From bf22a6976897977b0a3f1aeba6823c959fc4fdae Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Thu, 22 Apr 2021 21:44:23 +0200 Subject: futex: Provide FUTEX_LOCK_PI2 to support clock selection The FUTEX_LOCK_PI futex operand uses a CLOCK_REALTIME based absolute timeout since it was implemented, but it does not require that the FUTEX_CLOCK_REALTIME flag is set, because that was introduced later. In theory as none of the user space implementations can set the FUTEX_CLOCK_REALTIME flag on this operand, it would be possible to creatively abuse it and make the meaning invers, i.e. select CLOCK_REALTIME when not set and CLOCK_MONOTONIC when set. But that's a nasty hackery. Another option would be to have a new FUTEX_CLOCK_MONOTONIC flag only for FUTEX_LOCK_PI, but that's also awkward because it does not allow libraries to handle the timeout clock selection consistently. So provide a new FUTEX_LOCK_PI2 operand which implements the timeout semantics which the other operands use and leave FUTEX_LOCK_PI alone. Reported-by: Kurt Kanzenbach Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20210422194705.440773992@linutronix.de --- kernel/futex.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index f820439a8aae..f832b6434625 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -3707,12 +3707,14 @@ 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_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI && + cmd != FUTEX_LOCK_PI2) return -ENOSYS; } switch (cmd) { case FUTEX_LOCK_PI: + case FUTEX_LOCK_PI2: case FUTEX_UNLOCK_PI: case FUTEX_TRYLOCK_PI: case FUTEX_WAIT_REQUEUE_PI: @@ -3740,6 +3742,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3); case FUTEX_LOCK_PI: flags |= FLAGS_CLOCKRT; + fallthrough; + case FUTEX_LOCK_PI2: return futex_lock_pi(uaddr, flags, timeout, 0); case FUTEX_UNLOCK_PI: return futex_unlock_pi(uaddr, flags); @@ -3760,6 +3764,7 @@ static __always_inline bool futex_cmd_has_timeout(u32 cmd) switch (cmd) { case FUTEX_WAIT: case FUTEX_LOCK_PI: + case FUTEX_LOCK_PI2: case FUTEX_WAIT_BITSET: case FUTEX_WAIT_REQUEUE_PI: return true; -- cgit v1.2.3