1 //===- FuzzerMutate.cpp - Mutate a test input -----------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 // Mutate a test input.
9 //===----------------------------------------------------------------------===//
10 
11 #include "FuzzerDefs.h"
12 #include "FuzzerExtFunctions.h"
13 #include "FuzzerIO.h"
14 #include "FuzzerMutate.h"
15 #include "FuzzerOptions.h"
16 #include "FuzzerTracePC.h"
17 
18 namespace fuzzer {
19 
20 const size_t Dictionary::kMaxDictSize;
21 static const size_t kMaxMutationsToPrint = 10;
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())
65     return static_cast<char>(Rand(256));
66   const char Special[] = "!*'();:@&=+$,/?%#[]012Az-`~.\xff\x00";
67   return Special[Rand(sizeof(Special) - 1)];
68 }
69 
70 size_t MutationDispatcher::Mutate_Custom(uint8_t *Data, size_t Size,
71                                          size_t MaxSize) {
72   return EF->LLVMFuzzerCustomMutator(Data, Size, MaxSize,
73                                      Rand.Rand<unsigned int>());
74 }
75 
76 size_t MutationDispatcher::Mutate_CustomCrossOver(uint8_t *Data, size_t Size,
77                                                   size_t MaxSize) {
78   if (Size == 0)
79     return 0;
80   if (!CrossOverWith) return 0;
81   const Unit &Other = *CrossOverWith;
82   if (Other.empty())
83     return 0;
84   CustomCrossOverInPlaceHere.resize(MaxSize);
85   auto &U = CustomCrossOverInPlaceHere;
86   size_t NewSize = EF->LLVMFuzzerCustomCrossOver(
87       Data, Size, Other.data(), Other.size(), U.data(), U.size(),
88       Rand.Rand<unsigned int>());
89   if (!NewSize)
90     return 0;
91   assert(NewSize <= MaxSize && "CustomCrossOver returned overisized unit");
92   memcpy(Data, U.data(), NewSize);
93   return NewSize;
94 }
95 
96 size_t MutationDispatcher::Mutate_ShuffleBytes(uint8_t *Data, size_t Size,
97                                                size_t MaxSize) {
98   if (Size > MaxSize || Size == 0) return 0;
99   size_t ShuffleAmount =
100       Rand(std::min(Size, (size_t)8)) + 1; // [1,8] and <= Size.
101   size_t ShuffleStart = Rand(Size - ShuffleAmount);
102   assert(ShuffleStart + ShuffleAmount <= Size);
103   std::shuffle(Data + ShuffleStart, Data + ShuffleStart + ShuffleAmount, Rand);
104   return Size;
105 }
106 
107 size_t MutationDispatcher::Mutate_EraseBytes(uint8_t *Data, size_t Size,
108                                              size_t MaxSize) {
109   if (Size <= 1) return 0;
110   size_t N = Rand(Size / 2) + 1;
111   assert(N < Size);
112   size_t Idx = Rand(Size - N + 1);
113   // Erase Data[Idx:Idx+N].
114   memmove(Data + Idx, Data + Idx + N, Size - Idx - N);
115   // Printf("Erase: %zd %zd => %zd; Idx %zd\n", N, Size, Size - N, Idx);
116   return Size - N;
117 }
118 
119 size_t MutationDispatcher::Mutate_InsertByte(uint8_t *Data, size_t Size,
120                                              size_t MaxSize) {
121   if (Size >= MaxSize) return 0;
122   size_t Idx = Rand(Size + 1);
123   // Insert new value at Data[Idx].
124   memmove(Data + Idx + 1, Data + Idx, Size - Idx);
125   Data[Idx] = RandCh(Rand);
126   return Size + 1;
127 }
128 
129 size_t MutationDispatcher::Mutate_InsertRepeatedBytes(uint8_t *Data,
130                                                       size_t Size,
131                                                       size_t MaxSize) {
132   const size_t kMinBytesToInsert = 3;
133   if (Size + kMinBytesToInsert >= MaxSize) return 0;
134   size_t MaxBytesToInsert = std::min(MaxSize - Size, (size_t)128);
135   size_t N = Rand(MaxBytesToInsert - kMinBytesToInsert + 1) + kMinBytesToInsert;
136   assert(Size + N <= MaxSize && N);
137   size_t Idx = Rand(Size + 1);
138   // Insert new values at Data[Idx].
