2 files changed, 18 insertions, 1 deletions
diff --git a/lib/fuzzer/FuzzerMutate.cpp b/lib/fuzzer/FuzzerMutate.cpp
index e0e457852..865e598fd 100644
--- a/lib/fuzzer/FuzzerMutate.cpp
+++ b/lib/fuzzer/FuzzerMutate.cpp
@@ -339,7 +339,9 @@ size_t MutationDispatcher::InsertPartOf(const uint8_t *From, size_t FromSize,
 size_t MutationDispatcher::Mutate_CopyPart(uint8_t *Data, size_t Size,
                                            size_t MaxSize) {
   if (Size > MaxSize || Size == 0) return 0;
-  if (Rand.RandBool())
+  // If Size == MaxSize, `InsertPartOf(...)` will
+  // fail so there's no point using it in this case.
+  if (Size == MaxSize || Rand.RandBool())
     return CopyPartOf(Data, Size, Data, Size);
   else
     return InsertPartOf(Data, Size, Data, Size, MaxSize);
diff --git a/lib/fuzzer/tests/FuzzerUnittest.cpp b/lib/fuzzer/tests/FuzzerUnittest.cpp
index 3f57a5d60..c795eddc6 100644
--- a/lib/fuzzer/tests/FuzzerUnittest.cpp
+++ b/lib/fuzzer/tests/FuzzerUnittest.cpp
@@ -381,6 +381,21 @@ TEST(FuzzerMutate, CopyPart1) {
 TEST(FuzzerMutate, CopyPart2) {
   TestCopyPart(&MutationDispatcher::Mutate, 1 << 13);
 }
+TEST(FuzzerMutate, CopyPartNoInsertAtMaxSize) {
+  // This (non exhaustively) tests if `Mutate_CopyPart` tries to perform an
+  // insert on an input of size `MaxSize`.  Performing an insert in this case
+  // will lead to the mutation failing.
+  std::unique_ptr<ExternalFunctions> t(new ExternalFunctions());
+  fuzzer::EF = t.get();
+  Random Rand(0);
+  std::unique_ptr<MutationDispatcher> MD(new MutationDispatcher(Rand, {}));
+  uint8_t Data[8] = {0x00, 0x11, 0x22, 0x33, 0x44, 0x00, 0x11, 0x22};
+  size_t MaxSize = sizeof(Data);
+  for (int count = 0; count < (1 << 18); ++count) {
+    size_t NewSize = MD->Mutate_CopyPart(Data, MaxSize, MaxSize);
+    ASSERT_EQ(NewSize, MaxSize);
+  }
+}
 
 void TestAddWordFromDictionary(Mutator M, int NumIter) {
   std::unique_ptr<ExternalFunctions> t(new ExternalFunctions());