Fix clang UBSan trigger in sbo_resource.hpp

[lzulberti]

Running my application with UB sanitizers enabled, catched this line computing the offset from nullptr and then adding buffer + padding to nullptr again:

buffer_.p = static_cast<void*>(static_cast<char*>(nullptr) + buffer + padding);

The UBSan error:

/home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/detail/sbo_resource.hpp:49:66: runtime error: applying non-zero offset 124542660792792 to null pointer
    #0 0x5e31f7feaccb in boost::cobalt::detail::sbo_resource::align_as_max_() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/detail/sbo_resource.hpp:49:66
    #1 0x5e31f7feaccb in boost::cobalt::detail::sbo_resource::sbo_resource(void*, unsigned long, std::pmr::memory_resource*) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/detail/sbo_resource.hpp:74:5
    #2 0x5e31f7feaccb in boost::cobalt::op<boost::system::error_code>::awaitable::awaitable(boost::cobalt::op<boost::system::error_code>*) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/op.hpp:112:34
    #3 0x5e31f7feaccb in boost::cobalt::op<boost::system::error_code>::operator co_await() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/op.hpp:130:12
    #4 0x5e31f7feaccb in boost::cobalt::promise<void> (anonymous namespace)::push_worker_lock<std::shared_ptr<ingeniars::cobalt::Mutex>>(std::shared_ptr<ingeniars::cobalt::Mutex>, std::shared_ptr<std::vector<int, std::allocator<int>>>, std::chrono::duration<long, std::ratio<1l, 1000l>>)  <OMISSIS>
    #5 0x5e31f7ff16c3 in MutexTest_LockSerializesStress_Test::TestBody()::$_0::operator()() const (.resume) <OMISSIS>
    #6 0x5e31f804f1ef in std::__n4861::coroutine_handle<boost::cobalt::detail::task_promise<void>>::resume() const /usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/coroutine:247:29
    #7 0x5e31f804f1ef in boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>::operator()() && /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/unique_handle.hpp:76:15
    #8 0x5e31f804f1ef in boost::asio::detail::binder0<boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>>::operator()() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/bind_handler.hpp:57:5
    #9 0x5e31f804f1ef in void boost::asio::detail::executor_function::complete<boost::asio::detail::binder0<boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>>, std::allocator<void>>(boost::asio::detail::executor_function::impl_base*, bool) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/executor_function.hpp:114:7
    #10 0x5e31f8040516 in boost::asio::detail::executor_function::operator()() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/executor_function.hpp:62:7
    #11 0x5e31f8040516 in void boost::asio::io_context::basic_executor_type<std::allocator<void>, 4ul>::execute<boost::asio::detail::executor_function>(boost::asio::detail::executor_function&&) const /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/impl/io_context.hpp:218:7
    #12 0x5e31f804e99f in void boost::asio::execution::detail::any_executor_base::execute<boost::asio::detail::binder0<boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>>>(boost::asio::detail::binder0<boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>>&&) const /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/execution/any_executor.hpp:682:9
    #13 0x5e31f804e5bb in boost::asio::detail::work_dispatcher<boost::asio::detail::empty_work_function, boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>, boost::asio::any_io_executor, void>::operator()() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/work_dispatcher.hpp:238:65
    #14 0x5e31f804f7fc in void boost::asio::detail::executor_function::complete<boost::asio::detail::work_dispatcher<boost::asio::detail::empty_work_function, boost::cobalt::unique_handle<boost::cobalt::detail::task_promise<void>>, boost::asio::any_io_executor, void>, std::allocator<void>>(boost::asio::detail::executor_function::impl_base*, bool) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/executor_function.hpp:114:7
    #15 0x5e31f803e39f in boost::asio::detail::executor_function::operator()() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/executor_function.hpp:62:7
    #16 0x5e31f803e39f in boost::asio::detail::executor_op<boost::asio::detail::executor_function, std::allocator<void>, boost::asio::detail::scheduler_operation>::do_complete(void*, boost::asio::detail::scheduler_operation*, boost::system::error_code const&, unsigned long) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/executor_op.hpp:71:7
    #17 0x5e31f8039f72 in boost::asio::detail::scheduler_operation::complete(void*, boost::system::error_code const&, unsigned long) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/scheduler_operation.hpp:41:5
    #18 0x5e31f8039f72 in boost::asio::detail::scheduler::do_run_one(boost::asio::detail::conditionally_enabled_mutex<boost::asio::detail::posix_mutex>::scoped_lock&, boost::asio::detail::scheduler_thread_info&, boost::system::error_code const&) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/impl/scheduler.ipp:502:12
    #19 0x5e31f80393e2 in boost::asio::detail::scheduler::run(boost::system::error_code&) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/detail/impl/scheduler.ipp:218:10
    #20 0x5e31f7ffafe2 in boost::asio::io_context::run() /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/asio/impl/io_context.ipp:64:24
    #21 0x5e31f7ffafe2 in void boost::cobalt::run<void>(boost::cobalt::task<void>) /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/run.hpp:54:11
    #22 0x5e31f7fe7af3 in MutexTest_LockSerializesStress_Test::TestBody() <OMISSIS>
    #23 0x5e31f8223023 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:2664:10
    #24 0x5e31f8202325 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:2700:14
    #25 0x5e31f81dfbe6 in testing::Test::Run() /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:2739:5
    #26 0x5e31f81e063e in testing::TestInfo::Run() /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:2885:11
    #27 0x5e31f81e0e59 in testing::TestSuite::Run() /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:3063:30
    #28 0x5e31f81f21d1 in testing::internal::UnitTestImpl::RunAllTests() /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:6054:44
    #29 0x5e31f82253d3 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:2664:10
    #30 0x5e31f8204805 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:2700:14
    #31 0x5e31f81f1aa5 in testing::UnitTest::Run() /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/b/src/googletest/src/gtest.cc:5594:10
    #32 0x5e31f7fe43b0 in RUN_ALL_TESTS() /home/zulberti/.conan2/p/b/gtest579ad47de4fe4/p/include/gtest/gtest.h:2334:73
    
