add libfuzzer custom mutator, minor enhancements and fixes

custom_mutators/libfuzzer/FuzzerExtFunctionsDlsym.cpp

@@ -0,0 +1,60 @@
+//===- FuzzerExtFunctionsDlsym.cpp - Interface to external functions ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Implementation for operating systems that support dlsym(). We only use it on
+// Apple platforms for now. We don't use this approach on Linux because it
+// requires that clients of LibFuzzer pass ``--export-dynamic`` to the linker.
+// That is a complication we don't wish to expose to clients right now.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_APPLE
+
+  #include "FuzzerExtFunctions.h"
+  #include "FuzzerIO.h"
+  #include <dlfcn.h>
+
+using namespace fuzzer;
+
+template <typename T>
+static T GetFnPtr(const char *FnName, bool WarnIfMissing) {
+
+  dlerror();  // Clear any previous errors.
+  void *Fn = dlsym(RTLD_DEFAULT, FnName);
+  if (Fn == nullptr) {
+
+    if (WarnIfMissing) {
+
+      const char *ErrorMsg = dlerror();
+      Printf("WARNING: Failed to find function \"%s\".", FnName);
+      if (ErrorMsg) Printf(" Reason %s.", ErrorMsg);
+      Printf("\n");
+
+    }
+
+  }
+
+  return reinterpret_cast<T>(Fn);
+
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+\
+  #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN) \
+    this->NAME = GetFnPtr<decltype(ExternalFunctions::NAME)>(#NAME, WARN)
+
+  #include "FuzzerExtFunctions.def"
+
+  #undef EXT_FUNC
+
+}
+
+}  // namespace fuzzer
+
+#endif  // LIBFUZZER_APPLE
+