Don't pass null pointers to memcmp and memcpy in libFuzzer (#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 #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
GitOrigin-RevId: bde4ffe7521421cfa891c7d6e526566920326b3f
diff --git a/FuzzerDictionary.h b/FuzzerDictionary.h
index 48f063c..64eb35c 100644
--- a/FuzzerDictionary.h
+++ b/FuzzerDictionary.h
@@ -29,7 +29,9 @@
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);
}
diff --git a/FuzzerLoop.cpp b/FuzzerLoop.cpp
index 935dd23..6f415dd 100644
--- a/FuzzerLoop.cpp
+++ b/FuzzerLoop.cpp
@@ -579,6 +579,9 @@
// 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 @@
// 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)
