[compiler-rt] r358190 - [libFuzzer] Fallback to default Mutate when MutateWithMask fails.

[Max Moroz via llvm-commits]

Author: dor1s
Date: Thu Apr 11 09:24:53 2019
New Revision: 358190

URL: http://llvm.org/viewvc/llvm-project?rev=358190&view=rev
Log:
[libFuzzer] Fallback to default Mutate when MutateWithMask fails.

Summary:
In case the current corpus input doesn't have bytes going into the
focus function, MutateWithMask is useless and may fail gently, allowing the
default mutation routine happen, rather than crashing on an assertion.

For more context and the initial fix suggestion, see:
https://github.com/google/oss-fuzz/issues/1632#issuecomment-481862879

Reviewers: kcc, morehouse

Reviewed By: kcc

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60567

Modified:
    compiler-rt/trunk/lib/fuzzer/FuzzerLoop.cpp
    compiler-rt/trunk/lib/fuzzer/FuzzerMutate.cpp

Modified: compiler-rt/trunk/lib/fuzzer/FuzzerLoop.cpp
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/fuzzer/FuzzerLoop.cpp?rev=358190&r1=358189&r2=358190&view=diff
==============================================================================
--- compiler-rt/trunk/lib/fuzzer/FuzzerLoop.cpp (original)
+++ compiler-rt/trunk/lib/fuzzer/FuzzerLoop.cpp Thu Apr 11 09:24:53 2019
@@ -658,7 +658,9 @@ void Fuzzer::MutateAndTestOne() {
         Size <= CurrentMaxMutationLen)
       NewSize = MD.MutateWithMask(CurrentUnitData, Size, Size,
                                   II.DataFlowTraceForFocusFunction);
-    else
+    
+    // If MutateWithMask either failed or wasn't called, call default Mutate.
+    if (!NewSize)
       NewSize = MD.Mutate(CurrentUnitData, Size, CurrentMaxMutationLen);
     assert(NewSize > 0 && "Mutator returned empty unit");
     assert(NewSize <= CurrentMaxMutationLen && "Mutator return oversized unit");

Modified: compiler-rt/trunk/lib/fuzzer/FuzzerMutate.cpp
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/fuzzer/FuzzerMutate.cpp?rev=358190&r1=358189&r2=358190&view=diff
==============================================================================
--- compiler-rt/trunk/lib/fuzzer/FuzzerMutate.cpp (original)
+++ compiler-rt/trunk/lib/fuzzer/FuzzerMutate.cpp Thu Apr 11 09:24:53 2019
@@ -542,6 +542,7 @@ size_t MutationDispatcher::MutateWithMas
     if (Mask[I])
       T[OneBits++] = Data[I];
 
+  if (!OneBits) return 0;
   assert(!T.empty());
   size_t NewSize = Mutate(T.data(), OneBits, OneBits);
   assert(NewSize <= OneBits);