Don't pass null pointers to memcmp and memcpy in libFuzzer

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 #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 #96772

Author: davidben

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)