[libFuzzer] Preserve position hint in auto dictionary

[fmeum]

Summary

Currently, the position hint of an entry in the persistent auto
dictionary is fixed to 1. As a consequence, with a 50% chance, the entry
is applied right after the first byte of the input. As the position 1
does not appear to have any particular significance, this is likely a
bug that may have been caused by confusing the constructor parameter
with a success count.

This commit resolves the issue by preserving any existing position hint
or disabling the hint if the original entry didn't have one.

[fmeum]

compiler-rt/lib/fuzzer/FuzzerMutate.cpp

483

Maybe this can even be replaced by:

PersistentAutoDictionary.push_back(DE);

Or does the word in the entry need to be copied here?

[morehouse]

Thanks for the bug fix!

compiler-rt/lib/fuzzer/FuzzerMutate.cpp

483

AFAICT, DictionaryEntry is POD, so yes let's just do push_back(DE) unconditionally.

[fmeum]

Simplified the change by copying the DictionaryEntry POD.

[morehouse]

LGTM

compiler-rt/lib/fuzzer/FuzzerMutate.cpp

 // Copy successful dictionary entries to PersistentAutoDictionary.
 void MutationDispatcher::RecordSuccessfulMutationSequence() {
   for (auto DE : CurrentDictionaryEntrySequence) {
     // PersistentAutoDictionary.AddWithSuccessCountOne(DE);
     DE->IncSuccessCount();
     assert(DE->GetW().size());
     // Linear search is fine here as this happens seldom.
     if (!PersistentAutoDictionary.ContainsWord(DE->GetW()))
-      PersistentAutoDictionary.push_back({DE->GetW(), 1});
+      PersistentAutoDictionary.push_back(*DE);
   }
 }
 
 void MutationDispatcher::PrintRecommendedDictionary() {
   Vector<DictionaryEntry> V;
   for (auto &DE : PersistentAutoDictionary)
     if (!ManualDictionary.ContainsWord(DE.GetW()))
       V.push_back(DE);
   if (V.empty()) return;