futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

While reviewing Michael Kerrisk's recent futex manpage update, I noticed
that we allow the FUTEX_CLOCK_REALTIME flag for FUTEX_WAIT_BITSET but
not for FUTEX_WAIT.

FUTEX_WAIT is treated as a simple version for FUTEX_WAIT_BITSET
internally (with a bitmask of FUTEX_BITSET_MATCH_ANY). As such, I cannot
come up with a reason for this exclusion for FUTEX_WAIT.

This change does modify the behavior of the futex syscall, changing a
call with FUTEX_WAIT | FUTEX_CLOCK_REALTIME from returning -ENOSYS, to be
equivalent to FUTEX_WAIT_BITSET | FUTEX_CLOCK_REALTIME with a bitset of
FUTEX_BITSET_MATCH_ANY.

Reported-by: Michael Kerrisk <[email protected]>
Signed-off-by: Darren Hart <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Link: http://lkml.kernel.org/r/9f3bdc116d79d23f5ee72ceb9a2a857f5ff8fa29.1450474525.git.dvhart@linux.intel.com
Signed-off-by: Thomas Gleixner <[email protected]>

diff --git a/kernel/futex.c b/kernel/futex.c
index 461d438f48161542bee63ecb8979b9ba619a8f22..8a310e240cdaf60b27aaff432c5837bbd3e61d90 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3084,7 +3084,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 
        if (op & FUTEX_CLOCK_REALTIME) {
                flags |= FLAGS_CLOCKRT;
-               if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
+               if (cmd != FUTEX_WAIT && cmd != FUTEX_WAIT_BITSET && \
+                   cmd != FUTEX_WAIT_REQUEUE_PI)
                        return -ENOSYS;
        }