[sanitizer/coverage] Add AFL-style coverage counters (search heuristic for fuzzing).

Introduce -mllvm -sanitizer-coverage-8bit-counters=1
which adds imprecise thread-unfriendly 8-bit coverage counters.

The run-time library maps these 8-bit counters to 8-bit bitsets in the same way
AFL (http://lcamtuf.coredump.cx/afl/technical_details.txt) does:
counter values are divided into 8 ranges and based on the counter
value one of the bits in the bitset is set.
The AFL ranges are used here: 1, 2, 3, 4-7, 8-15, 16-31, 32-127, 128+.

These counters provide a search heuristic for single-threaded
coverage-guided fuzzers, we do not expect them to be useful for other purposes.

Depending on the value of -fsanitize-coverage=[123] flag,
these counters will be added to the function entry blocks (=1),
every basic block (=2), or every edge (=3).

Use these counters as an optional search heuristic in the Fuzzer library.
Add a test where this heuristic is critical.

llvm-svn: 231166

cmake/modules/HandleLLVMOptions.cmake

@@ -424,7 +424,7 @@ if(LLVM_USE_SANITIZER)
     message(WARNING "LLVM_USE_SANITIZER is not supported on this platform.")
   endif()
   if (LLVM_USE_SANITIZE_COVERAGE)
-    append("-fsanitize-coverage=4" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+    append("-fsanitize-coverage=4 -mllvm -sanitizer-coverage-8bit-counters=1" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
 endif()
 

lib/Fuzzer/FuzzerDriver.cpp

@@ -158,6 +158,7 @@ int FuzzerDriver(int argc, char **argv, UserCallback Callback) {
   Options.DoCrossOver = Flags.cross_over;
   Options.MutateDepth = Flags.mutate_depth;
   Options.ExitOnFirst = Flags.exit_on_first;
+  Options.UseCounters = Flags.use_counters;
   Options.UseFullCoverageSet = Flags.use_full_coverage_set;
   Options.UseCoveragePairs = Flags.use_coverage_pairs;
   Options.PreferSmallDuringInitialShuffle =

lib/Fuzzer/FuzzerFlags.def

@@ -32,6 +32,7 @@ FUZZER_FLAG(int, help, 0, "Print help.")
 FUZZER_FLAG(
     int, save_minimized_corpus, 0,
     "If 1, the minimized corpus is saved into the first input directory")
+FUZZER_FLAG(int, use_counters, 0, "Use coverage counters")
 FUZZER_FLAG(int, use_full_coverage_set, 0,
             "Experimental: Maximize the number of different full"
             " coverage sets as opposed to maximizing the total coverage."

lib/Fuzzer/FuzzerInternal.h

@@ -48,6 +48,7 @@ class Fuzzer {
     bool DoCrossOver = true;
     int  MutateDepth = 5;
     bool ExitOnFirst = false;
+    bool UseCounters = false;
     bool UseFullCoverageSet  = false;
     bool UseCoveragePairs = false;
     int PreferSmallDuringInitialShuffle = -1;
@@ -95,6 +96,15 @@ class Fuzzer {
   std::vector<Unit> Corpus;
   std::unordered_set<uintptr_t> FullCoverageSets;
   std::unordered_set<uint64_t>  CoveragePairs;
+
+  // For UseCounters
+  std::vector<uint8_t> CounterBitmap;
+  size_t TotalBits() {  // Slow. Call it only for printing stats.
+    size_t Res = 0;
+    for (auto x : CounterBitmap) Res += __builtin_popcount(x);
+    return Res;
+  }
+
   UserCallback Callback;
   FuzzingOptions Options;
   system_clock::time_point ProcessStartTime = system_clock::now();

lib/Fuzzer/FuzzerLoop.cpp

@@ -138,17 +138,28 @@ size_t Fuzzer::RunOneMaximizeFullCoverageSet(const Unit &U) {
 }
 