139   memmove(Data + Idx + N, Data + Idx, Size - Idx);
140   // Give preference to 0x00 and 0xff.
141   uint8_t Byte = static_cast<uint8_t>(
142       Rand.RandBool() ? Rand(256) : (Rand.RandBool() ? 0 : 255));
143   for (size_t i = 0; i < N; i++)
144     Data[Idx + i] = Byte;
145   return Size + N;
146 }
147 
148 size_t MutationDispatcher::Mutate_ChangeByte(uint8_t *Data, size_t Size,
149                                              size_t MaxSize) {
150   if (Size > MaxSize) return 0;
151   size_t Idx = Rand(Size);
152   Data[Idx] = RandCh(Rand);
153   return Size;
154 }
155 
156 size_t MutationDispatcher::Mutate_ChangeBit(uint8_t *Data, size_t Size,
157                                             size_t MaxSize) {
158   if (Size > MaxSize) return 0;
159   size_t Idx = Rand(Size);
160   Data[Idx] ^= 1 << Rand(8);
161   return Size;
162 }
163 
164 size_t MutationDispatcher::Mutate_AddWordFromManualDictionary(uint8_t *Data,
165                                                               size_t Size,
166                                                               size_t MaxSize) {
167   return AddWordFromDictionary(ManualDictionary, Data, Size, MaxSize);
168 }
169 
170 size_t MutationDispatcher::ApplyDictionaryEntry(uint8_t *Data, size_t Size,
171                                                 size_t MaxSize,
172                                                 DictionaryEntry &DE) {
173   const Word &W = DE.GetW();
174   bool UsePositionHint = DE.HasPositionHint() &&
175                          DE.GetPositionHint() + W.size() < Size &&
176                          Rand.RandBool();
177   if (Rand.RandBool()) {  // Insert W.
178     if (Size + W.size() > MaxSize) return 0;
179     size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size + 1);
180     memmove(Data + Idx + W.size(), Data + Idx, Size - Idx);
181     memcpy(Data + Idx, W.data(), W.size());
182     Size += W.size();
183   } else {  // Overwrite some bytes with W.
184     if (W.size() > Size) return 0;
185     size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size - W.size());
186     memcpy(Data + Idx, W.data(), W.size());
187   }
188   return Size;
189 }
190 
191 // Somewhere in the past we have observed a comparison instructions
192 // with arguments Arg1 Arg2. This function tries to guess a dictionary
193 // entry that will satisfy that comparison.
194 // It first tries to find one of the arguments (possibly swapped) in the
195 // input and if it succeeds it creates a DE with a position hint.
196 // Otherwise it creates a DE with one of the arguments w/o a position hint.
197 DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
198     const void *Arg1, const void *Arg2,
199     const void *Arg1Mutation, const void *Arg2Mutation,
200     size_t ArgSize, const uint8_t *Data,
201     size_t Size) {
202   bool HandleFirst = Rand.RandBool();
203   const void *ExistingBytes, *DesiredBytes;
204   Word W;
205   const uint8_t *End = Data + Size;
206   for (int Arg = 0; Arg < 2; Arg++) {
207     ExistingBytes = HandleFirst ? Arg1 : Arg2;
208     DesiredBytes = HandleFirst ? Arg2Mutation : Arg1Mutation;
209     HandleFirst = !HandleFirst;
210     W.Set(reinterpret_cast<const uint8_t*>(DesiredBytes), ArgSize);
211     const size_t kMaxNumPositions = 8;
212     size_t Positions[kMaxNumPositions];
213     size_t NumPositions = 0;
214     for (const uint8_t *Cur = Data;
215          Cur < End && NumPositions < kMaxNumPositions; Cur++) {
216       Cur =
