Created attachment 1122464 [details]
Failing program with annotations

changing any of lines with #if 1 to #if 0 causes valgrind not to report any errors

Created attachment 1122465 [details]
Preprocessed test.cpp

Created attachment 1122467 [details]
Output of valgrind

probably https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68921

(In reply to Jonathan Wakely from comment #5)
> probably https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68921

Oh, but that code isn't in gcc 4.8.5, so it can't be, sorry.

valgrind backtrace with debuginfo installed and linenumbers:

==5930== Syscall param futex(dummy) contains uninitialised byte(s)
==5930==    at 0x5663F8C: __lll_timedlock_wait (lowlevellock.S:190)
==5930==    by 0x5660A97: _L_timedlock_67 (pthread_mutex_timedlock.c:236)
==5930==    by 0x566032B: pthread_mutex_timedlock (pthread_mutex_timedlock.c:83)
==5930==    by 0x401108: __gthread_mutex_timedlock(pthread_mutex_t*, timespec const*) (gthr-default.h:768)
==5930==    by 0x4029A7: bool std::timed_mutex::_M_try_lock_until<std::chrono::duration<long, std::ratio<1l, 1000000000l> > >(std::chrono::time_point<std::chrono::_V2::steady_clock, std::chrono::duration<long, std::ratio<1l, 1000000000l> > > const&) (mutex:286)
==5930==    by 0x4023DE: bool std::timed_mutex::_M_try_lock_for<long, std::ratio<1l, 1000l> >(std::chrono::duration<long, std::ratio<1l, 1000l> > const&) (mutex:267)
==5930==    by 0x401FBE: bool std::timed_mutex::try_lock_for<long, std::ratio<1l, 1000l> >(std::chrono::duration<long, std::ratio<1l, 1000l> > const&) (mutex:240)
==5930==    by 0x401AFC: std::unique_lock<std::timed_mutex>::unique_lock<long, std::ratio<1l, 1000l> >(std::timed_mutex&, std::chrono::duration<long, std::ratio<1l, 1000l> > const&) (mutex:470)
==5930==    by 0x40128B: worker() (test.cpp:30)
==5930==    by 0x403504: void std::_Bind_simple<void (*())()>::_M_invoke<>(std::_Index_tuple<>) (functional:1732)
==5930==    by 0x40345E: std::_Bind_simple<void (*())()>::operator()() (functional:1720)
==5930==    by 0x4033F7: std::thread::_Impl<std::_Bind_simple<void (*())()> >::_M_run() (thread:115)

The check that valgrind does:

   case VKI_FUTEX_WAIT_BITSET:
      /* Check that the address at least begins in client-accessible area. */
      if (!VG_(am_is_valid_for_client)( ARG1, 1, VKI_PROT_READ )) {
            SET_STATUS_Failure( VKI_EFAULT );
            return;
      }
      if (*(vki_u32 *)ARG1 != ARG3) {
         PRE_REG_READ5(long, "futex",
                       vki_u32 *, futex, int, op, int, val,
                       struct timespec *, utime, int, dummy);
      } else {
         PRE_REG_READ6(long, "futex",
                       vki_u32 *, futex, int, op, int, val,
                       struct timespec *, utime, int, dummy, int, val3);
      }
      break;

It looks like argument 5 (uaddr2 aka dummy) really is not used. And for some reason in this particular case we call futex FUTEXT_WAIT_BITSET with that argument register really containing some uninitialized stuff.

The following makes the issue go away:

diff --git a/coregrind/m_syswrap/syswrap-linux.c b/coregrind/m_syswrap/syswrap-linux.c
index f796969..b57436c 100644
--- a/coregrind/m_syswrap/syswrap-linux.c
+++ b/coregrind/m_syswrap/syswrap-linux.c
@@ -1154,13 +1154,16 @@ PRE(sys_futex)
             return;
       }
       if (*(vki_u32 *)ARG1 != ARG3) {
-         PRE_REG_READ5(long, "futex",
+         PRE_REG_READ4(long, "futex",
                        vki_u32 *, futex, int, op, int, val,
-                       struct timespec *, utime, int, dummy);
+                       struct timespec *, utime);
       } else {
-         PRE_REG_READ6(long, "futex",
+        /* Note argument 5 is unused, but argument 6 is used.
+           So we cannot just PRE_REG_READ6. Read argument 6 separately.  */
+         PRE_REG_READ4(long, "futex",
                        vki_u32 *, futex, int, op, int, val,
-                       struct timespec *, utime, int, dummy, int, val3);
+                       struct timespec *, utime);
+         PRA6("futex",int,val3);
       }
       break;
    case VKI_FUTEX_WAKE_BITSET:

Will have to discuss upstream to see why the original code was written as it was. And whether this seems sane.

Patch upstream (valgrind svn r15793) and in fedora (valgrind-3.11.0-13.fc24)

Note that the fix as is in valgrind svn r15793 caused a regression in helgrind and drd. A small extra fix is necessary. See upstream valgrind svn r15795.

Verified for build valgrind-3.11.0-22.el7.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHEA-2016-2297.html