 size_t Fuzzer::RunOneMaximizeTotalCoverage(const Unit &U) {
+  size_t NumCounters = __sanitizer_get_number_of_counters();
+  if (Options.UseCounters) {
+    CounterBitmap.resize(NumCounters);
+    __sanitizer_update_counter_bitset_and_clear_counters(0);
+  }
   size_t OldCoverage = __sanitizer_get_total_unique_coverage();
   Callback(U.data(), U.size());
   size_t NewCoverage = __sanitizer_get_total_unique_coverage();
+  size_t NumNewBits = 0;
+  if (Options.UseCounters)
+    NumNewBits = __sanitizer_update_counter_bitset_and_clear_counters(
+        CounterBitmap.data());
+
   if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)) && Options.Verbosity) {
     size_t Seconds = secondsSinceProcessStartUp();
     std::cerr
         << "#" << TotalNumberOfRuns
         << "\tcov: " << NewCoverage
+        << "\tbits: " << TotalBits()
         << "\texec/s: " << (Seconds ? TotalNumberOfRuns / Seconds : 0) << "\n";
   }
-  if (NewCoverage > OldCoverage)
+  if (NewCoverage > OldCoverage || NumNewBits)
     return NewCoverage;
   return 0;
 }
@@ -189,6 +200,7 @@ size_t Fuzzer::MutateAndTestOne(Unit *U) {
       if (Options.Verbosity) {
         std::cerr << "#" << TotalNumberOfRuns
                   << "\tNEW: " << NewCoverage
+                  << " B: " << TotalBits()
                   << " L: " << U->size()
                   << " S: " << Corpus.size()
                   << " I: " << i

lib/Fuzzer/test/CMakeLists.txt

@@ -5,6 +5,7 @@
 set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -O0 -fsanitize-coverage=4")
 
 set(Tests
+  CounterTest
   FourIndependentBranchesTest
   FullCoverageSetTest
   InfiniteTest

lib/Fuzzer/test/CounterTest.cpp

@@ -0,0 +1,14 @@
+// Test for a fuzzer: must find the case where a particular basic block is
+// executed many times.
+#include <iostream>
+
+extern "C" void TestOneInput(const uint8_t *Data, size_t Size) {
+  int Num = 0;
+  for (size_t i = 0; i < Size; i++)
+    if (Data[i] == 'A' + i)
+      Num++;
+  if (Num >= 4) {
+    std::cerr <<  "BINGO!\n";
+    exit(1);
+  }
+}

lib/Fuzzer/test/fuzzer.test

@@ -17,3 +17,6 @@ FullCoverageSetTest: BINGO
 
 RUN: not ./LLVMFuzzer-FourIndependentBranchesTest -timeout=15 -seed=1 -use_coverage_pairs=1 2>&1 | FileCheck %s --check-prefix=FourIndependentBranchesTest
 FourIndependentBranchesTest: BINGO
+
+RUN: not ./LLVMFuzzer-CounterTest -use_counters=1 -max_len=6 -seed=1 -timeout=15 2>&1 | FileCheck %s --check-prefix=CounterTest
+CounterTest: BINGO

lib/Transforms/Instrumentation/SanitizerCoverage.cpp

@@ -80,6 +80,16 @@ static cl::opt<bool>
                                    "callbacks at every basic block"),
                           cl::Hidden, cl::init(false));
 
+// Experimental 8-bit counters used as an additional search heuristic during
+// coverage-guided fuzzing.
+// The counters are not thread-friendly:
+//   - contention on these counters may cause significant slowdown;
+//   - the counter updates are racy and the results may be inaccurate.
+// They are also inaccurate due to 8-bit integer overflow.
+static cl::opt<bool> ClUse8bitCounters("sanitizer-coverage-8bit-counters",
+                                       cl::desc("Experimental 8-bit counters"),
+                                       cl::Hidden, cl::init(false));
+
 namespace {
 
 class SanitizerCoverageModule : public ModulePass {
@@ -114,6 +124,7 @@ class SanitizerCoverageModule : public ModulePass {
   LLVMContext *C;
 
