[compiler-rt] bde4ffe - Don't pass null pointers to memcmp and memcpy in libFuzzer (#96775)

[via llvm-commits]

Author: David Benjamin
Date: 2024-08-13T15:26:40-07:00
New Revision: bde4ffe7521421cfa891c7d6e526566920326b3f

URL: https://github.com/llvm/llvm-project/commit/bde4ffe7521421cfa891c7d6e526566920326b3f
DIFF: https://github.com/llvm/llvm-project/commit/bde4ffe7521421cfa891c7d6e526566920326b3f.diff

LOG: 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

Added: 
    

Modified: 
    compiler-rt/lib/fuzzer/FuzzerDictionary.h
    compiler-rt/lib/fuzzer/FuzzerLoop.cpp

Removed: 
    


################################################################################
diff  --git a/compiler-rt/lib/fuzzer/FuzzerDictionary.h b/compiler-rt/lib/fuzzer/FuzzerDictionary.h
index 48f063c7ee4e6e..64eb35c57a5610 100644
--- a/compiler-rt/lib/fuzzer/FuzzerDictionary.h
+++ b/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);
   }
 

diff  --git a/compiler-rt/lib/fuzzer/FuzzerLoop.cpp b/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
index 935dd2342e18f8..6f415dd5763ac9 100644
--- a/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
+++ b/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)