linux-kernel - Re: [tip:timers/core] posix-timers: Zero out oldval itimerspec

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Date:   Mon, 12 Jun 2017 15:14:14 -0700
From:   Andrei Vagin <avagin@...tuozzo.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     tip-bot for Thomas Gleixner <tipbot@...or.com>,
        linux-tip-commits@...r.kernel.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, gorcunov@...nvz.org,
        john.stultz@...aro.org, mingo@...nel.org, peterz@...radead.org
Subject: Re: [tip:timers/core] posix-timers: Zero out oldval itimerspec

On Tue, Jun 13, 2017 at 12:01:17AM +0200, Thomas Gleixner wrote:
> On Mon, 12 Jun 2017, Andrei Vagin wrote:
> > > diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> > > index b53a0b5..88517dc 100644
> > > --- a/kernel/time/posix-timers.c
> > > +++ b/kernel/time/posix-timers.c
> > > @@ -828,6 +828,8 @@ SYSCALL_DEFINE4(timer_settime, timer_t, timer_id, int, flags,
> > >  	if (!timespec64_valid(&new_spec64.it_interval) ||
> > >  	    !timespec64_valid(&new_spec64.it_value))
> > >  		return -EINVAL;
> > > +	if (rtn)
> > > +		memset(rtn, 0, sizeof(*rtn));
> > 
> > Maybe we need to call memset after "retry:"?
> 
> That would be counter productive.
> 
> > common_timer_get() is called at the begining of common_timer_set(), then
> > common_timer_set() can return TIMER_RETRY. common_timer_get() will be
> > called again and some fields of rtn which have been touched first time
> > will not be touched.
> > 
> > At the end, rtn will contain data from two executions of
> > common_timer_get().
> 
> No. See the full code sequence:
> 
> retry:
>         timr = lock_timer(timer_id, &flag);
>         if (!timr)
>                 return -EINVAL;
> 
>         kc = clockid_to_kclock(timr->it_clock);
> 	if (WARN_ON_ONCE(!kc || !kc->timer_set))
>                 error = -EINVAL;
>         else
>                 error = kc->timer_set(timr, flags, &new_spec64, rtn);
> 
>         unlock_timer(timr, flag);
>         if (error == TIMER_RETRY) {
>                 rtn = NULL;     // We already got the old time...
>                 goto retry;
>         }
> 
> If you clear it after retry, you'll get all zeros in the retry case. Not
> what you really want.

Yes, you are right. Sorry for the noise.
> 
> Thanks,
> 
> 	tglx