...

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/zulberti/.conan2/p/b/boostbc3d9ea54563f/p/include/boost/cobalt/detail/sbo_resource.hpp:49:6

[klemens-morgenstern]

Thanks. Can you change the PR so it only changes the affected lines?

[lzulberti]

Sure, done ;-)

Is the offset to nullptr needed?

[klemens-morgenstern]

You can't use a reinterpret_cast in a constexpr function :)

[lzulberti]

I see, if contstexpr is needed and the function needs to perform that UB operation, I do not see solutions. Do you?

Should the alignment check be done in the constexpr function? Can it be enforced to programmer doing something like

alignas(std::max_align_t) unsigned char storage[1024];

constexpr auto make_resource()
{
  return boost::cobalt::detail::sbo_resource(
      storage,
      sizeof(storage));
}

Constexpr buffer should be already aligned, while runtime behavior can be preserved.

[lzulberti]

Hi @klemens-morgenstern, you're right that reinterpret_cast can't be evaluated during constant evaluation, that's exactly why the bare-cast version is rejected. With -Winvalid-constexpr (an error by default on recent clang) it fails to compile even when the function is never called at compile time:

error: constexpr function never produces a constant expression [-Winvalid-constexpr]
note: reinterpret_cast is not allowed in a constant expression

But a constexpr function is only ill-formed when no invocation can be a constant expression. Guarding the cast with std::is_constant_evaluated() and returning early restores that. During constant evaluation the function just returns, and at runtime it does the normal integer-arithmetic alignment:

constexpr void align_as_max_()
{
  if (std::is_constant_evaluated())
    return;

  const auto buffer = reinterpret_cast<std::uintptr_t>(buffer_.p);
  ...
}

Skipping alignment at compile time is fine here: no real buffer is ever aligned during constant evaluation. The only ctor that calls align_as_max_() takes a (void*, size) and its sole users construct it at runtime (op::awaitable lives in a coroutine frame, which can't be constexpr; composition gets its resource from handler.get_allocator()); the default ctor used by get_null_sbo_resource() doesn't touch it.

Verified compile + run clean, and UBSan + ASan clean, under both gcc 15 and clang 21 in C++20 — covering the misaligned-start (the originally-flagged branch), too-small, and empty-buffer paths. Also confirmed valid constexpr under clang -Werror=invalid-constexpr and g++.