[PATCH] futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

[Darren Hart]

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.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Signed-off-by: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
---
 kernel/futex.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 684d754..3c8c6d6 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3046,7 +3046,8 @@ 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;
 	}
 
-- 
2.1.4


-- 
Darren Hart
Intel Open Source Technology Center
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