Messages in this thread | | | Date | Thu, 19 Sep 2013 12:15:10 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clockevents: Sanitize ticks to nsec conversion |
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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.
Thanks,
tglx
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