[LibFuzzer] Introduce a portable WeakAlias implementation.

Windows doesn't really support weak aliases, but with some
linker magic we can get something that's pretty close on
Windows.  This introduces an interface to accessing weakly
aliased symbols that will work on any platform.  Linker
magic changes to come in a separate patch.

Patch by Marcos Pividori
Differential Revision: https://reviews.llvm.org/D27235

llvm-svn: 288530

2 files changed, 55 insertions, 0 deletions

diff --git a/llvm/lib/Fuzzer/CMakeLists.txt b/llvm/lib/Fuzzer/CMakeLists.txt
index ff59f88d506..0276db5fdc3 100644
--- a/llvm/lib/Fuzzer/CMakeLists.txt
+++ b/llvm/lib/Fuzzer/CMakeLists.txt
@@ -13,6 +13,7 @@ if( LLVM_USE_SANITIZE_COVERAGE )
     FuzzerDriver.cpp
     FuzzerExtFunctionsDlsym.cpp
     FuzzerExtFunctionsWeak.cpp
+    FuzzerExtFunctionsWeakAlias.cpp
     FuzzerIO.cpp
     FuzzerIOPosix.cpp
     FuzzerIOWindows.cpp
diff --git a/llvm/lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp b/llvm/lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
new file mode 100644
index 00000000000..3c42e539fcc
--- /dev/null
+++ b/llvm/lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
@@ -0,0 +1,54 @@
+//===- 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.
+//===----------------------------------------------------------------------===//
+#include "FuzzerDefs.h"
+#if LIBFUZZER_WINDOWS
+
+#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) {
+  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