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libstdc++: Avoid recursion in __nothrow_wait_cv::wait [PR105730]

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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.
This commit is contained in:
Jonathan Wakely 2022-12-22 09:56:47 +00:00
parent f99b94865f
commit ee4af2ed0b
1 changed files with 21 additions and 1 deletions
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22
libstdc++-v3/src/c++11/compatibility-condvar.cc
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@ -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