add libfuzzer custom mutator, minor enhancements and fixes

custom_mutators/libfuzzer/FuzzerExtFunctionsWeak.cpp

@@ -0,0 +1,63 @@
+//===- FuzzerExtFunctionsWeak.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 Linux. This relies on the linker's support for weak
+// symbols. We don't use this approach on Apple platforms because it requires
+// clients of LibFuzzer to pass ``-U _<symbol_name>`` to the linker to allow
+// weak symbols to be undefined. That is a complication we don't want to expose
+// to clients right now.
+//===----------------------------------------------------------------------===//
+#include "FuzzerPlatform.h"
+#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_FUCHSIA || \
+    LIBFUZZER_FREEBSD || LIBFUZZER_OPENBSD || LIBFUZZER_EMSCRIPTEN
+
+  #include "FuzzerExtFunctions.h"
+  #include "FuzzerIO.h"
+
+extern "C" {
+
+  // Declare these symbols as weak to allow them to be optionally defined.
+  #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN) \
+    __attribute__((weak, visibility("default"))) RETURN_TYPE NAME FUNC_SIG
+
+  #include "FuzzerExtFunctions.def"
+
+  #undef EXT_FUNC
+
+}
+
+using namespace fuzzer;
+
+static void CheckFnPtr(void *FnPtr, const char *FnName, bool WarnIfMissing) {
+
+  if (FnPtr == nullptr && WarnIfMissing) {
+
+    Printf("WARNING: Failed to find function \"%s\".\n", FnName);
+
+  }
+
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+\
+  #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                         \
+    this->NAME = ::NAME;                                                      \
+    CheckFnPtr(reinterpret_cast<void *>(reinterpret_cast<uintptr_t>(::NAME)), \
+               #NAME, WARN);
+
+  #include "FuzzerExtFunctions.def"
+
+  #undef EXT_FUNC
+
+}
+
+}  // namespace fuzzer
+
+#endif
+