Re: [GCC 12] libstdc++: Fix lifetime bugs for non-TLS eh_globals [PR105880]

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Thu, 21 Jul 2022, 08:16 Sebastian Huber, <
sebastian.huber@embedded-brains.de> wrote:

> From: Jonathan Wakely <jwakely@redhat.com>
>
> 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)
> ---
>
> Would it be acceptable to back port this fix to GCC 12?
>

Yes, it's ok for gcc-12




>  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
>
> --
> 2.35.3
>
>