Don't pass null pointers to memcmp and memcpy in libFuzzer (llvm#96775)

In C, it is UB to call `memcmp(NULL, NULL, 0)`, `memcpy(NULL, NULL, 0)`,
etc. Unfortunately, `(NULL, 0)` is the natural representation of an
empty sequence of objects and extremely common in real world code. As a
result, all C code, and C++ code which calls into C functions, must
carefully guard all calls to `memcpy`.

This is a serious, real world usability issue in C and should be fixed
in the language (see llvm#49459). In the meantime, pay the cost of the extra
branch to avoid tripping UBSan in libFuzzer. Once the usability problem
in C has been fixed, these checks can be removed.

Fixes llvm#96772

compiler-rt/lib/fuzzer/FuzzerDictionary.h

@@ -29,7 +29,9 @@ template <size_t kMaxSizeT> class FixedWord {
  static_assert(kMaxSizeT <= std::numeric_limits<uint8_t>::max(),
  "FixedWord::kMaxSizeT cannot fit in a uint8_t.");
  assert(S <= kMaxSize);
- memcpy(Data, B, S);
+ // memcpy cannot take null pointer arguments even if Size is 0.
+ if (S)
+ memcpy(Data, B, S);
  Size = static_cast<uint8_t>(S);
  }
 

compiler-rt/lib/fuzzer/FuzzerLoop.cpp

@@ -579,6 +579,9 @@ void Fuzzer::CrashOnOverwrittenData() {
 // Compare two arrays, but not all bytes if the arrays are large.
 static bool LooseMemeq(const uint8_t *A, const uint8_t *B, size_t Size) {
  const size_t Limit = 64;
+ // memcmp cannot take null pointer arguments even if Size is 0.
+ if (!Size)
+ return true;
  if (Size <= 64)
  return !memcmp(A, B, Size);
  // Compare first and last Limit/2 bytes.
@@ -596,7 +599,9 @@ ATTRIBUTE_NOINLINE bool Fuzzer::ExecuteCallback(const uint8_t *Data,
  // We copy the contents of Unit into a separate heap buffer
  // so that we reliably find buffer overflows in it.
  uint8_t *DataCopy = new uint8_t[Size];
- memcpy(DataCopy, Data, Size);
+ // memcpy cannot take null pointer arguments even if Size is 0.
+ if (Size)
+ memcpy(DataCopy, Data, Size);
  if (EF->__msan_unpoison)
  EF->__msan_unpoison(DataCopy, Size);
  if (EF->__msan_unpoison_param)