.data(), NewSize);
  90   return NewSize;
  91 }
  92
  93 size_t MutationDispatcher::Mutate_ShuffleBytes(uint8_t *Data, size_t Size,
  94                                                size_t MaxSize) {
  95   if (Size > MaxSize || Size == 0) return 0;
  96   size_t ShuffleAmount =
  97       Rand(std::min(Size, (size_t)8)) + 1; // [1,8] and <= Size.
  98   size_t ShuffleStart = Rand(Size - ShuffleAmount);
  99   assert(ShuffleStart + ShuffleAmount <= Size);
 100   std::shuffle(Data + ShuffleStart, Data + ShuffleStart + ShuffleAmount, Rand);
 101   return Size;
 102 }
 103
 104 size_t MutationDispatcher::Mutate_EraseBytes(uint8_t *Data, size_t Size,
 105                                              size_t MaxSize) {
 106   if (Size <= 1) return 0;
 107   size_t N = Rand(Size / 2) + 1;
 108   assert(N < Size);
 109   size_t Idx = Rand(Size - N + 1);
 110   // Erase Data[Idx:Idx+N].
 111   memmove(Data + Idx, Data + Idx + N, Size - Idx - N);
 112   // Printf("Erase: %zd %zd => %zd; Idx %zd\n", N, Size, Size - N, Idx);
 113   return Size - N;
 114 }
 115
 116 size_t MutationDispatcher::Mutate_InsertByte(uint8_t *Data, size_t Size,
 117                                              size_t MaxSize) {
 118   if (Size >= MaxSize) return 0;
 119   size_t Idx = Rand(Size + 1);
 120   // Insert new value at Data[Idx].
 121   memmove(Data + Idx + 1, Data + Idx, Size - Idx);
 122   Data[Idx] = RandCh(Rand);
 123   return Size + 1;
 124 }
 125
 126 size_t MutationDispatcher::Mutate_InsertRepeatedBytes(uint8_t *Data,
 127                                                       size_t Size,
 128                                                       size_t MaxSize) {
 129   const size_t kMinBytesToInsert = 3;
 130   if (Size + kMinBytesToInsert >= MaxSize) return 0;
 131   size_t MaxBytesToInsert = std::min(MaxSize - Size, (size_t)128);
 132   size_t N = Rand(MaxBytesToInsert - kMinBytesToInsert + 1) + kMinBytesToInsert;
 133   assert(Size + N <= MaxSize && N);
 134   size_t Idx = Rand(Size + 1);
 135   // Insert new values at Data[Idx].
 136   memmove(Data + Idx + N, Data + Idx, Size - Idx);
 137   // Give preference to 0x00 and 0xff.
 138   uint8_t Byte = Rand.RandBool() ? Rand(256) : (Rand.RandBool() ? 0 : 255);
 139   for (size_t i = 0; i < N; i++)
 140     Data[Idx + i] = Byte;
 141   return Size + N;
 142 }
 143
 144 size_t MutationDispatcher::Mutate_ChangeByte(uint8_t *Data, size_t Size,
 145                                              size_t MaxSize) {
 146   if (Size > MaxSize) return 0;
 147   size_t Idx = Rand(Size);
 148   Data[Idx] = RandCh(Rand);
 149   return Size;
 150 }
 151
 152 size_t MutationDispatcher::Mutate_ChangeBit(uint8_t *Data, size_t Size,
 153                                             size_t MaxSize) {
 154   if (Size > MaxSize) return 0;
 155   size_t Idx = Rand(Size);
 156   Data[Idx] ^= 1 << Rand(8);
 157   return Size;
 158 }
 159
 160 size_t MutationDispatcher::Mutate_AddWordFromManualDictionary(uint8_t *Data,
 161                                                               size_t Size,
 162                                                               size_t MaxSize) {
 163   return AddWordFromDictionary(ManualDictionary, Data, Size, MaxSize);
 164 }
 165
 166 size_t MutationDispatcher::ApplyDictionaryEntry(uint8_t *Data, size_t Size,
 167                                                 size_t MaxSize,
 168                                                 DictionaryEntry &DE) {
 169   const Word &W = DE.GetW();
 170   bool UsePositionHint = DE.HasPositionHint() &&
 171                          DE.GetPositionHint() + W.size() < Size &&
 172                          Rand.RandBool();
 173   if (Rand.RandBool()) {  // Insert W.