217           (const uint8_t *)SearchMemory(Cur, End - Cur, ExistingBytes, ArgSize);
218       if (!Cur) break;
219       Positions[NumPositions++] = Cur - Data;
220     }
221     if (!NumPositions) continue;
222     return DictionaryEntry(W, Positions[Rand(NumPositions)]);
223   }
224   DictionaryEntry DE(W);
225   return DE;
226 }
227 
228 
229 template <class T>
230 DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
231     T Arg1, T Arg2, const uint8_t *Data, size_t Size) {
232   if (Rand.RandBool()) Arg1 = Bswap(Arg1);
233   if (Rand.RandBool()) Arg2 = Bswap(Arg2);
234   T Arg1Mutation = static_cast<T>(Arg1 + Rand(-1, 1));
235   T Arg2Mutation = static_cast<T>(Arg2 + Rand(-1, 1));
236   return MakeDictionaryEntryFromCMP(&Arg1, &Arg2, &Arg1Mutation, &Arg2Mutation,
237                                     sizeof(Arg1), Data, Size);
238 }
239 
240 DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
241     const Word &Arg1, const Word &Arg2, const uint8_t *Data, size_t Size) {
242   return MakeDictionaryEntryFromCMP(Arg1.data(), Arg2.data(), Arg1.data(),
243                                     Arg2.data(), Arg1.size(), Data, Size);
244 }
245 
246 size_t MutationDispatcher::Mutate_AddWordFromTORC(
247     uint8_t *Data, size_t Size, size_t MaxSize) {
248   Word W;
249   DictionaryEntry DE;
250   switch (Rand(4)) {
251   case 0: {
252     auto X = TPC.TORC8.Get(Rand.Rand<size_t>());
253     DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
254   } break;
255   case 1: {
256     auto X = TPC.TORC4.Get(Rand.Rand<size_t>());
257     if ((X.A >> 16) == 0 && (X.B >> 16) == 0 && Rand.RandBool())
258       DE = MakeDictionaryEntryFromCMP((uint16_t)X.A, (uint16_t)X.B, Data, Size);
259     else
260       DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
261   } break;
262   case 2: {
263     auto X = TPC.TORCW.Get(Rand.Rand<size_t>());
264     DE = MakeDictionaryEntryFromCMP(X.A, X.B, Data, Size);
265   } break;
266   case 3: if (Options.UseMemmem) {
267       auto X = TPC.MMT.Get(Rand.Rand<size_t>());
268       DE = DictionaryEntry(X);
269   } break;
270   default:
271     assert(0);
272   }
273   if (!DE.GetW().size()) return 0;
274   Size = ApplyDictionaryEntry(Data, Size, MaxSize, DE);
275   if (!Size) return 0;
276   DictionaryEntry &DERef =
277       CmpDictionaryEntriesDeque[CmpDictionaryEntriesDequeIdx++ %
278                                 kCmpDictionaryEntriesDequeSize];
279   DERef = DE;
280   CurrentDictionaryEntrySequence.push_back(&DERef);
281   return Size;
282 }
283 
284 size_t MutationDispatcher::Mutate_AddWordFromPersistentAutoDictionary(
285     uint8_t *Data, size_t Size, size_t MaxSize) {
286   return AddWordFromDictionary(PersistentAutoDictionary, Data, Size, MaxSize);
287 }
288 
289 size_t MutationDispatcher::AddWordFromDictionary(Dictionary &D, uint8_t *Data,
290                                                  size_t Size, size_t MaxSize) {
291   if (Size > MaxSize) return 0;
292   if (D.empty()) return 0;
293   DictionaryEntry &DE = D[Rand(D.size())];
294   Size = ApplyDictionaryEntry(Data, Size, MaxSize, DE);
295   if (!Size) return 0;
296   DE.IncUseCount();
297   CurrentDictionaryEntrySequence.push_back(&DE);
298   return Size;
299 }
300 
301 // Overwrites part of To[0,ToSize) with a part of From[0,FromSize).
302 // Returns ToSize.
303 size_t MutationDispatcher::CopyPartOf(const uint8_t *From, size_t FromSize,
304                                       uint8_t *To, size_t ToSize) {
305   // Copy From[FromBeg, FromBeg + CopySize) into To[ToBeg, ToBeg + CopySize).
