libsanitizer/sanitizer_common/sanitizer_win_defs.h

   1 //===-- sanitizer_win_defs.h ------------------------------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // Common definitions for Windows-specific code.
  10 //
  11 //===----------------------------------------------------------------------===//
  12 #ifndef SANITIZER_WIN_DEFS_H
  13 #define SANITIZER_WIN_DEFS_H
  14
  15 #include "sanitizer_platform.h"
  16 #if SANITIZER_WINDOWS
  17
  18 #ifndef WINAPI
  19 #if defined(_M_IX86) || defined(__i386__)
  20 #define WINAPI __stdcall
  21 #else
  22 #define WINAPI
  23 #endif
  24 #endif
  25
  26 #if defined(_M_IX86) || defined(__i386__)
  27 #define WIN_SYM_PREFIX "_"
  28 #else
  29 #define WIN_SYM_PREFIX
  30 #endif
  31
  32 // For MinGW, the /export: directives contain undecorated symbols, contrary to
  33 // link/lld-link. The GNU linker doesn't support /alternatename and /include
  34 // though, thus lld-link in MinGW mode interprets them in the same way as
  35 // in the default mode.
  36 #ifdef __MINGW32__
  37 #define WIN_EXPORT_PREFIX
  38 #else
  39 #define WIN_EXPORT_PREFIX WIN_SYM_PREFIX
  40 #endif
  41
  42 // Intermediate macro to ensure the parameter is expanded before stringified.
  43 #define STRINGIFY_(A) #A
  44 #define STRINGIFY(A) STRINGIFY_(A)
  45
  46 #if !SANITIZER_GO
  47
  48 // ----------------- A workaround for the absence of weak symbols --------------
  49 // We don't have a direct equivalent of weak symbols when using MSVC, but we can
  50 // use the /alternatename directive to tell the linker to default a specific
  51 // symbol to a specific value.
  52 // Take into account that this is a pragma directive for the linker, so it will
  53 // be ignored by the compiler and the function will be marked as UNDEF in the
  54 // symbol table of the resulting object file. The linker won't find the default
  55 // implementation until it links with that object file.
  56 // So, suppose we provide a default implementation "fundef" for "fun", and this
  57 // is compiled into the object file "test.obj" including the pragma directive.
  58 // If we have some code with references to "fun" and we link that code with
  59 // "test.obj", it will work because the linker always link object files.
  60 // But, if "test.obj" is included in a static library, like "test.lib", then the
  61 // liker will only link to "test.obj" if necessary. If we only included the
  62 // definition of "fun", it won't link to "test.obj" (from test.lib) because
  63 // "fun" appears as UNDEF, so it doesn't resolve the symbol "fun", and will
  64 // result in a link error (the linker doesn't find the pragma directive).
  65 // So, a workaround is to force linkage with the modules that include weak
  66 // definitions, with the following macro: WIN_FORCE_LINK()
  67
  68 #define WIN_WEAK_ALIAS(Name, Default)                                          \
  69   __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY(Name) "="\
  70                                              WIN_SYM_PREFIX STRINGIFY(Default)))
  71
  72 #define WIN_FORCE_LINK(Name)                                                   \
  73   __pragma(comment(linker, "/include:" WIN_SYM_PREFIX STRINGIFY(Name)))
  74
  75 #define WIN_EXPORT(ExportedName, Name)                                         \
  76   __pragma(comment(linker, "/export:" WIN_EXPORT_PREFIX STRINGIFY(ExportedName)\
  77                                   "=" WIN_EXPORT_PREFIX STRINGIFY(Name)))
  78
  79 // We cannot define weak functions on Windows, but we can use WIN_WEAK_ALIAS()
  80 // which defines an alias to a default implementation, and only works when
  81 // linking statically.
  82 // So, to define a weak function "fun", we define a default implementation with
  83 // a different name "fun__def" and we create a "weak alias" fun = fun__def.
  84 // Then, users can override it just defining "fun".
  85 // We impose "extern "C"" because otherwise WIN_WEAK_ALIAS() will fail because
  86 // of name mangling.
