[libFuzzer] Handle unstable edges by using minimum hit counts

Summary:
Created unstable_handle flag that takes 1 or 2, depending on the handling type.
Modified RunOne to accommodate the following heuristic:
    Use the first CollectFeatures to count how many features there are.
    If no new features, CollectFeatures like before.
    If there is new feature, we run CB 2 more times,
        Check which edges are unstable per input and we store the least amount of hit counts for each edge.
        Apply these hit counts back to inline8bitcounters so that CollectFeatures can work as intended.
Modified UnstableCounters to 8int_t and created a bitset UnstableSet to tell which edges are unstable.

Patch by Kyungtak Woo (@kevinwkt).

Reviewers: Dor1s, metzman, morehouse

Reviewed By: Dor1s, morehouse

Subscribers: delcypher, #sanitizers, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D49525

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@337696 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 7 deletions

diff --git a/lib/fuzzer/FuzzerLoop.cpp b/lib/fuzzer/FuzzerLoop.cpp
index ffcd3419c..e63ee7361 100644
--- a/lib/fuzzer/FuzzerLoop.cpp
+++ b/lib/fuzzer/FuzzerLoop.cpp
@@ -465,11 +465,15 @@ void Fuzzer::CheckForUnstableCounters(const uint8_t *Data, size_t Size) {
 
   // First Rerun
   CBSetupAndRun();
-  TPC.UpdateUnstableCounters();
+  TPC.UpdateUnstableCounters(Options.HandleUnstable);
 
   // Second Rerun
   CBSetupAndRun();
-  TPC.UpdateUnstableCounters();
+  TPC.UpdateUnstableCounters(Options.HandleUnstable);
+
+  // Move minimum hit counts back to ModuleInline8bitCounters
+  if (Options.HandleUnstable)
+    TPC.ApplyUnstableCounters();
 }
 
 bool Fuzzer::RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile,
@@ -482,6 +486,17 @@ bool Fuzzer::RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile,
   UniqFeatureSetTmp.clear();
   size_t FoundUniqFeaturesOfII = 0;
   size_t NumUpdatesBefore = Corpus.NumFeatureUpdates();
+  bool NewFeaturesUnstable = false;
+
+  if (Options.HandleUnstable || Options.PrintUnstableStats) {
+    TPC.CollectFeatures([&](size_t Feature) {
+      if (Corpus.IsFeatureNew(Feature, Size, Options.Shrink))
+        NewFeaturesUnstable = true;
+    });
+    if (NewFeaturesUnstable)
+      CheckForUnstableCounters(Data, Size);
+  }
+
   TPC.CollectFeatures([&](size_t Feature) {
     if (Corpus.AddFeature(Feature, Size, Options.Shrink))
       UniqFeatureSetTmp.push_back(Feature);
@@ -490,16 +505,12 @@ bool Fuzzer::RunOne(const uint8_t *Data, size_t Size, bool MayDeleteFile,
                              II->UniqFeatureSet.end(), Feature))
         FoundUniqFeaturesOfII++;
   });
+
   if (FoundUniqFeatures)
     *FoundUniqFeatures = FoundUniqFeaturesOfII;
   PrintPulseAndReportSlowInput(Data, Size);
   size_t NumNewFeatures = Corpus.NumFeatureUpdates() - NumUpdatesBefore;
 
-  // If print_unstable_stats, execute the same input two more times to detect
-  // unstable edges.
-  if (NumNewFeatures && Options.PrintUnstableStats)
-    CheckForUnstableCounters(Data, Size);
-
   if (NumNewFeatures) {
     TPC.UpdateObservedPCs();
     Corpus.AddToCorpus({Data, Data + Size}, NumNewFeatures, MayDeleteFile,