[libFuzzer] Make dataflow and focus functions more user friendly.

Summary:
- Fail loudly if SetFocusFunction failed when it should not. For more info see
  - https://github.com/google/oss-fuzz/issues/3311
  - https://github.com/google/sanitizers/issues/1190
- Fail loudly if CollectDataFlow is called without seed corpus.

Reviewers: kcc, metzman

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73813

compiler-rt/lib/fuzzer/FuzzerDataFlowTrace.cpp

@@ -248,6 +248,11 @@ int CollectDataFlow(const std::string &DFTBinary, const std::string &DirPath,
                     const Vector<SizedFile> &CorporaFiles) {
   Printf("INFO: collecting data flow: bin: %s dir: %s files: %zd\n",
          DFTBinary.c_str(), DirPath.c_str(), CorporaFiles.size());
+  if (CorporaFiles.empty()) {
+    Printf("ERROR: can't collect data flow without corpus provided.");
+    return 1;
+  }
+
   static char DFSanEnv[] = "DFSAN_OPTIONS=fast16labels=1:warn_unimplemented=0";
   putenv(DFSanEnv);
   MkDir(DirPath);

compiler-rt/lib/fuzzer/FuzzerTracePC.cpp

@@ -240,7 +240,9 @@ void TracePC::IterateCoveredFunctions(CallBack CB) {
 void TracePC::SetFocusFunction(const std::string &FuncName) {
   // This function should be called once.
   assert(!FocusFunctionCounterPtr);
-  if (FuncName.empty())
+  // "auto" is not a valid function name. If this function is called with "auto"
+  // that means the auto focus functionality failed.
+  if (FuncName.empty() || FuncName == "auto")
     return;
   for (size_t M = 0; M < NumModules; M++) {
     auto &PCTE = ModulePCTable[M];
@@ -256,6 +258,10 @@ void TracePC::SetFocusFunction(const std::string &FuncName) {
       return;
     }
   }
+
+  Printf("ERROR: Failed to set focus function. Make sure the function name is "
+         "valid (%s) and symbolization is enabled.\n", FuncName.c_str());
+  exit(1);
 }
 
 bool TracePC::ObservedFocusFunction() {

compiler-rt/test/fuzzer/dataflow.test

@@ -118,3 +118,9 @@ RUN: printf "%0.sA" {1..150001} > %t/IN/very_long_input
 RUN: rm -rf %t/OUT
 RUN: %t-ThreeFunctionsTest  -collect_data_flow=%t-ThreeFunctionsTestDF -data_flow_trace=%t/OUT %t/IN/very_long_input
 RUN: rm %t/IN/very_long_input
+
+# Test that it fails explicitly when an empty corpus is provided.
+RUN: rm -rf %t/IN && mkdir %t/IN
+RUN: not %t-ThreeFunctionsTest  -collect_data_flow=%t-ThreeFunctionsTestDF -data_flow_trace=%t/OUT %t/IN 2>&1 | FileCheck %s --check-prefix=EMPTY_CORPUS
+
+EMPTY_CORPUS: ERROR: can't collect data flow without corpus provided

compiler-rt/test/fuzzer/focus-function.test

@@ -10,9 +10,9 @@ RUN: %t-exe -runs=100 2>&1 | FileCheck %s --check-prefix=FOCUS_NONE
 FOCUS_NONE-NOT: INFO: Focus function is set to
 FOCUS_NONE-NOT: INFO: {{.*}} inputs touch the focus function
 
-RUN: %t-exe -runs=100 -focus_function=WRONG 2>&1 | FileCheck %s --check-prefix=FOCUS_WRONG
+RUN: not %t-exe -runs=100 -focus_function=WRONG 2>&1 | FileCheck %s --check-prefix=FOCUS_WRONG
 FOCUS_WRONG-NOT: INFO: Focus function is set to
-FOCUS_WRONG: INFO: 0/1 inputs touch the focus function
+FOCUS_WRONG: ERROR: Failed to set focus function
 
 RUN: %t-exe -runs=100 -focus_function=f0 2>&1 | FileCheck %s --check-prefix=FOCUS_F0
 FOCUS_F0: INFO: Focus function is set to 'f0'