[PATCH] clockevents: Sanitize ticks to nsec conversion

[Thomas Gleixner]

On Thu, 19 Sep 2013, Uwe Kleine-König wrote:
> On Thu, Sep 19, 2013 at 12:15:10PM +0200, Thomas Gleixner wrote:
> > On Thu, 19 Sep 2013, Uwe Kleine-König wrote:
> > > On Thu, Sep 19, 2013 at 12:01:25AM +0200, Thomas Gleixner wrote:
> > > > Versus the 64bit overflow check, we need to be even more careful. We
> > > > need to check for overflowing (1 << 63) - 1 (i.e. the max positive
> > > > value which fits into a s64). See clockevents_program_event().
> > > 
> > > That is because you interpret times < 0 as in the past, right? But note
> > > that the interim result we're talking about here is still to be divided
> > > by evt->mult. So assuming mult > 1, that check is too strict unless you
> > > move it below the do_div in clockevent_delta2ns. For sure it makes sense
> > > to use the same value for a and b in the handling:
> > 
> > No, it's not too strict.
> > 
> >     nsec = (latch << shift) / mult;
> > 
> > Now the backwards conversion does:
> > 
> >     latch = (nsec * mult) >> shift;
> >
> > So we want nsec * mult to be in the positive range of s64. Which
> > means, that latch << shift must be in that range as well.
> The backwards conversion is in clockevents_program_event(), right? There
> is:
> 
> 	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
> 
> So I don't see a problem if nsec * mult overflows (1 << 63) - 1 as long
> as it still fits into an unsigned long long (i.e. a 64 bit value).

Right. It doesn't matter.