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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r13-4857] libstdc++: Avoid recursion in __nothrow_wait_cv::wait [PR105730] Date: Thu, 22 Dec 2022 23:35:03 +0000 (GMT) [thread overview] Message-ID: <20221222233503.9B44B385B53D@sourceware.org> (raw) https://gcc.gnu.org/g:ee4af2ed0b7322884ec4ff537564683c3749b813 commit r13-4857-gee4af2ed0b7322884ec4ff537564683c3749b813 Author: Jonathan Wakely <jwakely@redhat.com> Date: Thu Dec 22 09:56:47 2022 +0000 libstdc++: Avoid recursion in __nothrow_wait_cv::wait [PR105730] The commit r12-5877-g9e18a25331fa25 removed the incorrect noexcept-specifier from std::condition_variable::wait and gave the new symbol version @@GLIBCXX_3.4.30. It also redefined the original symbol std::condition_variable::wait(unique_lock<mutex>&)@GLIBCXX_3.4.11 as an alias for a new symbol, __gnu_cxx::__nothrow_wait_cv::wait, which still has the incorrect noexcept guarantee. That __nothrow_wait_cv::wait is just a wrapper around the real condition_variable::wait which adds noexcept and so terminates on a __forced_unwind exception. This doesn't work on uclibc, possibly due to a dynamic linker bug. When __nothrow_wait_cv::wait calls the condition_variable::wait function it binds to the alias symbol, which means it just calls itself recursively until the stack overflows. This change avoids the possibility of a recursive call by changing the __nothrow_wait_cv::wait function so that instead of calling condition_variable::wait it re-implements it. This requires accessing the private _M_cond member of condition_variable, so we need to use the trick of instantiating a template with the member-pointer of the private member. libstdc++-v3/ChangeLog: PR libstdc++/105730 * src/c++11/compatibility-condvar.cc (__nothrow_wait_cv::wait): Access private data member of base class and call its wait member. Diff: --- libstdc++-v3/src/c++11/compatibility-condvar.cc | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) diff --git a/libstdc++-v3/src/c++11/compatibility-condvar.cc b/libstdc++-v3/src/c++11/compatibility-condvar.cc index e3a8b8403ca..3cef3bc0714 100644 --- a/libstdc++-v3/src/c++11/compatibility-condvar.cc +++ b/libstdc++-v3/src/c++11/compatibility-condvar.cc @@ -67,6 +67,24 @@ _GLIBCXX_END_NAMESPACE_VERSION && defined(_GLIBCXX_HAVE_SYMVER_SYMBOL_RENAMING_RUNTIME_SUPPORT) namespace __gnu_cxx _GLIBCXX_VISIBILITY(default) { +namespace +{ + // Pointer-to-member for private std::condition_variable::_M_cond member. + std::__condvar std::condition_variable::* __base_member; + + template<std::__condvar std::condition_variable::*X> + struct cracker + { static std::__condvar std::condition_variable::* value; }; + + // Initializer for this static member also initializes __base_member. + template<std::__condvar std::condition_variable::*X> + std::__condvar std::condition_variable::* + cracker<X>::value = __base_member = X; + + // Explicit instantiation is allowed to access the private member. + template class cracker<&std::condition_variable::_M_cond>; +} + struct __nothrow_wait_cv : std::condition_variable { void wait(std::unique_lock<std::mutex>&) noexcept; @@ -76,7 +94,9 @@ __attribute__((used)) void __nothrow_wait_cv::wait(std::unique_lock<std::mutex>& lock) noexcept { - this->condition_variable::wait(lock); + // In theory this could be simply this->std::condition_variable::wait(lock) + // but with uclibc that binds to the @GLIBCXX_3.4.11 symbol, see PR 105730. + (this->*__base_member).wait(*lock.mutex()); } } // namespace __gnu_cxx
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