   GlobalVariable *GuardArray;
+  GlobalVariable *EightBitCounterArray;
 
   int CoverageLevel;
 };
@@ -152,9 +163,9 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
       M.getOrInsertFunction(kSanCovWithCheckName, VoidTy, Int32PtrTy, nullptr));
   SanCovIndirCallFunction = checkInterfaceFunction(M.getOrInsertFunction(
       kSanCovIndirCallName, VoidTy, IntptrTy, IntptrTy, nullptr));
-  SanCovModuleInit = checkInterfaceFunction(
-      M.getOrInsertFunction(kSanCovModuleInitName, Type::getVoidTy(*C),
-                            Int32PtrTy, IntptrTy, Int8PtrTy, nullptr));
+  SanCovModuleInit = checkInterfaceFunction(M.getOrInsertFunction(
+      kSanCovModuleInitName, Type::getVoidTy(*C), Int32PtrTy, IntptrTy,
+      Int8PtrTy, Int8PtrTy, nullptr));
   SanCovModuleInit->setLinkage(Function::ExternalLinkage);
   // We insert an empty inline asm after cov callbacks to avoid callback merge.
   EmptyAsm = InlineAsm::get(FunctionType::get(IRB.getVoidTy(), false),
@@ -171,9 +182,15 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
   // At this point we create a dummy array of guards because we don't
   // know how many elements we will need.
   Type *Int32Ty = IRB.getInt32Ty();
+  Type *Int8Ty = IRB.getInt8Ty();
+
   GuardArray =
       new GlobalVariable(M, Int32Ty, false, GlobalValue::ExternalLinkage,
                          nullptr, "__sancov_gen_cov_tmp");
+  if (ClUse8bitCounters)
+    EightBitCounterArray =
+        new GlobalVariable(M, Int8Ty, false, GlobalVariable::ExternalLinkage,
+                           nullptr, "__sancov_gen_cov_tmp");
 
   for (auto &F : M)
     runOnFunction(F);
@@ -186,11 +203,28 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
       M, Int32ArrayNTy, false, GlobalValue::PrivateLinkage,
       Constant::getNullValue(Int32ArrayNTy), "__sancov_gen_cov");
 
+
   // Replace the dummy array with the real one.
   GuardArray->replaceAllUsesWith(
       IRB.CreatePointerCast(RealGuardArray, Int32PtrTy));
   GuardArray->eraseFromParent();
 
+  GlobalVariable *RealEightBitCounterArray;
+  if (ClUse8bitCounters) {
+    // Make sure the array is 16-aligned.
+    static const int kCounterAlignment = 16;
+    Type *Int8ArrayNTy =
+        ArrayType::get(Int8Ty, RoundUpToAlignment(SanCovFunction->getNumUses(),
+                                                  kCounterAlignment));
+    RealEightBitCounterArray = new GlobalVariable(
+        M, Int8ArrayNTy, false, GlobalValue::PrivateLinkage,
+        Constant::getNullValue(Int8ArrayNTy), "__sancov_gen_cov_counter");
+    RealEightBitCounterArray->setAlignment(kCounterAlignment);
+    EightBitCounterArray->replaceAllUsesWith(
+        IRB.CreatePointerCast(RealEightBitCounterArray, Int8PtrTy));
+    EightBitCounterArray->eraseFromParent();
+  }
+
   // Create variable for module (compilation unit) name
   Constant *ModNameStrConst =
       ConstantDataArray::getString(M.getContext(), M.getName(), true);
@@ -200,10 +234,13 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
 
   // Call __sanitizer_cov_module_init
   IRB.SetInsertPoint(CtorFunc->getEntryBlock().getTerminator());
-  IRB.CreateCall3(SanCovModuleInit,
-                  IRB.CreatePointerCast(RealGuardArray, Int32PtrTy),
-                  ConstantInt::get(IntptrTy, SanCovFunction->getNumUses()),
-                  IRB.CreatePointerCast(ModuleName, Int8PtrTy));
+  IRB.CreateCall4(
+      SanCovModuleInit, IRB.CreatePointerCast(RealGuardArray, Int32PtrTy),
+      ConstantInt::get(IntptrTy, SanCovFunction->getNumUses()),
+      ClUse8bitCounters
+          ? IRB.CreatePointerCast(RealEightBitCounterArray, Int8PtrTy)
+          : Constant::getNullValue(Int8PtrTy),
+      IRB.CreatePointerCast(ModuleName, Int8PtrTy));
   return true;
 }
 
