[LibFuzzer] Tweak `MutationDispatcher::Mutate_CopyPart` mutation.

It doesn't make sense to non-deterministically choose between
`CopyPart(..)` and `InsertPart(..)` when it is known that
`InsertPart(..)` will fail.

This upstream's a change from JFS solver's fork of LibFuzzer.

Differential Revision: https://reviews.llvm.org/D45693

llvm-svn: 330687

1 files changed, 15 insertions, 0 deletions

diff --git a/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp b/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp
index 3f57a5d60c2..c795eddc656 100644
--- a/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp
+++ b/compiler-rt/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());