[libFuzzer] Fix FuzzerExtFunctions when working with weak functions.

[mpividori]

Summary

For weak functions we also need to check if "fun__dll" is present.
So, we were not checking properly for weak functions. This did not generate any problem before because the only weak function that we include is __sanitizer_print_memory_profile, but could generate problem in the future.
So, in this diff I include the changes to fix that.

lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp

 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 
 using namespace fuzzer;
 
 extern "C" {
 // Declare these symbols as weak to allow them to be optionally defined.
+// Default implementation of weak functions are exported with the __dll suffix,
+// so we need to check if "fun" or "fun__dll" is present.
 #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
   RETURN_TYPE NAME##Def FUNC_SIG {                                             \
     Printf("ERROR: Function \"%s\" not defined.\n", #NAME);                    \
     exit(1);                                                                   \
   }                                                                            \
-  RETURN_TYPE NAME FUNC_SIG __attribute__((weak, alias(#NAME "Def")));
+  RETURN_TYPE NAME FUNC_SIG __attribute__((weak, alias(#NAME "Def")));         \
+  RETURN_TYPE NAME##__dll FUNC_SIG __attribute__((weak, alias(#NAME "Def")));
 
 #include "FuzzerExtFunctions.def"
 
 #undef EXT_FUNC
 }
 
 template <typename T>
-static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
-  if (Fun == FunDef) {
-    if (WarnIfMissing)
-      Printf("WARNING: Failed to find function \"%s\".\n", FnName);
-    return nullptr;
-  }
-  return Fun;
-}
+static T *GetFnPtr(T *Fun, T *FunDll, T *FunDef, const char *FnName,
+    bool WarnIfMissing) {
+  if (Fun != FunDef)
+    return Fun;
+  if (FunDll != FunDef)
+    return FunDll;
 
 namespace fuzzer {
 
 ExternalFunctions::ExternalFunctions() {
 #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
-  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
+  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##__dll, ::NAME##Def,  \
+      #NAME, WARN);
 
 #include "FuzzerExtFunctions.def"
 
 #undef EXT_FUNC
 }
 
 } // namespace fuzzer
 
 #endif // LIBFUZZER_WINDOWS