Re: [PATCH] libstdc++: Ensure __gthread_self doesn't call undefined weak symbol [PR 95989]

[Jonathan Wakely <jwakely@redhat.com>]

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On 19/11/20 21:42 +0000, Jonathan Wakely wrote:
>On 12/11/20 17:34 +0000, Jonathan Wakely wrote:
>>On 11/11/20 19:08 +0100, Jakub Jelinek via Libstdc++ wrote:
>>>On Wed, Nov 11, 2020 at 05:24:42PM +0000, Jonathan Wakely wrote:
>>>>--- a/libgcc/gthr-posix.h
>>>>+++ b/libgcc/gthr-posix.h
>>>>@@ -684,7 +684,14 @@ __gthread_equal (__gthread_t __t1, __gthread_t __t2)
>>>>static inline __gthread_t
>>>>__gthread_self (void)
>>>>{
>>>>+#if __GLIBC_PREREQ(2, 27)
>>>
>>>What if it is a non-glibc system where __GLIBC_PREREQ macro isn't defined?
>>>I think you'd get then
>>>error: missing binary operator before token "("
>>>So I think you want
>>>#if defined __GLIBC__ && defined __GLIBC_PREREQ
>>>#if __GLIBC_PREREQ(2, 27)
>>>return pthread_self ();
>>>#else
>>>return __gthrw_(pthread_self) ();
>>>#else
>>>return __gthrw_(pthread_self) ();
>>>#endif
>>>or similar.
>>
>>
>>Here's a fixed version of the patch.
>>
>>I've moved the glibc-specific code in this_thread::get_id() into a new
>>macro defined in config/os/gnu-linux/os_defines.h (where we already
>>know we are dealing with glibc). That means we don't do the
>>__GLIBC_PREREQ check directly in <thread>, it's hidden away in a
>>target-specific header.
>>
>>Tested powerpc64le-linux (glibc 2.17 and 2.32), sparc-solaris2.11 and
>>powerpc-aix.
>
>I've committed this version which only fixes this_thread::get_id() in
>libstdc++, and doesn't change __gthread_self in gthr-posix.h
>
>Due to a recent change to replace other uses of __gthread_self with
>calls to this_thread::get_id(), fixing it there fixes all uses in
>libstdc++.

Here's the backport for gcc-10, where we still use __gthread_self in
two places in <stop_token>.

Tested x86_64-linux, committed to gcc-10 branch.




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commit b11cbbbb74b0e357652a1f1f0d937455310a6389
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 19 21:07:06 2020

    libstdc++: Avoid calling undefined __gthread_self weak symbol [PR 95989]
    
    Since glibc 2.27 the pthread_self symbol has been defined in libc rather
    than libpthread. Because we only call pthread_self through a weak alias
    it's possible for statically linked executables to end up without a
    definition of pthread_self. This crashes when trying to call an
    undefined weak symbol.
    
    We can use the __GLIBC_PREREQ version check to detect the version of
    glibc where pthread_self is no longer in libpthread, and call it
    directly rather than through the weak reference.
    
    It would be better to check for pthread_self in libc during configure
    instead of hardcoding the __GLIBC_PREREQ check. That would be
    complicated by the fact that prior to glibc 2.27 libc.a didn't have the
    pthread_self symbol, but libc.so.6 did.  The configure checks would need
    to try to link both statically and dynamically, and the result would
    depend on whether the static libc.a happens to be installed during
    configure (which could vary between different systems using the same
    version of glibc). Doing it properly is left for a future date, as that
    will be needed anyway after glibc moves all pthread symbols from
    libpthread to libc. When that happens we should revisit the whole
    approach of using weak symbols for pthread symbols.
    
    For the purposes of std::this_thread::get_id() we call
    pthread_self() directly when using glibc 2.27 or later. Otherwise, if
    __gthread_active_p() is true then we know the libpthread symbol is
    available so we call that. Otherwise, we are single-threaded and just
    use ((__gthread_t)1) as the thread ID.
    
    An undesirable consequence of this change is that code compiled prior to
    the change might inline the old definition of this_thread::get_id()
    which always returns (__gthread_t)1 in a program that isn't linked to
    libpthread. Code compiled after the change will use pthread_self() and
    so get a real TID. That could result in the main thread having different
    thread::id values in different translation units. This seems acceptable,
    as there are not expected to be many uses of thread::id in programs
    that aren't linked to libpthread.
    
    An earlier version of this patch also changed __gthread_self() to use
    __GLIBC_PREREQ(2, 27) and only use the weak symbol for older glibc. Tha
    might still make sense to do, but isn't needed by libstdc++ now.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/95989
            * config/os/gnu-linux/os_defines.h (_GLIBCXX_NATIVE_THREAD_ID):
            Define new macro to get reliable thread ID.
            * include/std/stop_token (_Stop_state_t::_M_request_stop):
            Use new macro if it's defined.
            (_Stop_state_t::_M_remove_callback): Likewise.
            * include/std/thread (this_thread::get_id): Likewise.
            * testsuite/30_threads/jthread/95989.cc: New test.
            * testsuite/30_threads/this_thread/95989.cc: New test.
    