306   size_t ToBeg = Rand(ToSize);
307   size_t CopySize = Rand(ToSize - ToBeg) + 1;
308   assert(ToBeg + CopySize <= ToSize);
309   CopySize = std::min(CopySize, FromSize);
310   size_t FromBeg = Rand(FromSize - CopySize + 1);
311   assert(FromBeg + CopySize <= FromSize);
312   memmove(To + ToBeg, From + FromBeg, CopySize);
313   return ToSize;
314 }
315 
316 // Inserts part of From[0,ToSize) into To.
317 // Returns new size of To on success or 0 on failure.
318 size_t MutationDispatcher::InsertPartOf(const uint8_t *From, size_t FromSize,
319                                         uint8_t *To, size_t ToSize,
320                                         size_t MaxToSize) {
321   if (ToSize >= MaxToSize) return 0;
322   size_t AvailableSpace = MaxToSize - ToSize;
323   size_t MaxCopySize = std::min(AvailableSpace, FromSize);
324   size_t CopySize = Rand(MaxCopySize) + 1;
325   size_t FromBeg = Rand(FromSize - CopySize + 1);
326   assert(FromBeg + CopySize <= FromSize);
327   size_t ToInsertPos = Rand(ToSize + 1);
328   assert(ToInsertPos + CopySize <= MaxToSize);
329   size_t TailSize = ToSize - ToInsertPos;
330   if (To == From) {
331     MutateInPlaceHere.resize(MaxToSize);
332     memcpy(MutateInPlaceHere.data(), From + FromBeg, CopySize);
333     memmove(To + ToInsertPos + CopySize, To + ToInsertPos, TailSize);
334     memmove(To + ToInsertPos, MutateInPlaceHere.data(), CopySize);
335   } else {
336     memmove(To + ToInsertPos + CopySize, To + ToInsertPos, TailSize);
337     memmove(To + ToInsertPos, From + FromBeg, CopySize);
338   }
339   return ToSize + CopySize;
340 }
341 
342 size_t MutationDispatcher::Mutate_CopyPart(uint8_t *Data, size_t Size,
343                                            size_t MaxSize) {
344   if (Size > MaxSize || Size == 0) return 0;
345   // If Size == MaxSize, `InsertPartOf(...)` will
346   // fail so there's no point using it in this case.
347   if (Size == MaxSize || Rand.RandBool())
348     return CopyPartOf(Data, Size, Data, Size);
349   else
350     return InsertPartOf(Data, Size, Data, Size, MaxSize);
351 }
352 
353 size_t MutationDispatcher::Mutate_ChangeASCIIInteger(uint8_t *Data, size_t Size,
354                                                      size_t MaxSize) {
355   if (Size > MaxSize) return 0;
356   size_t B = Rand(Size);
357   while (B < Size && !isdigit(Data[B])) B++;
358   if (B == Size) return 0;
359   size_t E = B;
360   while (E < Size && isdigit(Data[E])) E++;
361   assert(B < E);
362   // now we have digits in [B, E).
363   // strtol and friends don't accept non-zero-teminated data, parse it manually.
364   uint64_t Val = Data[B] - '0';
365   for (size_t i = B + 1; i < E; i++)
366     Val = Val * 10 + Data[i] - '0';
367 
368   // Mutate the integer value.
369   switch(Rand(5)) {
370     case 0: Val++; break;
371     case 1: Val--; break;
372     case 2: Val /= 2; break;
373     case 3: Val *= 2; break;
374     case 4: Val = Rand(Val * Val); break;
375     default: assert(0);
376   }
377   // Just replace the bytes with the new ones, don't bother moving bytes.
