[gcc r12-8589] libstdc++: Fix lifetime bugs for non-TLS eh_globals [PR105880]

[gcc r12-8589] libstdc++: Fix lifetime bugs for non-TLS eh_globals [PR105880]

[Sebastian Huber]

https://gcc.gnu.org/g:ade3197134cc9ed2b7e589a7b58c73110ae13f1c

commit r12-8589-gade3197134cc9ed2b7e589a7b58c73110ae13f1c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 8 10:43:57 2022 +0100

    libstdc++: Fix lifetime bugs for non-TLS eh_globals [PR105880]
    
    This ensures that the single-threaded fallback buffer eh_globals is not
    destroyed during program termination, using the same immortalization
    technique used for error category objects.
    
    Also ensure that init._M_init can still be read after init has been
    destroyed, by making it a static data member.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/105880
            * libsupc++/eh_globals.cc (eh_globals): Ensure constant init and
            prevent destruction during termination.
            (__eh_globals_init::_M_init): Replace with static member _S_init.
            (__cxxabiv1::__cxa_get_globals_fast): Update.
            (__cxxabiv1::__cxa_get_globals): Likewise.
    
    (cherry picked from commit 1e65f2ed99024f23c56f7b6a961898bcaa882a92)

Diff:
---
 libstdc++-v3/libsupc++/eh_globals.cc | 51 ++++++++++++++++++++++++++----------
 1 file changed, 37 insertions(+), 14 deletions(-)

diff --git a/libstdc++-v3/libsupc++/eh_globals.cc b/libstdc++-v3/libsupc++/eh_globals.cc
index 3a003b89edf..768425c0f40 100644
--- a/libstdc++-v3/libsupc++/eh_globals.cc
+++ b/libstdc++-v3/libsupc++/eh_globals.cc
@@ -64,8 +64,26 @@ __cxxabiv1::__cxa_get_globals() _GLIBCXX_NOTHROW
 
 #else
 
-// Single-threaded fallback buffer.
-static __cxa_eh_globals eh_globals;
+#if __has_cpp_attribute(clang::require_constant_initialization)
+#  define __constinit [[clang::require_constant_initialization]]
+#endif
+
+namespace
+{
+  struct constant_init
+  {
+    union {
+      unsigned char unused;
+      __cxa_eh_globals obj;
+    };
+    constexpr constant_init() : obj() { }
+
+    ~constant_init() { /* do nothing, union member is not destroyed */ }
+  };
+
+  // Single-threaded fallback buffer.
+  __constinit constant_init eh_globals;
+}
 
 #if __GTHREADS
 
@@ -90,32 +108,37 @@ eh_globals_dtor(void* ptr)
 struct __eh_globals_init
 {
   __gthread_key_t  	_M_key;
-  bool 			_M_init;
+  static bool 		_S_init;
 
-  __eh_globals_init() : _M_init(false)
-  { 
+  __eh_globals_init()
+  {
     if (__gthread_active_p())
-      _M_init = __gthread_key_create(&_M_key, eh_globals_dtor) == 0; 
+      _S_init = __gthread_key_create(&_M_key, eh_globals_dtor) == 0;
   }
 
   ~__eh_globals_init()
   {
-    if (_M_init)
+    if (_S_init)
       __gthread_key_delete(_M_key);
-    _M_init = false;
+    _S_init = false;
   }
+
+  __eh_globals_init(const __eh_globals_init&) = delete;
+  __eh_globals_init& operator=(const __eh_globals_init&) = delete;
 };
 
+bool __eh_globals_init::_S_init = false;
+
 static __eh_globals_init init;
 
 extern "C" __cxa_eh_globals*
 __cxxabiv1::__cxa_get_globals_fast() _GLIBCXX_NOTHROW
 {
   __cxa_eh_globals* g;
-  if (init._M_init)
+  if (init._S_init)
     g = static_cast<__cxa_eh_globals*>(__gthread_getspecific(init._M_key));
   else
-    g = &eh_globals;
+    g = &eh_globals.obj;
   return g;
 }
 
@@ -123,7 +146,7 @@ extern "C" __cxa_eh_globals*
 __cxxabiv1::__cxa_get_globals() _GLIBCXX_NOTHROW
 {
   __cxa_eh_globals* g;
-  if (init._M_init)
+  if (init._S_init)
     {
       g = static_cast<__cxa_eh_globals*>(__gthread_getspecific(init._M_key));
       if (!g)
@@ -140,7 +163,7 @@ __cxxabiv1::__cxa_get_globals() _GLIBCXX_NOTHROW
 	}
     }
   else
-    g = &eh_globals;
+    g = &eh_globals.obj;
   return g;
 }
 
@@ -148,11 +171,11 @@ __cxxabiv1::__cxa_get_globals() _GLIBCXX_NOTHROW
 
 extern "C" __cxa_eh_globals*
 __cxxabiv1::__cxa_get_globals_fast() _GLIBCXX_NOTHROW
-{ return &eh_globals; }
+{ return &eh_globals.obj; }
 
 extern "C" __cxa_eh_globals*
 __cxxabiv1::__cxa_get_globals() _GLIBCXX_NOTHROW
-{ return &eh_globals; }
+{ return &eh_globals.obj; }
 
 #endif