  87
  88 // Dummy name for default implementation of weak function.
  89 # define WEAK_DEFAULT_NAME(Name) Name##__def
  90 // Name for exported implementation of weak function.
  91 # define WEAK_EXPORT_NAME(Name) Name##__dll
  92
  93 // Use this macro when you need to define and export a weak function from a
  94 // library. For example:
  95 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
  96 # define WIN_WEAK_EXPORT_DEF(ReturnType, Name, ...)                            \
  97   WIN_WEAK_ALIAS(Name, WEAK_DEFAULT_NAME(Name))                                \
  98   WIN_EXPORT(WEAK_EXPORT_NAME(Name), Name)                                     \
  99   extern "C" ReturnType Name(__VA_ARGS__);                                     \
 100   extern "C" ReturnType WEAK_DEFAULT_NAME(Name)(__VA_ARGS__)
 101
 102 // Use this macro when you need to import a weak function from a library. It
 103 // defines a weak alias to the imported function from the dll. For example:
 104 //   WIN_WEAK_IMPORT_DEF(compare)
 105 # define WIN_WEAK_IMPORT_DEF(Name)                                             \
 106   WIN_WEAK_ALIAS(Name, WEAK_EXPORT_NAME(Name))
 107
 108 // So, for Windows we provide something similar to weak symbols in Linux, with
 109 // some differences:
 110 // + A default implementation must always be provided.
 111 //
 112 // + When linking statically it works quite similarly. For example:
 113 //
 114 //   // libExample.cc
 115 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 116 //
 117 //   // client.cc
 118 //   // We can use the default implementation from the library:
 119 //   compare(1, 2);
 120 //   // Or we can override it:
 121 //   extern "C" bool compare (int a, int b) { return a >= b; }
 122 //
 123 //  And it will work fine. If we don't override the function, we need to ensure
 124 //  that the linker includes the object file with the default implementation.
 125 //  We can do so with the linker option "-wholearchive:".
 126 //
 127 // + When linking dynamically with a library (dll), weak functions are exported
 128 //  with "__dll" suffix. Clients can use the macro WIN_WEAK_IMPORT_DEF(fun)
 129 //  which defines a "weak alias" fun = fun__dll.
 130 //
 131 //   // libExample.cc
 132 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 133 //
 134 //   // client.cc
 135 //   WIN_WEAK_IMPORT_DEF(compare)
 136 //   // We can use the default implementation from the library:
 137 //   compare(1, 2);
 138 //   // Or we can override it:
 139 //   extern "C" bool compare (int a, int b) { return a >= b; }
 140 //
 141 //  But if we override the function, the dlls don't have access to it (which
 142 //  is different in linux). If that is desired, the strong definition must be
 143 //  exported and interception can be used from the rest of the dlls.
 144 //
 145 //   // libExample.cc
 146 //   WIN_WEAK_EXPORT_DEF(bool, compare, int a, int b) { return a > b; }
 147 //   // When initialized, check if the main executable defined "compare".
 148 //   int libExample_init() {
 149 //     uptr fnptr = __interception::InternalGetProcAddress(
 150 //         (void *)GetModuleHandleA(0), "compare");
 151 //     if (fnptr && !__interception::OverrideFunction((uptr)compare, fnptr, 0))
 152 //       abort();
 153 //     return 0;
 154 //   }
 155 //
 156 //   // client.cc
 157 //   WIN_WEAK_IMPORT_DEF(compare)
 158 //   // We override and export compare:
 159 //   extern "C" __declspec(dllexport) bool compare (int a, int b) {
 160 //     return a >= b;
 161 //   }
 162 //
 163
 164 #else // SANITIZER_GO
 165
 166 // Go neither needs nor wants weak references.
 167 // The shenanigans above don't work for gcc.
 168 # define WIN_WEAK_EXPORT_DEF(ReturnType, Name, ...)                            \
 169   extern "C" ReturnType Name(__VA_ARGS__)
 170
 171 #endif // SANITIZER_GO
 172
 173 #endif // SANITIZER_WINDOWS
 174 #endif // SANITIZER_WIN_DEFS_H