 174     if (Size + W.size() > MaxSize) return 0;
 175     size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size + 1);
 176     memmove(Data + Idx + W.size(), Data + Idx, Size - Idx);
 177     memcpy(Data + Idx, W.data(), W.size());
 178     Size += W.size();
 179   } else {  // Overwrite some bytes with W.
 180     if (W.size() > Size) return 0;
 181     size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size - W.size());
 182     memcpy(Data + Idx, W.data(), W.size());
 183   }
 184   return Size;
 185 }
 186
 187 // Somewhere in the past we have observed a comparison instructions
 188 // with arguments Arg1 Arg2. This function tries to guess a dictionary
 189 // entry that will satisfy that comparison.
 190 // It first tries to find one of the arguments (possibly swapped) in the
 191 // input and if it succeeds it creates a DE with a position hint.
 192 // Otherwise it creates a DE with one of the arguments w/o a position hint.
 193 DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
 194     const void *Arg1, const void *Arg2,
 195     const void *Arg1Mutation, const void *Arg2Mutation,
 196     size_t ArgSize, const uint8_t *Data,
 197     size_t Size) {
 198   ScopedDoingMyOwnMemOrStr scoped_doing_my_own_mem_os_str;
 199   bool HandleFirst = Rand.RandBool();
 200   const void *ExistingBytes, *DesiredBytes;
 201   Word W;
 202   const uint8_t *End = Data + Size;
 203   for (int Arg = 0; Arg < 2; Arg++) {
 204     ExistingBytes = HandleFirst ? Arg1 : Arg2;
 205     DesiredBytes = HandleFirst ? Arg2Mutation : Arg1Mutation;
 206     HandleFirst = !HandleFirst;
 207     W.Set(reinterpret_cast<const uint8_t*>(DesiredBytes), ArgSize);
 208     const size_t kMaxNumPositions = 8;
 209     size_t Positions[kMaxNumPositions];
 210     size_t NumPositions = 0;
 211     for (const uint8_t *Cur = Data;
 212          Cur < End && NumPositions < kMaxNumPositions; Cur++) {
 213       Cur =
 214           (const uint8_t *)SearchMemory(Cur, End - Cur, ExistingBytes, ArgSize);
 215       if (!Cur) break;
 216       Positions[NumPositions++] = Cur - Data;
 217     }
 218     if (!NumPositions) continue;
 219     return DictionaryEntry(W, Positions[Rand(NumPositions)]);
 220   }
 221   DictionaryEntry DE(W);
 222   return DE;
 223 }
 224
 225
 226 template <class T>
 227 DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
 228     T Arg1, T Arg2, const uint8_t *Data, size_t Size) {
 229   if (Rand.RandBool()) Arg1 = Bswap(Arg1);
 230   if (Rand.RandBool()) Arg2 = Bswap(Arg2);
 231   T Arg1Mutation = Arg1 + Rand(-1, 1);
 232   T Arg2Mutation = Arg2 + Rand(-1, 1);
 233   return MakeDictionaryEntryFromCMP(&Arg1, &Arg2, &Arg1Mutation, &Arg2Mutation,
 234                                     sizeof(Arg1), Data, Size);
 235 }
 236
 237 DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
 238     const Word &Arg1, const Word &Arg2, const uint8_t *Data, size_t Size) {
 239   return MakeDictionaryEntryFromCMP(Arg1.data(), Arg2.data(), Arg1.data(),
 240                                     Arg2.data(), Arg1.size(), Data, Size);
 241 }
 242
 243 size_t MutationDispatcher::Mutate_AddWordFromTORC(
 244     uint8_t *Data, size_t Size, size_t MaxSize) {
 245   Word W;
 246   DictionaryEntry DE;
 247   switch (Rand(4)) {
 248   case 0: {
 249     auto X = TPC.TORC8.Get(Rand.Rand());
 250     DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
 251   } break;
 252   case 1: {
 253     auto X = TPC.TORC4.Get(Rand.Rand());
 254     if ((X.A >> 16) == 0 && (X.B >> 16) == 0 && Rand.RandBool())
 255       DE = MakeDictionaryEntryFromCMP((uint16_t)X.A, (uint16_t)X.B, Data, Size);
 256     else
 257       DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
 258   } break;
 259   case 2: {
 260     auto X = TPC.TORCW.Get(Rand.Rand());
 261     DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
 262   } break;
 263   case 3: if (Options.UseMemmem) {
 264     auto X = TPC.MMT.Get(Rand.Rand());
 265     DE = DictionaryEntry(X);
