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.

(cherry picked from commit 8ade3c3ea77e166f2873fb7ae57f9690e2b8d0e0)

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index dd65417487396bc1105ab14a5f9f79f4c3aa4086..390c3e500dede9ca63767b7a54e4dae1eb1ab81a 100644 (file)
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -170,11 +170,13 @@ _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>
@@ -183,11 +185,13 @@ _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 ada2fdf3c4175e46a2c49c3dbf302426daece4eb..6139fc354c617a6d59be044ee30bd19f0a0f068d 100644 (file)
--- 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 (file)
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();
+}