@@ -314,6 +351,17 @@ void SanitizerCoverageModule::InjectCoverageAtBlock(Function &F, BasicBlock &BB,
     IRB.CreateCall(EmptyAsm);  // Avoids callback merge.
   }
 
+  if(ClUse8bitCounters) {
+    IRB.SetInsertPoint(IP);
+    Value *P = IRB.CreateAdd(
+        IRB.CreatePointerCast(EightBitCounterArray, IntptrTy),
+        ConstantInt::get(IntptrTy, SanCovFunction->getNumUses() - 1));
+    P = IRB.CreateIntToPtr(P, IRB.getInt8PtrTy());
+    Value *LI = IRB.CreateLoad(P);
+    Value *Inc = IRB.CreateAdd(LI, ConstantInt::get(IRB.getInt8Ty(), 1));
+    IRB.CreateStore(Inc, P);
+  }
+
   if (ClExperimentalTracing) {
     // Experimental support for tracing.
     // Insert a callback with the same guard variable as used for coverage.

test/Instrumentation/SanitizerCoverage/coverage.ll

@@ -5,10 +5,8 @@
 ; RUN: opt < %s -sancov -sanitizer-coverage-level=2 -sanitizer-coverage-block-threshold=1  -S | FileCheck %s --check-prefix=CHECK_WITH_CHECK
 ; RUN: opt < %s -sancov -sanitizer-coverage-level=3 -sanitizer-coverage-block-threshold=10 -S | FileCheck %s --check-prefix=CHECK3
 ; RUN: opt < %s -sancov -sanitizer-coverage-level=4 -S | FileCheck %s --check-prefix=CHECK4
+; RUN: opt < %s -sancov -sanitizer-coverage-level=3 -sanitizer-coverage-8bit-counters=1  -S | FileCheck %s --check-prefix=CHECK-8BIT
 
-; RUN: opt < %s -sancov -sanitizer-coverage-level=0  -S | FileCheck %s --check-prefix=CHECK0
-; RUN: opt < %s -sancov -sanitizer-coverage-level=1  -S | FileCheck %s --check-prefix=CHECK1
-; RUN: opt < %s -sancov -sanitizer-coverage-level=2  -S | FileCheck %s --check-prefix=CHECK2
 ; RUN: opt < %s -sancov -sanitizer-coverage-level=2 -sanitizer-coverage-block-threshold=10 \
 ; RUN:      -S | FileCheck %s --check-prefix=CHECK2
 ; RUN: opt < %s -sancov -sanitizer-coverage-level=2 -sanitizer-coverage-block-threshold=1 \
@@ -78,6 +76,24 @@ entry:
 ; CHECK3-NOT: call void @__sanitizer_cov
 ; CHECK3: ret void
 
+; test -sanitizer-coverage-8bit-counters=1
+; CHECK-8BIT-LABEL: define void @foo
+
+; CHECK-8BIT: [[V11:%[0-9]*]] = load i8
+; CHECK-8BIT: [[V12:%[0-9]*]] = add i8 [[V11]], 1
+; CHECK-8BIT: store i8 [[V12]]
+; CHECK-8BIT: [[V21:%[0-9]*]] = load i8
+; CHECK-8BIT: [[V22:%[0-9]*]] = add i8 [[V21]], 1
+; CHECK-8BIT: store i8 [[V22]]
+; CHECK-8BIT: [[V31:%[0-9]*]] = load i8
+; CHECK-8BIT: [[V32:%[0-9]*]] = add i8 [[V31]], 1
+; CHECK-8BIT: store i8 [[V32]]
+; CHECK-8BIT: [[V41:%[0-9]*]] = load i8
+; CHECK-8BIT: [[V42:%[0-9]*]] = add i8 [[V41]], 1
+; CHECK-8BIT: store i8 [[V42]]
+
+; CHECK-8BIT: ret void
+
 
 %struct.StructWithVptr = type { i32 (...)** }