378   for (size_t i = B; i < E; i++) {
379     size_t Idx = E + B - i - 1;
380     assert(Idx >= B && Idx < E);
381     Data[Idx] = (Val % 10) + '0';
382     Val /= 10;
383   }
384   return Size;
385 }
386 
387 template<class T>
388 size_t ChangeBinaryInteger(uint8_t *Data, size_t Size, Random &Rand) {
389   if (Size < sizeof(T)) return 0;
390   size_t Off = Rand(Size - sizeof(T) + 1);
391   assert(Off + sizeof(T) <= Size);
392   T Val;
393   if (Off < 64 && !Rand(4)) {
394     Val = static_cast<T>(Size);
395     if (Rand.RandBool())
396       Val = Bswap(Val);
397   } else {
398     memcpy(&Val, Data + Off, sizeof(Val));
399     T Add = static_cast<T>(Rand(21));
400     Add -= 10;
401     if (Rand.RandBool())
402       Val = Bswap(T(Bswap(Val) + Add)); // Add assuming different endiannes.
403     else
404       Val = Val + Add;               // Add assuming current endiannes.
405     if (Add == 0 || Rand.RandBool()) // Maybe negate.
406       Val = -Val;
407   }
408   memcpy(Data + Off, &Val, sizeof(Val));
409   return Size;
410 }
411 
412 size_t MutationDispatcher::Mutate_ChangeBinaryInteger(uint8_t *Data,
413                                                       size_t Size,
414                                                       size_t MaxSize) {
415   if (Size > MaxSize) return 0;
416   switch (Rand(4)) {
417     case 3: return ChangeBinaryInteger<uint64_t>(Data, Size, Rand);
418     case 2: return ChangeBinaryInteger<uint32_t>(Data, Size, Rand);
419     case 1: return ChangeBinaryInteger<uint16_t>(Data, Size, Rand);
420     case 0: return ChangeBinaryInteger<uint8_t>(Data, Size, Rand);
421     default: assert(0);
422   }
423   return 0;
424 }
425 
426 size_t MutationDispatcher::Mutate_CrossOver(uint8_t *Data, size_t Size,
427                                             size_t MaxSize) {
428   if (Size > MaxSize) return 0;
429   if (Size == 0) return 0;
430   if (!CrossOverWith) return 0;
431   const Unit &O = *CrossOverWith;
432   if (O.empty()) return 0;
433   size_t NewSize = 0;
434   switch(Rand(3)) {
435     case 0:
436       MutateInPlaceHere.resize(MaxSize);
437       NewSize = CrossOver(Data, Size, O.data(), O.size(),
438                           MutateInPlaceHere.data(), MaxSize);
439       memcpy(Data, MutateInPlaceHere.data(), NewSize);
440       break;
441     case 1:
442       NewSize = InsertPartOf(O.data(), O.size(), Data, Size, MaxSize);
443       if (!NewSize)
444         NewSize = CopyPartOf(O.data(), O.size(), Data, Size);
445       break;
446     case 2:
447       NewSize = CopyPartOf(O.data(), O.size(), Data, Size);
448       break;
449     default: assert(0);
450   }
451   assert(NewSize > 0 && "CrossOver returned empty unit");
452   assert(NewSize <= MaxSize && "CrossOver returned overisized unit");
453   return NewSize;
454 }
455 
456 void MutationDispatcher::StartMutationSequence() {
457   CurrentMutatorSequence.clear();
458   CurrentDictionaryEntrySequence.clear();
459 }
460 
461 // Copy successful dictionary entries to PersistentAutoDictionary.
462 void MutationDispatcher::RecordSuccessfulMutationSequence() {
463   for (auto DE : CurrentDictionaryEntrySequence) {
464     // PersistentAutoDictionary.AddWithSuccessCountOne(DE);
465     DE->IncSuccessCount();
466     assert(DE->GetW().size());
467     // Linear search is fine here as this happens seldom.
