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    Subject[PATCH 5.4 160/184] Revert 337f13046ff0 ("futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op")
    Date
    From: Thomas Gleixner <tglx@linutronix.de>

    commit 4fbf5d6837bf81fd7a27d771358f4ee6c4f243f8 upstream.

    The FUTEX_WAIT operand has historically a relative timeout which means that
    the clock id is irrelevant as relative timeouts on CLOCK_REALTIME are not
    subject to wall clock changes and therefore are mapped by the kernel to
    CLOCK_MONOTONIC for simplicity.

    If a caller would set FUTEX_CLOCK_REALTIME for FUTEX_WAIT the timeout is
    still treated relative vs. CLOCK_MONOTONIC and then the wait arms that
    timeout based on CLOCK_REALTIME which is broken and obviously has never
    been used or even tested.

    Reject any attempt to use FUTEX_CLOCK_REALTIME with FUTEX_WAIT again.

    The desired functionality can be achieved with FUTEX_WAIT_BITSET and a
    FUTEX_BITSET_MATCH_ANY argument.

    Fixes: 337f13046ff0 ("futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op")
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/r/20210422194704.834797921@linutronix.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/futex.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -3862,8 +3862,7 @@ long do_futex(u32 __user *uaddr, int op,

    if (op & FUTEX_CLOCK_REALTIME) {
    flags |= FLAGS_CLOCKRT;
    - if (cmd != FUTEX_WAIT && cmd != FUTEX_WAIT_BITSET && \
    - cmd != FUTEX_WAIT_REQUEUE_PI)
    + if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
    return -ENOSYS;
    }


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