 266   } break;
 267   default:
 268     assert(0);
 269   }
 270   if (!DE.GetW().size()) return 0;
 271   Size = ApplyDictionaryEntry(Data, Size, MaxSize, DE);
 272   if (!Size) return 0;
 273   DictionaryEntry &DERef =
 274       CmpDictionaryEntriesDeque[CmpDictionaryEntriesDequeIdx++ %
 275                                 kCmpDictionaryEntriesDequeSize];
 276   DERef = DE;
 277   CurrentDictionaryEntrySequence.push_back(&DERef);
 278   return Size;
 279 }
 280
 281 size_t MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary(
 282     uint8_t *Data, size_t Size, size_t MaxSize) {
 283   return AddWordFromDictionary(PersistentAutoDictionary, Data, Size, MaxSize);
 284 }
 285
 286 size_t MutationDispatcher::AddWordFromDictionary(Dictionary &D, uint8_t *Data,
 287                                                  size_t Size, size_t MaxSize) {
 288   if (Size > MaxSize) return 0;
 289   if (D.empty()) return 0;
 290   DictionaryEntry &DE = D[Rand(D.size())];
 291   Size = ApplyDictionaryEntry(Data, Size, MaxSize, DE);
 292   if (!Size) return 0;
 293   DE.IncUseCount();
 294   CurrentDictionaryEntrySequence.push_back(&DE);
 295   return Size;
 296 }
 297
 298 // Overwrites part of To[0,ToSize) with a part of From[0,FromSize).
 299 // Returns ToSize.
 300 size_t MutationDispatcher::CopyPartOf(const uint8_t *From, size_t FromSize,
 301                                       uint8_t *To, size_t ToSize) {
 302   // Copy From[FromBeg, FromBeg + CopySize) into To[ToBeg, ToBeg + CopySize).
 303   size_t ToBeg = Rand(ToSize);
 304   size_t CopySize = Rand(ToSize - ToBeg) + 1;
 305   assert(ToBeg + CopySize <= ToSize);
 306   CopySize = std::min(CopySize, FromSize);
 307   size_t FromBeg = Rand(FromSize - CopySize + 1);
 308   assert(FromBeg + CopySize <= FromSize);
 309   memmove(To + ToBeg, From + FromBeg, CopySize);
 310   return ToSize;
 311 }
 312
 313 // Inserts part of From[0,ToSize) into To.
 314 // Returns new size of To on success or 0 on failure.
 315 size_t MutationDispatcher::InsertPartOf(const uint8_t *From, size_t FromSize,
 316                                         uint8_t *To, size_t ToSize,
 317                                         size_t MaxToSize) {
 318   if (ToSize >= MaxToSize) return 0;
 319   size_t AvailableSpace = MaxToSize - ToSize;
 320   size_t MaxCopySize = std::min(AvailableSpace, FromSize);
 321   size_t CopySize = Rand(MaxCopySize) + 1;
 322   size_t FromBeg = Rand(FromSize - CopySize + 1);
 323   assert(FromBeg + CopySize <= FromSize);
 324   size_t ToInsertPos = Rand(ToSize + 1);
 325   assert(ToInsertPos + CopySize <= MaxToSize);
 326   size_t TailSize = ToSize - ToInsertPos;
 327   if (To == From) {
 328     MutateInPlaceHere.resize(MaxToSize);
 329     memcpy(MutateInPlaceHere.data(), From + FromBeg, CopySize);
 330     memmove(To + ToInsertPos + CopySize, To + ToInsertPos, TailSize);
 331     memmove(To + ToInsertPos, MutateInPlaceHere.data(), CopySize);
 332   } else {
 333     memmove(To + ToInsertPos + CopySize, To + ToInsertPos, TailSize);
 334     memmove(To + ToInsertPos, From + FromBeg, CopySize);
 335   }
 336   return ToSize + CopySize;
 337 }
 338
 339 size_t MutationDispatcher::Mutate_CopyPart(uint8_t *Data, size_t Size,
 340                                            size_t MaxSize) {
 341   if (Size > MaxSize || Size == 0) return 0;
 342   if (Rand.RandBool())
 343     return CopyPartOf(Data, Size, Data, Size);
 344   else
 345     return InsertPartOf(Data, Size, Data, Size, MaxSize);
 346 }
 347
 348 size_t MutationDispatcher::Mutate_ChangeASCIIInteger(uint8_t *Data, size_t Size,
 349                                                      size_t MaxSize) {
 350   if (Size > MaxSize) return 0;
 351   size_t B = Rand(Size);
 352   while (B < Size && !isdigit(Data[B])) B++;
 353   if (B == Size) return 0;
 354   size_t E = B;
 355   while (E < Size && isdigit(Data[E])) E++;
 356   assert(B < E);
 357   // now we have digits in [B, E).
