[Bug libstdc++/96817] __cxa_guard_acquire unsafe against dynamically loaded pthread

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96817

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-08-27
             Status|UNCONFIRMED                 |NEW

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Something like this should work with the latest glibc:

--- a/libstdc++-v3/libsupc++/guard.cc
+++ b/libstdc++-v3/libsupc++/guard.cc
@@ -252,7 +252,22 @@ namespace __cxxabiv1
 # ifdef _GLIBCXX_USE_FUTEX
     // If __atomic_* and futex syscall are supported, don't use any global
     // mutex.
+#if __has_include(<sys/single_threaded.h>)
+    // Prefer to rely on __libc_single_threaded instead of __gthread_active_p
+    if (__libc_single_threaded)
+      {
+       int *gi = (int *) (void *) g;
+       if (*gi == 0)
+         {
+           *gi = _GLIBCXX_GUARD_PENDING_BIT;
+           return 1;
+         }
+       else
+         throw_recursive_init_exception();
+      }
+#else
     if (__gthread_active_p ())
+#endif
       {
        int *gi = (int *) (void *) g;
        const int guard_bit = _GLIBCXX_GUARD_BIT;
@@ -390,7 +405,17 @@ namespace __cxxabiv1
 #ifdef _GLIBCXX_USE_FUTEX
     // If __atomic_* and futex syscall are supported, don't use any global
     // mutex.
+#if __has_include(<sys/single_threaded.h>)
+    // Prefer to rely on __libc_single_threaded instead of __gthread_active_p
+    if (__libc_single_threaded)
+      {
+       int *gi = (int *) (void *) g;
+       *gi = _GLIBCXX_GUARD_BIT;
+       return;
+      }
+#else
     if (__gthread_active_p ())
+#endif
       {
        int *gi = (int *) (void *) g;
        const int guard_bit = _GLIBCXX_GUARD_BIT;