base/win/dllmain.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // Windows doesn't support pthread_key_create's destr_function, and in fact
   6 // it's a bit tricky to get code to run when a thread exits.  This is
   7 // cargo-cult magic from http://www.codeproject.com/threads/tls.asp.
   8 // We are trying to be compatible with both a LoadLibrary style invocation, as
   9 // well as static linking. This code only needs to be included if we use
  10 // LoadLibrary, but it hooks into the "standard" set of TLS callbacks that are
  11 // provided for static linking.
  12
  13 // This code is deliberately written to match the style of calls seen in
  14 // base/threading/thread_local_storage_win.cc.  Please keep the two in sync if
  15 // coding conventions are changed.
  16
  17 // WARNING: Do *NOT* try to include this in the construction of the base
  18 // library, even though it potentially drives code in
  19 // base/threading/thread_local_storage_win.cc.  If you do, some users will end
  20 // up getting duplicate definition of DllMain() in some of their later links.
  21
  22 // Force a reference to _tls_used to make the linker create the TLS directory
  23 // if it's not already there (that is, even if __declspec(thread) is not used).
  24 // Force a reference to p_thread_callback_dllmain_typical_entry to prevent whole
  25 // program optimization from discarding the variables.
  26
  27 #include <windows.h>
  28
  29 #include "base/compiler_specific.h"
  30 #include "base/win/win_util.h"
  31
  32 // Indicate if another service is scanning the callbacks.  When this becomes
  33 // set to true, then DllMain() will stop supporting the callback service. This
  34 // value is set to true the first time any of our callbacks are called, as that
  35 // shows that some other service is handling callbacks.
  36 static bool linker_notifications_are_active = false;
  37
  38 // This will be our mostly no-op callback that we'll list.  We won't
  39 // deliberately call it, and if it is called, that means we don't need to do any
  40 // of the callbacks anymore.  We expect such a call to arrive via a
  41 // THREAD_ATTACH message, long before we'd have to perform our THREAD_DETACH
  42 // callbacks.
  43 static void NTAPI on_callback(PVOID h, DWORD reason, PVOID reserved);
  44
  45 #ifdef _WIN64
  46
  47 #pragma comment(linker, "/INCLUDE:_tls_used")
  48 #pragma comment(linker, "/INCLUDE:p_thread_callback_dllmain_typical_entry")
  49
  50 #else  // _WIN64
  51
  52 #pragma comment(linker, "/INCLUDE:__tls_used")
  53 #pragma comment(linker, "/INCLUDE:_p_thread_callback_dllmain_typical_entry")
  54
  55 #endif  // _WIN64
  56
  57 // Explicitly depend on tlssup.cc variable to bracket the list of TLS callbacks.
  58 extern "C" PIMAGE_TLS_CALLBACK __xl_a;
  59 extern "C" PIMAGE_TLS_CALLBACK __xl_z;
  60
  61 // extern "C" suppresses C++ name mangling so we know the symbol names for the
  62 // linker /INCLUDE:symbol pragmas above.
  63 extern "C" {
  64 #ifdef _WIN64
  65
  66 // .CRT section is merged with .rdata on x64 so it must be constant data.
  67 #pragma data_seg(push, old_seg)
  68 // Use a typical possible name in the .CRT$XL? list of segments.
  69 #pragma const_seg(".CRT$XLB")
  70 // When defining a const variable, it must have external linkage to be sure the
  71 // linker doesn't discard it.
  72 extern const PIMAGE_TLS_CALLBACK p_thread_callback_dllmain_typical_entry;
  73 const PIMAGE_TLS_CALLBACK p_thread_callback_dllmain_typical_entry = on_callback;
  74 #pragma data_seg(pop, old_seg)
  75
  76 #else  // _WIN64
  77
  78 #pragma data_seg(push, old_seg)
  79 // Use a typical possible name in the .CRT$XL? list of segments.
  80 #pragma data_seg(".CRT$XLB")
  81 PIMAGE_TLS_CALLBACK p_thread_callback_dllmain_typical_entry = on_callback;
  82 #pragma data_seg(pop, old_seg)
  83
  84 #endif  // _WIN64
  85 }  // extern "C"
  86
  87 // Custom crash code to get a unique entry in crash reports.
  88 NOINLINE static void CrashOnProcessDetach() {
  89   *static_cast<volatile int*>(0) = 0x356;
  90 }
  91
  92 // Make DllMain call the listed callbacks.  This way any third parties that are
  93 // linked in will also be called.
  94 BOOL WINAPI DllMain(PVOID h, DWORD reason, PVOID reserved) {
  95   if (DLL_PROCESS_DETACH == reason && base::win::ShouldCrashOnProcessDetach())
  96     CrashOnProcessDetach();
  97
  98   if (DLL_THREAD_DETACH != reason && DLL_PROCESS_DETACH != reason)
  99     return true;  // We won't service THREAD_ATTACH calls.
 100
 101   if (linker_notifications_are_active)
 102     return true;  // Some other service is doing this work.
 103
 104   for (PIMAGE_TLS_CALLBACK* it = &__xl_a; it < &__xl_z; ++it) {
 105     if (*it == NULL || *it == on_callback)
 106       continue;  // Don't bother to call our own callback.
 107     (*it)(h, reason, reserved);
 108   }
 109   return true;
 110 }
 111
 112 static void NTAPI on_callback(PVOID h, DWORD reason, PVOID reserved) {
 113   // Do nothing.  We were just a place holder in the list used to test that we
 114   // call all items.
 115   // If we are called, it means that some other system is scanning the callbacks
 116   // and we don't need to do so in DllMain().
 117   linker_notifications_are_active = true;
 118   // Note: If some other routine some how plays this same game... we could both
 119   // decide not to do the scanning <sigh>, but this trick should suppress
 120   // duplicate calls on Vista, where the runtime takes care of the callbacks,
 121   // and allow us to do the callbacks on XP, where we are currently devoid of
 122   // callbacks (due to an explicit LoadLibrary call).
 123 }