[libFuzzer] Add INFO output when LLVMFuzzerCustomMutator is found.

[Dor1s]

Summary

there is an ongoing work on interchangeable custom mutators
(https://github.com/google/clusterfuzz/pull/1333/files#r367706283)
and having some sort of signalling from libFuzzer that it has loaded
a custom mutator would be helpful.

The initial idea was to make the mutator to print something, but given
the anticipated variety of different mutators, it does not seem possible
to make all of them print the same message to signal their execution.

[metzman]

LGTM. This seems like a generic way of handling this and is especially useful when using LD_PRELOAD to load custom mutators since iirc LD_PRELOAD doesn't make so much noise when it fails.

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

201

Maybe drop "by default" I don't think they can opt out right?

[Dor1s]

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

201

Actually it can be overwritten by a value passed via cmd and there is a test for that!

[metzman]

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

201

ACK. LGTM

[Dor1s]

Given that this is a simple change and Kostya is a busy man I'm going to go ahead and land this. We can always revert if there is any concern.

[Dor1s]

rebase

[Dor1s]

one rebase is never enough

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

for (size_t F = 0; F < kNumFlags; F++) {
     if (FlagDescriptions[F].UIntFlag)
       *FlagDescriptions[F].UIntFlag =
           static_cast<unsigned int>(FlagDescriptions[F].Default);
     if (FlagDescriptions[F].StrFlag)
       *FlagDescriptions[F].StrFlag = nullptr;
   }
 
   // Disable len_control by default, if LLVMFuzzerCustomMutator is used.
-  if (EF->LLVMFuzzerCustomMutator)
+  if (EF->LLVMFuzzerCustomMutator) {
     Flags.len_control = 0;
+    Printf("INFO: found LLVMFuzzerCustomMutator (%p). "
+           "Disabling -len_control by default.\n", EF->LLVMFuzzerCustomMutator);
+  }
 
   Inputs = new Vector<std::string>;
   for (size_t A = 1; A < Args.size(); A++) {
     if (ParseOneFlag(Args[A].c_str())) {
       if (Flags.ignore_remaining_args)
         break;
       continue;
     }

compiler-rt/test/fuzzer/fuzzer-custommutator.test

 RUN: %cpp_compiler %S/CustomMutatorTest.cpp -o %t-CustomMutatorTest
 RUN: not %run %t-CustomMutatorTest 2>&1 | FileCheck %s --check-prefix=LLVMFuzzerCustomMutator
+LLVMFuzzerCustomMutator: INFO: found LLVMFuzzerCustomMutator
 LLVMFuzzerCustomMutator: In LLVMFuzzerCustomMutator
 LLVMFuzzerCustomMutator: {{.*}} lim: 4096 {{.*}}
 LLVMFuzzerCustomMutator: BINGO
 
 # len_control is disabled for custom mutators by default, test that it can be enabled.
 RUN: not %run %t-CustomMutatorTest -len_control=100 2>&1 | FileCheck %s --check-prefix=LLVMFuzzerCustomMutatorWithLenControl
+LLVMFuzzerCustomMutatorWithLenControl: INFO: found LLVMFuzzerCustomMutator
 LLVMFuzzerCustomMutatorWithLenControl: In LLVMFuzzerCustomMutator
 LLVMFuzzerCustomMutatorWithLenControl: {{.*}} lim: {{[1-9][0-9]?}} {{.*}}
 LLVMFuzzerCustomMutatorWithLenControl: BINGO