libstdc++: Fix std::future::wait_until for subsecond negative times [PR118093]

The current check for negative times (i.e. before the epoch) only checks
for a negative number of seconds. For a time 1ms before the epoch the
seconds part will be zero, but the futex syscall will still fail with an
EINVAL error. Extend the check to handle this case.

This change adds a redundant check in the headers too, so that we avoid
even calling into the library for negative times. Both checks can be
marked [[unlikely]]. The check in the headers avoids the cost of
splitting the time into seconds and nanoseconds and then making a PLT
call. The check inside the library matches where we were checking
already, and fixes existing binaries that were compiled against older
headers but use a newer libstdc++.so.6 at runtime.

libstdc++-v3/ChangeLog:

	PR libstdc++/118093
	* include/bits/atomic_futex.h (_M_load_and_test_until_impl):
	Return false for times before the epoch.
	* src/c++11/futex.cc (_M_futex_wait_until): Extend check for
	negative times to check for subsecond times. Add unlikely
	attribute.
	(_M_futex_wait_until_steady): Likewise.
	* testsuite/30_threads/future/members/118093.cc: New test.

3 files changed, 40 insertions, 10 deletions

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index 1146f0a..e69420d 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -172,11 +172,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	bool __equal, memory_order __mo,
 	const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
     {
-      auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
-      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
-      // XXX correct?
+      auto __d = __atime.time_since_epoch();
+      if (__d < __d.zero()) [[__unlikely__]]
+	return false;
+      auto __s = chrono::duration_cast<chrono::seconds>(__d);
+      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__d - __s);
       return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
-	  true, __s.time_since_epoch(), __ns);
+				    true, __s, __ns);
     }
 
     template<typename _Dur>
@@ -185,11 +187,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	bool __equal, memory_order __mo,
 	const chrono::time_point<std::chrono::steady_clock, _Dur>& __atime)
     {
-      auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
-      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
-      // XXX correct?
+      auto __d = __atime.time_since_epoch();
+      if (__d < __d.zero()) [[__unlikely__]]
+	return false;
+      auto __s = chrono::duration_cast<chrono::seconds>(__d);
+      auto __ns = chrono::duration_cast<chrono::nanoseconds>(__d - __s);
       return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
-	  true, __s.time_since_epoch(), __ns);
+	  true, __s, __ns);
     }
 
   public:
diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc
index 902b821..d3a7ca3 100644
--- a/libstdc++-v3/src/c++11/futex.cc
+++ b/libstdc++-v3/src/c++11/futex.cc
@@ -128,7 +128,7 @@ namespace
 	if (!futex_clock_realtime_unavailable.load(std::memory_order_relaxed))
 	  {
 	    // futex sets errno=EINVAL for absolute timeouts before the epoch.
-	    if (__s.count() < 0)
+	    if (__s.count() < 0 || __ns.count() < 0) [[unlikely]]
 	      return false;
 
 	    syscall_timespec rt;
@@ -204,7 +204,7 @@ namespace
 	if (!futex_clock_monotonic_unavailable.load(std::memory_order_relaxed))
 	  {
 	    // futex sets errno=EINVAL for absolute timeouts before the epoch.
-	    if (__s.count() < 0) [[unlikely]]
+	    if (__s.count() < 0 || __ns.count() < 0) [[unlikely]]
 	      return false;
 
 	    syscall_timespec rt;
diff --git a/libstdc++-v3/testsuite/30_threads/future/members/118093.cc b/libstdc++-v3/testsuite/30_threads/future/members/118093.cc
new file mode 100644
index 0000000..2bb1e1c
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/future/members/118093.cc
@@ -0,0 +1,26 @@
+// { dg-do run { target c++11 } }
+
+#include <chrono>
+#include <future>
+
+void
+test_sys()
+{
+  std::promise<void> p;
+  std::chrono::system_clock::time_point tp(std::chrono::milliseconds{-10});
+  (void) p.get_future().wait_until(tp);
+}
+
+void
+test_steady()
+{
+  std::promise<void> p;
+  std::chrono::steady_clock::time_point tp(std::chrono::milliseconds{-10});
+  (void) p.get_future().wait_until(tp);
+}
+
+int main()
+{
+  test_sys();
+  test_steady();
+}