[lib/Fuzzer] make assertions more informative and update comments for the user-supplied mutator

llvm-svn: 238658

Author: kcc

llvm/lib/Fuzzer/FuzzerInterface.h

@@ -69,12 +69,12 @@ class UserSuppliedFuzzer {
   /// Executes the target function on 'Size' bytes of 'Data'.
   virtual void TargetFunction(const uint8_t *Data, size_t Size) = 0;
   /// Mutates 'Size' bytes of data in 'Data' inplace into up to 'MaxSize' bytes,
-  /// returns the new size of the data.
+  /// returns the new size of the data, which should be positive.
   virtual size_t Mutate(uint8_t *Data, size_t Size, size_t MaxSize) {
     return BasicMutate(Data, Size, MaxSize);
   }
   /// Crosses 'Data1' and 'Data2', writes up to 'MaxOutSize' bytes into Out,
-  /// returns the number of bytes written.
+  /// returns the number of bytes written, which should be positive.
   virtual size_t CrossOver(const uint8_t *Data1, size_t Size1,
                            const uint8_t *Data2, size_t Size2,
                            uint8_t *Out, size_t MaxOutSize) {

llvm/lib/Fuzzer/FuzzerLoop.cpp

@@ -289,7 +289,9 @@ void Fuzzer::MutateAndTestOne(Unit *U) {
     size_t Size = U->size();
     U->resize(Options.MaxLen);
     size_t NewSize = USF.Mutate(U->data(), Size, U->size());
-    assert(NewSize > 0 && NewSize <= (size_t)Options.MaxLen);
+    assert(NewSize > 0 && "Mutator returned empty unit");
+    assert(NewSize <= (size_t)Options.MaxLen &&
+           "Mutator return overisized unit");
     U->resize(NewSize);
     RunOneAndUpdateCorpus(*U);
     size_t NumTraceBasedMutations = StopTraceRecording();
@@ -317,7 +319,9 @@ void Fuzzer::Loop(size_t NumIterations) {
           size_t NewSize = USF.CrossOver(
               Corpus[J1].data(), Corpus[J1].size(), Corpus[J2].data(),
               Corpus[J2].size(), CurrentUnit.data(), CurrentUnit.size());
-          assert(NewSize > 0 && NewSize <= (size_t)Options.MaxLen);
+          assert(NewSize > 0 && "CrossOver returned empty unit");
+          assert(NewSize <= (size_t)Options.MaxLen &&
+                 "CrossOver return overisized unit");
           CurrentUnit.resize(NewSize);
           MutateAndTestOne(&CurrentUnit);
         }