Libfuzzer: do not pass null into user function

[krasin]

LGTM with a nit

lib/Fuzzer/test/SimpleTest.cpp

11

Alternatively, you could #include "gtest/gtest.h" and use EXPECT_NE:

EXPECT_NE(NULL, Data) << "Pointer to the data must not be NULL"

EXPECT_NE will work even if LLVM is built without asserts, while assert will just pass.

[kcc]

lib/Fuzzer/FuzzerLoop.cpp

242

Looks weird.
I would prefer to have assert(!U.empty()) and enforce it in callers.

lib/Fuzzer/test/SimpleTest.cpp

11

Oh no, we don't want these simple tests to depend on gtest.

[aizatsky]

lib/Fuzzer/FuzzerLoop.cpp

242

There are too many paths that lead here. Plus, didn't we agree that its ok to call it with empty unit?

[kcc]

lib/Fuzzer/FuzzerLoop.cpp

242

Emm. I am probably not 100% convinced we want to call this with empty units. But ok, let's assume we can.
Now, remind me, why we can't call the target function as f(NULL, 0) ?

[kcc]

LGTM with two nits.

lib/Fuzzer/FuzzerLoop.cpp

242

no static?

243

no {} ?

[aizatsky]

review

[aizatsky]

thanks. running check-all and submitting.

lib/Fuzzer/FuzzerLoop.cpp

if (TotalNumberOfRuns >= Options.MaxNumberOfRuns)
     return;
   if (Options.OnlyASCII)
     ToASCII(U);
   if (RunOne(U))
     ReportNewCoverage(U);
 }
 
 void Fuzzer::ExecuteCallback(const Unit &U) {
-  int Res = USF.TargetFunction(U.data(), U.size());
+  const uint8_t *Data = U.data();
+  static uint8_t EmptyData;
+  if (!Data) {
+    Data = &EmptyData;
+  }
+  int Res = USF.TargetFunction(Data, U.size());
   (void)Res;
   assert(Res == 0);
 }
 
 size_t Fuzzer::RecordBlockCoverage() {
   CHECK_WEAK_API_FUNCTION(__sanitizer_get_total_unique_coverage);
   return LastRecordedBlockCoverage = __sanitizer_get_total_unique_coverage();
 }

lib/Fuzzer/test/SimpleTest.cpp

 // Simple test for a fuzzer. The fuzzer must find the string "Hi!".
+#include <assert.h>
 #include <cstdint>
 #include <cstdlib>
 #include <cstddef>
 #include <iostream>
 
 static volatile int Sink;
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  assert(Data);
   if (Size > 0 && Data[0] == 'H') {
     Sink = 1;
     if (Size > 1 && Data[1] == 'i') {
       Sink = 2;
       if (Size > 2 && Data[2] == '!') {
         std::cout << "BINGO; Found the target, exiting\n";
         exit(0);
       }
     }
   }
   return 0;
 }