On Fri, 16 Mar 2018, Sebastian Andrzej Siewior wrote:
> cpu_chill() uses __hrtimer_nanosleep() with a hack to get
> TASK_UNINTERRUPTIBLE state. It seems to work but it does not.
> The problem is that we may have a signal pending and receive a sporadic
> wake up _before_ the timer expires. At that point the code will look at
_We_ have nothing pending. Please describe it in a factual way and not like
you are running the code yourself.
> ->restart_block and use it based on its value from the previous syscall
> resulting in a possible copy-to-user.
What you want to describe is:
If a task calls cpu_chill() and gets woken up by a regular or spurious
wakeup and has a signal pending, then it exits the sleep loop in
do_nanosleep() and sets up the restart block. If restart->nanosleep.type is
not TI_NONE then this results in accessing a stale user pointer from a
previously interrupted syscall and a copy to user based on the stale
pointer or a BUG() when 'type' is not supported in nanosleep_copyout().
> instead all this trouble, we could use schedule_hrtimeout() which is not
Sentences start with an uppercase letter.
> instead all this trouble, we could use schedule_hrtimeout() which is not
s/we could// ....
> an user-space interface and does not have this side effects.
hmm?
> Cc: stable-rt@xxxxxxxxxxxxxxx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> kernel/time/hrtimer.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 6c77643eaf02..c0efc22ba635 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1873,14 +1873,13 @@ COMPAT_SYSCALL_DEFINE2(nanosleep, struct compat_timespec __user *, rqtp,
> */
> void cpu_chill(void)
> {
> - struct timespec64 tu = {
> - .tv_nsec = NSEC_PER_MSEC,
> - };
> + ktime_t chill_time;
> unsigned int freeze_flag = current->flags & PF_NOFREEZE;
>
> + chill_time = ktime_set(0, NSEC_PER_MSEC);
> + set_current_state(TASK_UNINTERRUPTIBLE);
> current->flags |= PF_NOFREEZE;
> - __hrtimer_nanosleep(&tu, HRTIMER_MODE_REL_HARD, CLOCK_MONOTONIC,
> - TASK_UNINTERRUPTIBLE);
> + schedule_hrtimeout(&chill_time, HRTIMER_MODE_REL_HARD);
> if (!freeze_flag)
> current->flags &= ~PF_NOFREEZE;
> }
> --
> 2.16.2
>
>
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