    (cherry picked from commit 08b4d325711d5c6f68ac29443aba3fd7aa173ac8)

diff --git a/libstdc++-v3/config/os/gnu-linux/os_defines.h b/libstdc++-v3/config/os/gnu-linux/os_defines.h
index f821486ec8f5..01bfa9ddd4f2 100644
--- a/libstdc++-v3/config/os/gnu-linux/os_defines.h
+++ b/libstdc++-v3/config/os/gnu-linux/os_defines.h
@@ -49,4 +49,16 @@
 // version dynamically in case it has changed since libstdc++ was configured.
 #define _GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC __GLIBC_PREREQ(2,23)
 
+#if __GLIBC_PREREQ(2, 27)
+// Since glibc 2.27 pthread_self() is usable without linking to libpthread.
+# define _GLIBCXX_NATIVE_THREAD_ID pthread_self()
+#else
+// Before then it was in libc.so.6 but not libc.a, and always returns 0,
+// which breaks the invariant this_thread::get_id() != thread::id{}.
+// So only use it if we know the libpthread version is available.
+// Otherwise use (__gthread_t)1 as the ID of the main (and only) thread.
+# define _GLIBCXX_NATIVE_THREAD_ID \
+  (__gthread_active_p() ? __gthread_self() : (__gthread_t)1)
+#endif
+
 #endif
diff --git a/libstdc++-v3/include/std/stop_token b/libstdc++-v3/include/std/stop_token
index 847d12f7454e..76709dd59ebd 100644
--- a/libstdc++-v3/include/std/stop_token
+++ b/libstdc++-v3/include/std/stop_token
@@ -238,7 +238,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	while (!_M_try_lock_and_stop(__old));
 
 #if _GLIBCXX_HAS_GTHREADS
+#ifdef _GLIBCXX_NATIVE_THREAD_ID
+	_M_requester = _GLIBCXX_NATIVE_THREAD_ID;
+#else
 	_M_requester = __gthread_self();
+#endif
 #endif
 
 	while (_M_head)
@@ -344,10 +348,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	// _M_request_stop.
 
 #if _GLIBCXX_HAS_GTHREADS
+#ifdef _GLIBCXX_NATIVE_THREAD_ID
+	auto __tid = _GLIBCXX_NATIVE_THREAD_ID;
+#else
+	auto __tid = __gthread_self();
+#endif
 	// Despite appearances there is no data race on _M_requester. The only
 	// write to it happens before the callback is removed from the list,
 	// and removing it from the list happens before this read.
-	if (!__gthread_equal(_M_requester, __gthread_self()))
+	if (!__gthread_equal(_M_requester, __tid))
 	  {
 	    // Synchronize with completion of callback.
 	    __cb->_M_done.acquire();
diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index 96e5d1ed4e21..b1ed458ba212 100644
--- a/libstdc++-v3/include/std/thread
+++ b/libstdc++-v3/include/std/thread
@@ -364,15 +364,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline thread::id
     get_id() noexcept
     {
-#ifdef __GLIBC__
-      // For the GNU C library pthread_self() is usable without linking to
-      // libpthread.so but returns 0, so we cannot use it in single-threaded
-      // programs, because this_thread::get_id() != thread::id{} must be true.
-      // We know that pthread_t is an integral type in the GNU C library.
-      if (!__gthread_active_p())
-	return thread::id(1);
-#endif
+#ifdef _GLIBCXX_NATIVE_THREAD_ID
+      return thread::id(_GLIBCXX_NATIVE_THREAD_ID);
+#else
       return thread::id(__gthread_self());
+#endif
     }
 
     /// yield
diff --git a/libstdc++-v3/testsuite/30_threads/jthread/95989.cc b/libstdc++-v3/testsuite/30_threads/jthread/95989.cc
new file mode 100644
index 000000000000..46444b5ccabc
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/jthread/95989.cc
@@ -0,0 +1,54 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-gthreads {} }
+// { dg-additional-options "-pthread" { target pthread } }
+// { dg-additional-options "-static" { target static } }
+
+#include <thread>
+
+// PR libstdc++/95989
+// Segfault compiling with static libraries and using jthread::request_stop
+
+void
+test01()
+{
+  std::jthread t{ [] () {} };
+}
+
+void
+test02()
+{
+  std::jthread t{ [] () {} };
+  t.request_stop();
+}
+
+void
+test03()
+{
+  std::jthread t{ [] {} };
+  std::stop_callback cb(t.get_stop_token(), [] () {});
+}
+
+int
+main()
+{
+  test01();
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/30_threads/this_thread/95989.cc b/libstdc++-v3/testsuite/30_threads/this_thread/95989.cc
new file mode 100644
index 000000000000..16535af8b743
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/this_thread/95989.cc
@@ -0,0 +1,51 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do run { target c++11 } }
+// { dg-require-gthreads {} }
+// { dg-additional-options "-pthread" { target pthread } }
+// { dg-additional-options "-static" { target static } }
+
+#include <thread>
+#include <testsuite_hooks.h>
+
+__attribute__((noinline,noipa))
+void
+join(std::thread& t)
+{
+  if (!t.joinable())
+    return;
+
+  // Using thread::join() creates a dependency on libpthread symbols
+  // so that __gthread_active_p is true, and we use pthread_self.
+  t.join();
+}
+
+void
+test01()
+{
+  std::thread t;
+  // PR libstdc++/95989
+  auto id = std::this_thread::get_id();
+  VERIFY (t.get_id() != id );
+}
+
+int
+main()
+{
+  test01();
+}