llvm-project/llvm/lib/Fuzzer/FuzzerMutate.cpp

//===- FuzzerMutate.cpp - Mutate a test input -----------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
// Mutate a test input.
//===----------------------------------------------------------------------===//

#include <cstring>

#include "FuzzerInternal.h"

namespace fuzzer {

static char FlipRandomBit(char X, FuzzerRandomBase &Rand) {
  int Bit = Rand(8);
  char Mask = 1 << Bit;
  char R;
  if (X & (1 << Bit))
    R = X & ~Mask;
  else
    R = X | Mask;
  assert(R != X);
  return R;
}

static char RandCh(FuzzerRandomBase &Rand) {
  if (Rand.RandBool()) return Rand(256);
  const char *Special = "!*'();:@&=+$,/?%#[]123ABCxyz-`~.";
  return Special[Rand(sizeof(Special) - 1)];
}

size_t Mutate_EraseByte(uint8_t *Data, size_t Size, size_t MaxSize,
                        FuzzerRandomBase &Rand) {
  assert(Size);
  if (Size == 1) return Size;
  size_t Idx = Rand(Size);
  // Erase Data[Idx].
  memmove(Data + Idx, Data + Idx + 1, Size - Idx - 1);
  return Size - 1;
}

size_t Mutate_InsertByte(uint8_t *Data, size_t Size, size_t MaxSize,
                         FuzzerRandomBase &Rand) {
  if (Size == MaxSize) return Size;
  size_t Idx = Rand(Size + 1);
  // Insert new value at Data[Idx].
  memmove(Data + Idx + 1, Data + Idx, Size - Idx);
  Data[Idx] = RandCh(Rand);
  return Size + 1;
}

size_t Mutate_ChangeByte(uint8_t *Data, size_t Size, size_t MaxSize,
                         FuzzerRandomBase &Rand) {
  size_t Idx = Rand(Size);
  Data[Idx] = RandCh(Rand);
  return Size;
}

size_t Mutate_ChangeBit(uint8_t *Data, size_t Size, size_t MaxSize,
                        FuzzerRandomBase &Rand) {
  size_t Idx = Rand(Size);
  Data[Idx] = FlipRandomBit(Data[Idx], Rand);
  return Size;
}

// Mutates Data in place, returns new size.
size_t Mutate(uint8_t *Data, size_t Size, size_t MaxSize,
              FuzzerRandomBase &Rand) {
  assert(MaxSize > 0);
  assert(Size <= MaxSize);
  if (Size == 0) {
    for (size_t i = 0; i < MaxSize; i++)
      Data[i] = RandCh(Rand);
    return MaxSize;
  }
  assert(Size > 0);
  switch (Rand(4)) {
  case 0: Size = Mutate_EraseByte(Data, Size, MaxSize, Rand); break;
  case 1: Size = Mutate_InsertByte(Data, Size, MaxSize, Rand); break;
  case 2: Size = Mutate_ChangeByte(Data, Size, MaxSize, Rand); break;
  case 3: Size = Mutate_ChangeBit(Data, Size, MaxSize, Rand); break;
  }
  assert(Size > 0);
  return Size;
}

}  // namespace fuzzer