 358   // strtol and friends don't accept non-zero-teminated data, parse it manually.
 359   uint64_t Val = Data[B] - '0';
 360   for (size_t i = B + 1; i < E; i++)
 361     Val = Val * 10 + Data[i] - '0';
 362
 363   // Mutate the integer value.
 364   switch(Rand(5)) {
 365     case 0: Val++; break;
 366     case 1: Val--; break;
 367     case 2: Val /= 2; break;
 368     case 3: Val *= 2; break;
 369     case 4: Val = Rand(Val * Val); break;
 370     default: assert(0);
 371   }
 372   // Just replace the bytes with the new ones, don't bother moving bytes.
 373   for (size_t i = B; i < E; i++) {
 374     size_t Idx = E + B - i - 1;
 375     assert(Idx >= B && Idx < E);
 376     Data[Idx] = (Val % 10) + '0';
 377     Val /= 10;
 378   }
 379   return Size;
 380 }
 381
 382 template<class T>
 383 size_t ChangeBinaryInteger(uint8_t *Data, size_t Size, Random &Rand) {
 384   if (Size < sizeof(T)) return 0;
 385   size_t Off = Rand(Size - sizeof(T) + 1);
 386   assert(Off + sizeof(T) <= Size);
 387   T Val;
 388   if (Off < 64 && !Rand(4)) {
 389     Val = Size;
 390     if (Rand.RandBool())
 391       Val = Bswap(Val);
 392   } else {
 393     memcpy(&Val, Data + Off, sizeof(Val));
 394     T Add = Rand(21);
 395     Add -= 10;
 396     if (Rand.RandBool())
 397       Val = Bswap(T(Bswap(Val) + Add)); // Add assuming different endiannes.
 398     else
 399       Val = Val + Add;               // Add assuming current endiannes.
 400     if (Add == 0 || Rand.RandBool()) // Maybe negate.
 401       Val = -Val;
 402   }
 403   memcpy(Data + Off, &Val, sizeof(Val));
 404   return Size;
 405 }
 406
 407 size_t MutationDispatcher::Mutate_ChangeBinaryInteger(uint8_t *Data,
 408                                                       size_t Size,
 409                                                       size_t MaxSize) {
 410   if (Size > MaxSize) return 0;
 411   switch (Rand(4)) {
 412     case 3: return ChangeBinaryInteger<uint64_t>(Data, Size, Rand);
 413     case 2: return ChangeBinaryInteger<uint32_t>(Data, Size, Rand);
 414     case 1: return ChangeBinaryInteger<uint16_t>(Data, Size, Rand);
 415     case 0: return ChangeBinaryInteger<uint8_t>(Data, Size, Rand);
 416     default: assert(0);
 417   }
 418   return 0;
 419 }
 420
 421 size_t MutationDispatcher::Mutate_CrossOver(uint8_t *Data, size_t Size,
 422                                             size_t MaxSize) {
 423   if (Size > MaxSize) return 0;
 424   if (!Corpus || Corpus->size() < 2 || Size == 0) return 0;
 425   size_t Idx = Rand(Corpus->size());
 426   const Unit &O = (*Corpus)[Idx];
 427   if (O.empty()) return 0;
 428   MutateInPlaceHere.resize(MaxSize);
 429   auto &U = MutateInPlaceHere;
 430   size_t NewSize = 0;
 431   switch(Rand(3)) {
 432     case 0:
 433       NewSize = CrossOver(Data, Size, O.data(), O.size(), U.data(), U.size());
 434       break;
 435     case 1:
 436       NewSize = InsertPartOf(O.data(), O.size(), U.data(), U.size(), MaxSize);
 437       if (!NewSize)
 438         NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
 439       break;
 440     case 2:
 441       NewSize = CopyPartOf(O.data(), O.size(), U.data(), U.size());
 442       break;
 443     default: assert(0);
 444   }
 445   assert(NewSize > 0 && "CrossOver returned empty unit");
 446   assert(NewSize <= MaxSize && "CrossOver returned overisized unit");
 447   memcpy(Data, U.data(), NewSize);
 448   return NewSize;
 449 }
 450
 451 void MutationDispatcher::StartMutationSequence() {
 452   CurrentMutatorSequence.clear();
 453   CurrentDictionaryEntrySequence.clear();
 454 }
 455
 456 // Copy successful dictionary entries to PersistentAutoDictionary.
