6 files changed, 32 insertions, 9 deletions
diff --git a/llvm/lib/Fuzzer/FuzzerCorpus.h b/llvm/lib/Fuzzer/FuzzerCorpus.h
index 663c5854b4c..42b3d2482fa 100644
--- a/llvm/lib/Fuzzer/FuzzerCorpus.h
+++ b/llvm/lib/Fuzzer/FuzzerCorpus.h
@@ -59,6 +59,12 @@ class InputCorpus {
       Res += !II->U.empty();
     return Res;
   }
+  size_t MaxInputSize() const {
+    size_t Res = 0;
+    for (auto II : Inputs)
+        Res = std::max(Res, II->U.size());
+    return Res;
+  }
   bool empty() const { return Inputs.empty(); }
   const Unit &operator[] (size_t Idx) const { return Inputs[Idx]->U; }
   void AddToCorpus(const Unit &U, size_t NumFeatures, bool MayDeleteFile = false) {
diff --git a/llvm/lib/Fuzzer/FuzzerDriver.cpp b/llvm/lib/Fuzzer/FuzzerDriver.cpp
index 8a8c12fd6cb..30cdb8063bb 100644
--- a/llvm/lib/Fuzzer/FuzzerDriver.cpp
+++ b/llvm/lib/Fuzzer/FuzzerDriver.cpp
@@ -390,6 +390,9 @@ int FuzzerDriver(int *argc, char ***argv, UserCallback Callback) {
   FuzzingOptions Options;
   Options.Verbosity = Flags.verbosity;
   Options.MaxLen = Flags.max_len;
+  Options.ExperimentalLenControl = Flags.experimental_len_control;
+  if (Flags.experimental_len_control && Flags.max_len == 64)
+    Options.MaxLen = 1 << 20;
   Options.UnitTimeoutSec = Flags.timeout;
   Options.ErrorExitCode = Flags.error_exitcode;
   Options.TimeoutExitCode = Flags.timeout_exitcode;
diff --git a/llvm/lib/Fuzzer/FuzzerFlags.def b/llvm/lib/Fuzzer/FuzzerFlags.def
index 68dc820caa3..16917ac6b65 100644
--- a/llvm/lib/Fuzzer/FuzzerFlags.def
+++ b/llvm/lib/Fuzzer/FuzzerFlags.def
@@ -17,6 +17,7 @@ FUZZER_FLAG_INT(runs, -1,
 FUZZER_FLAG_INT(max_len, 0, "Maximum length of the test input. "
     "If 0, libFuzzer tries to guess a good value based on the corpus "
     "and reports it. ")
+FUZZER_FLAG_INT(experimental_len_control, 0, "experimental flag")
 FUZZER_FLAG_INT(cross_over, 1, "If 1, cross over inputs.")
 FUZZER_FLAG_INT(mutate_depth, 5,
             "Apply this number of consecutive mutations to each input.")
diff --git a/llvm/lib/Fuzzer/FuzzerLoop.cpp b/llvm/lib/Fuzzer/FuzzerLoop.cpp
index f161cc7cda6..c666de8f226 100644
--- a/llvm/lib/Fuzzer/FuzzerLoop.cpp
+++ b/llvm/lib/Fuzzer/FuzzerLoop.cpp
@@ -697,6 +697,19 @@ void Fuzzer::TryDetectingAMemoryLeak(const uint8_t *Data, size_t Size,
   }
 }
 
+static size_t ComputeMutationLen(size_t MaxInputSize, size_t MaxMutationLen,
+                                 Random &Rand) {
+  assert(MaxInputSize <= MaxMutationLen);
+  if (MaxInputSize == MaxMutationLen) return MaxMutationLen;
+  size_t Result = MaxInputSize;
+  size_t R = Rand.Rand();
+  if ((R % (1U << 7)) == 0)
+    Result++;
+  if ((R % (1U << 15)) == 0)
+    Result += 10 + Result / 2;
+  return Min(Result, MaxMutationLen);
+}
+
 void Fuzzer::MutateAndTestOne() {
   MD.StartMutationSequence();
 
@@ -710,13 +723,19 @@ void Fuzzer::MutateAndTestOne() {
 
   assert(MaxMutationLen > 0);
 
+  size_t CurrentMaxMutationLen =
+      Options.ExperimentalLenControl
+          ? ComputeMutationLen(Corpus.MaxInputSize(), MaxMutationLen,
+                               MD.GetRand())
+          : MaxMutationLen;
+
   for (int i = 0; i < Options.MutateDepth; i++) {
     if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
       break;
     size_t NewSize = 0;
-    NewSize = MD.Mutate(CurrentUnitData, Size, MaxMutationLen);
+    NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
     assert(NewSize > 0 && "Mutator returned empty unit");
-    assert(NewSize <= MaxMutationLen && "Mutator return overisized unit");
+    assert(NewSize <= CurrentMaxMutationLen && "Mutator return overisized unit");
     Size = NewSize;
     if (i == 0)
       StartTraceRecording();
diff --git a/llvm/lib/Fuzzer/FuzzerMutate.cpp b/llvm/lib/Fuzzer/FuzzerMutate.cpp
index 80b7a5f961f..2b130cc291e 100644
--- a/llvm/lib/Fuzzer/FuzzerMutate.cpp
+++ b/llvm/lib/Fuzzer/FuzzerMutate.cpp
@@ -485,13 +485,6 @@ size_t MutationDispatcher::MutateImpl(uint8_t *Data, size_t Size,
                                       size_t MaxSize,
                                       const std::vector<Mutator> &Mutators) {
   assert(MaxSize > 0);
-  if (Size == 0) {
-    for (size_t i = 0; i < MaxSize; i++)
-      Data[i] = RandCh(Rand);
-    if (Options.OnlyASCII)
-      ToASCII(Data, MaxSize);
-    return MaxSize;
-  }
   assert(Size > 0);
   // Some mutations may fail (e.g. can't insert more bytes if Size == MaxSize),
   // in which case they will return 0.
diff --git a/llvm/lib/Fuzzer/FuzzerOptions.h b/llvm/lib/Fuzzer/FuzzerOptions.h
index f1cd4935be9..1f6167326d2 100644
--- a/llvm/lib/Fuzzer/FuzzerOptions.h
+++ b/llvm/lib/Fuzzer/FuzzerOptions.h
@@ -19,6 +19,7 @@ namespace fuzzer {
 struct FuzzingOptions {
   int Verbosity = 1;
   size_t MaxLen = 0;
+  bool ExperimentalLenControl = false;
   int UnitTimeoutSec = 300;
   int TimeoutExitCode = 77;
   int ErrorExitCode = 77;