Messages in this thread | | | Date | Wed, 30 Apr 2014 12:00:01 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On Wed, 30 Apr 2014 10:41:14 -0400 Rik van Riel <riel@redhat.com> wrote:
> It is possible for "limit - setpoint + 1" to equal zero, leading to a > divide by zero error. Blindly adding 1 to "limit - setpoint" is not > working, so we need to actually test the divisor before calling div64. > > ... > > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, > unsigned long limit) > { > long long pos_ratio; > + long divisor; > long x; > > + divisor = limit - setpoint; > + if (!(s32)divisor) > + divisor = 1; /* Avoid div-by-zero */ > + > x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, > - limit - setpoint + 1); > + (s32)divisor);
Doesn't this just paper over the bug one time in four billion? The other 3999999999 times, pos_ratio_polynom() returns an incorect result?
If it is indeed the case that pos_ratio_polynom() callers are legitimately passing a setpoint which is more than 2^32 less than limit then it would be better to handle that input correctly.
Writing a new suite of div functions sounds overkillish. At some loss of precision could we do something like
if (divisor > 2^32) { divisor >>= log2(divisor) - 32; dividend >>= log2(divisor) - 32; } x = div(dividend, divisor);
?
And let's uninline the sorry thing while we're in there ;)
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