[libFuzzer][MSVC] Use alternatename for ext functions

[metzman]

Summary

Use alternatename for external functions only when using
MSVC since Clang doesn't support it and MSVC doesn't support
Clang's method (weak aliases).

[metzman]

trivial fix

[metzman]

• close paren

[metzman]

Undo accidental change

[metzman]

Matt could you please take a look?

For context, this patch is part of a series that I am trying to land to get libFuzzer working when compiled by MSVC.

@rnk or @thakis what do you think of this workaround for this bug I came up with?

I tried using things like volatile loads of the function pointers to prevent the optimizer from getting in the way, but I couldn't come up with a reliable solution, so I decided not to use alternatename when using clang.

[morehouse]

compiler-rt/lib/fuzzer/FuzzerExtFunctionsAlternatename.cpp

39

Maybe move the above macros into FuzzerDefs.h or create a FuzzerWinDefs.h. Then FuzzerExtFunctionsAlternatename.cpp is unneeded, and we can reuse FuzzerExtFunctionsWeakAlias.cpp.

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp

26

This attribute could also become a macro in FuzzerDefs.h or FuzzerWinDefs.h

[metzman]

compiler-rt/lib/fuzzer/FuzzerExtFunctionsAlternatename.cpp

39

What do you think of moving them both into a single file FuzzerExtFunctionsWin.cpp?
It seems better to define a macro in the only file it will ever be used than to define it in a header where there is less context and it could be leaked.
Your call of course though.

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp

26

If you agree with my above comment to create FuzzerExtFunctionsWin.cpp, then what do you think of just defining this as a macro in that file?

[morehouse]

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp

26

SGTM

[metzman]

• move to one file

[metzman]

• add comma

[metzman]

• fmt

[morehouse]

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWindows.cpp

37 ↗

(On Diff #181640)

Let's move all the macros outside of the extern "C" block except for the EXT_FUNC definition.

56 ↗

(On Diff #181640)

You can make the definition of WIN_WEAK_ALIAS depend on LIBFUZZER_MSVC and then use the same EXT_FUNC as above.

[metzman]

• make suggested changes

[morehouse]

LGTM

compiler-rt/lib/fuzzer/CMakeLists.txt

 set(LIBFUZZER_SOURCES
   FuzzerCrossOver.cpp
   FuzzerDataFlowTrace.cpp
   FuzzerDriver.cpp
+  FuzzerExtFunctionsAlternatename.cpp
   FuzzerExtFunctionsDlsym.cpp
   FuzzerExtFunctionsWeakAlias.cpp
   FuzzerExtFunctionsWeak.cpp
   FuzzerExtraCounters.cpp
   FuzzerIO.cpp
   FuzzerIOPosix.cpp
   FuzzerIOWindows.cpp
   FuzzerLoop.cpp

compiler-rt/lib/fuzzer/FuzzerExtFunctionsAlternatename.cpp

+//=== FuzzerExtFunctionsAlternatename.cpp - Interface to external functions ===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Implementation using alternatenames. Works for MSVC on Windows.
+// Does not work with clang (see https://bugs.llvm.org/show_bug.cgi?id=40218#c0
+// for more details.
+//===----------------------------------------------------------------------===//
+#include "FuzzerDefs.h"
+#if LIBFUZZER_WINDOWS && LIBFUZZER_MSVC
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+
+using namespace fuzzer;
+
+extern "C" {
+// Declare these symbols as weak to allow them to default to a specified
+// function if not defined explicitly.
+// Copied from compiler-rt/lib/sanitizer_common/sanitizer_win_defs.h
+#if defined(_M_IX86) || defined(__i386__)
+#define WIN_SYM_PREFIX "_"
+#else
+#define WIN_SYM_PREFIX
+#endif
+
+// Intermediate macro to ensure the parameter is expanded before stringified.
+#define STRINGIFY_(A) #A
+#define STRINGIFY(A) STRINGIFY_(A)
+
+#define WIN_WEAK_ALIAS(Name, Default)                                  \
+  __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY( \
+                               Name) "=" WIN_SYM_PREFIX STRINGIFY(Default)))
+
+#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);                                                \
+  }                                                         \
+  WIN_WEAK_ALIAS(NAME, NAME##Def) RETURN_TYPE NAME FUNC_SIG;
+#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;
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+} // namespace fuzzer
+
+#endif // LIBFUZZER_WINDOWS && LIBFUZZER_MSVC

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp

 //===- FuzzerExtFunctionsWeakAlias.cpp - Interface to external functions --===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-// Implementation using weak aliases. Works for Windows.
+// Implementation using weak aliases. Works for clang on Windows.
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
-#if LIBFUZZER_WINDOWS
+#if LIBFUZZER_WINDOWS && !LIBFUZZER_MSVC
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 
 using namespace fuzzer;
 
 extern "C" {
 // Declare these symbols as weak to allow them to be optionally defined.
 #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")));
 
 #include "FuzzerExtFunctions.def"
 
 #undef EXT_FUNC
 }
 
 template <typename T>
 static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {

 
 #include "FuzzerExtFunctions.def"
 
 #undef EXT_FUNC
 }
 
 } // namespace fuzzer
 
-#endif // LIBFUZZER_WINDOWS
+#endif // LIBFUZZER_WINDOWS && !LIBFUZZER_MSVC