 457 void MutationDispatcher::RecordSuccessfulMutationSequence() {
 458   for (auto DE : CurrentDictionaryEntrySequence) {
 459     // PersistentAutoDictionary.AddWithSuccessCountOne(DE);
 460     DE->IncSuccessCount();
 461     assert(DE->GetW().size());
 462     // Linear search is fine here as this happens seldom.
 463     if (!PersistentAutoDictionary.ContainsWord(DE->GetW()))
 464       PersistentAutoDictionary.push_back({DE->GetW(), 1});
 465   }
 466 }
 467
 468 void MutationDispatcher::PrintRecommendedDictionary() {
 469   std::vector<DictionaryEntry> V;
 470   for (auto &DE : PersistentAutoDictionary)
 471     if (!ManualDictionary.ContainsWord(DE.GetW()))
 472       V.push_back(DE);
 473   if (V.empty()) return;
 474   Printf("###### Recommended dictionary. ######\n");
 475   for (auto &DE: V) {
 476     assert(DE.GetW().size());
 477     Printf("\"");
 478     PrintASCII(DE.GetW(), "\"");
 479     Printf(" # Uses: %zd\n", DE.GetUseCount());
 480   }
 481   Printf("###### End of recommended dictionary. ######\n");
 482 }
 483
 484 void MutationDispatcher::PrintMutationSequence() {
 485   Printf("MS: %zd ", CurrentMutatorSequence.size());
 486   for (auto M : CurrentMutatorSequence)
 487     Printf("%s-", M.Name);
 488   if (!CurrentDictionaryEntrySequence.empty()) {
 489     Printf(" DE: ");
 490     for (auto DE : CurrentDictionaryEntrySequence) {
 491       Printf("\"");
 492       PrintASCII(DE->GetW(), "\"-");
 493     }
 494   }
 495 }
 496
 497 size_t MutationDispatcher::Mutate(uint8_t *Data, size_t Size, size_t MaxSize) {
 498   return MutateImpl(Data, Size, MaxSize, Mutators);
 499 }
 500
 501 size_t MutationDispatcher::DefaultMutate(uint8_t *Data, size_t Size,
 502                                          size_t MaxSize) {
 503   return MutateImpl(Data, Size, MaxSize, DefaultMutators);
 504 }
 505
 506 // Mutates Data in place, returns new size.
 507 size_t MutationDispatcher::MutateImpl(uint8_t *Data, size_t Size,
 508                                       size_t MaxSize,
 509                                       const std::vector<Mutator> &Mutators) {
 510   assert(MaxSize > 0);
 511   // Some mutations may fail (e.g. can't insert more bytes if Size == MaxSize),
 512   // in which case they will return 0.
 513   // Try several times before returning un-mutated data.
 514   for (int Iter = 0; Iter < 100; Iter++) {
 515     auto M = Mutators[Rand(Mutators.size())];
 516     size_t NewSize = (this->*(M.Fn))(Data, Size, MaxSize);
 517     if (NewSize && NewSize <= MaxSize) {
 518       if (Options.OnlyASCII)
 519         ToASCII(Data, NewSize);
 520       CurrentMutatorSequence.push_back(M);
 521       return NewSize;
 522     }
 523   }
 524   *Data = ' ';
 525   return 1;   // Fallback, should not happen frequently.
 526 }
 527
 528 void MutationDispatcher::AddWordToManualDictionary(const Word &W) {
 529   ManualDictionary.push_back(
 530       {W, std::numeric_limits<size_t>::max()});
 531 }
 532
 533 }  // namespace fuzzer