468     if (!PersistentAutoDictionary.ContainsWord(DE->GetW()))
469       PersistentAutoDictionary.push_back({DE->GetW(), 1});
470   }
471 }
472 
473 void MutationDispatcher::PrintRecommendedDictionary() {
474   Vector<DictionaryEntry> V;
475   for (auto &DE : PersistentAutoDictionary)
476     if (!ManualDictionary.ContainsWord(DE.GetW()))
477       V.push_back(DE);
478   if (V.empty()) return;
479   Printf("###### Recommended dictionary. ######\n");
480   for (auto &DE: V) {
481     assert(DE.GetW().size());
482     Printf("\"");
483     PrintASCII(DE.GetW(), "\"");
484     Printf(" # Uses: %zd\n", DE.GetUseCount());
485   }
486   Printf("###### End of recommended dictionary. ######\n");
487 }
488 
489 void MutationDispatcher::PrintMutationSequence(bool Verbose) {
490   Printf("MS: %zd ", CurrentMutatorSequence.size());
491   size_t EntriesToPrint =
492       Verbose ? CurrentMutatorSequence.size()
493               : std::min(kMaxMutationsToPrint, CurrentMutatorSequence.size());
494   for (size_t i = 0; i < EntriesToPrint; i++)
495     Printf("%s-", CurrentMutatorSequence[i].Name);
496   if (!CurrentDictionaryEntrySequence.empty()) {
497     Printf(" DE: ");
498     EntriesToPrint = Verbose ? CurrentDictionaryEntrySequence.size()
499                              : std::min(kMaxMutationsToPrint,
500                                         CurrentDictionaryEntrySequence.size());
501     for (size_t i = 0; i < EntriesToPrint; i++) {
502       Printf("\"");
503       PrintASCII(CurrentDictionaryEntrySequence[i]->GetW(), "\"-");
504     }
505   }
506 }
507 
508 std::string MutationDispatcher::MutationSequence() {
509   std::string MS;
510   for (auto M : CurrentMutatorSequence) {
511     MS += M.Name;
512     MS += "-";
513   }
514   return MS;
515 }
516 
517 size_t MutationDispatcher::Mutate(uint8_t *Data, size_t Size, size_t MaxSize) {
518   return MutateImpl(Data, Size, MaxSize, Mutators);
519 }
520 
521 size_t MutationDispatcher::DefaultMutate(uint8_t *Data, size_t Size,
522                                          size_t MaxSize) {
523   return MutateImpl(Data, Size, MaxSize, DefaultMutators);
524 }
525 
526 // Mutates Data in place, returns new size.
527 size_t MutationDispatcher::MutateImpl(uint8_t *Data, size_t Size,
528                                       size_t MaxSize,
529                                       Vector<Mutator> &Mutators) {
530   assert(MaxSize > 0);
531   // Some mutations may fail (e.g. can't insert more bytes if Size == MaxSize),
532   // in which case they will return 0.
533   // Try several times before returning un-mutated data.
534   for (int Iter = 0; Iter < 100; Iter++) {
535     auto M = Mutators[Rand(Mutators.size())];
536     size_t NewSize = (this->*(M.Fn))(Data, Size, MaxSize);
537     if (NewSize && NewSize <= MaxSize) {
538       if (Options.OnlyASCII)
539         ToASCII(Data, NewSize);
540       CurrentMutatorSequence.push_back(M);
541       return NewSize;
542     }
543   }
544   *Data = ' ';
545   return 1;   // Fallback, should not happen frequently.
546 }
547 
548 // Mask represents the set of Data bytes that are worth mutating.
549 size_t MutationDispatcher::MutateWithMask(uint8_t *Data, size_t Size,
550                                           size_t MaxSize,
551                                           const Vector<uint8_t> &Mask) {
552   size_t MaskedSize = std::min(Size, Mask.size());
553   // * Copy the worthy bytes into a temporary array T
554   // * Mutate T
555   // * Copy T back.
556   // This is totally unoptimized.
557   auto &T = MutateWithMaskTemp;
558   if (T.size() < Size)
559     T.resize(Size);
560   size_t OneBits = 0;
561   for (size_t I = 0; I < MaskedSize; I++)
562     if (Mask[I])
563       T[OneBits++] = Data[I];
564 
565   if (!OneBits) return 0;
566   assert(!T.empty());
567   size_t NewSize = Mutate(T.data(), OneBits, OneBits);
568   assert(NewSize <= OneBits);
569   (void)NewSize;
570   // Even if NewSize < OneBits we still use all OneBits bytes.
571   for (size_t I = 0, J = 0; I < MaskedSize; I++)
572     if (Mask[I])
573       Data[I] = T[J++];
574   return Size;
575 }
576 
577 void MutationDispatcher::AddWordToManualDictionary(const Word &W) {
578   ManualDictionary.push_back(
579       {W, std::numeric_limits<size_t>::max()});
580 }
581 
582 }  // namespace fuzzer
583