[gcc r15-6690] libstdc++: Fix std::future::wait_until for subsecond negative times [PR118093]

[Jonathan Wakely]

https://gcc.gnu.org/g:8ade3c3ea77e166f2873fb7ae57f9690e2b8d0e0

commit r15-6690-g8ade3c3ea77e166f2873fb7ae57f9690e2b8d0e0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Dec 17 21:32:19 2024 +0000

    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.

Diff:
---
 libstdc++-v3/include/bits/atomic_futex.h           | 20 ++++++++++-------
 libstdc++-v3/src/c++11/futex.cc                    |  4 ++--
 .../testsuite/30_threads/future/members/118093.cc  | 26 ++++++++++++++++++++++
 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 1146f0a78a11..e69420d23055 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 902b821843cb..d3a7ca3783d2 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 000000000000..2bb1e1c6dadc
--- /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();
+}