linux-kernel - Re: [PATCH] Fix chance of sign extension to nsec after its msb is set during calculation.

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Date:   Fri, 2 Sep 2016 20:30:34 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Liav Rehana <liavr@...lanox.com>
cc:     linux-kernel@...r.kernel.org, john.stultz@...aro.org,
        noamca@...lanox.com, eladkan@...lanox.com
Subject: Re: [PATCH] Fix chance of sign extension to nsec after its msb is
 set during calculation.

On Fri, 2 Sep 2016, Thomas Gleixner wrote:
> On Thu, 1 Sep 2016, Liav Rehana wrote:
> > From: Liav Rehana <liavr@...lanox.com>
> > 
> > During the calculation of the nsec variable, "delta * tkr->mult" may cause
> > overflow to the msb, if the suspended time is too long.
> > In that case, we need to guarantee that the variable will not go through a
> > sign extension during its shift, and thus it will result in a much higher
> > value - close to the larget value of 64 bits.
> > The following commit fixes this problem, which causes the following bug:
> > Trying to connect through ftp to the os after a long enough suspended time
> > will cause the nsec variable to get a much higher value after its shift
> > because of sign extension, and thus the loop that follows some instructions
> > afterwards, implemented in the inline function __iter_div_u64_rem, will
> > take too long.
> > 
> > Signed-off-by: Liav Rehana <liavr@...lanox.com>
> > ---
> >  kernel/time/timekeeping.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> > index 479d25c..ddf56a5 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -305,7 +305,7 @@ static inline s64 timekeeping_delta_to_ns(struct tk_read_base *tkr,
> >  	s64 nsec;
> >  
> >  	nsec = delta * tkr->mult + tkr->xtime_nsec;
> > -	nsec >>= tkr->shift;
> > +	nsec = ((u64) nsec) >> tkr->shift;
> 
> This typecast is just a baindaid. What happens if you double the suspend time?
> The multiplication will simply overflow. So the proper fix is to sanity check
> delta and do multiple conversions if delta is big enough. Preferrably this
> happens somewhere at the call site and not in this hotpath function.

As a side note. John, why is that stuff unsigned at all? Shouldn't we use
u64 for all of this?

Thanks,

	tglx