Messages in this thread | | | Date | Wed, 30 Apr 2014 15:30:04 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On 04/30/2014 03:00 PM, Andrew Morton wrote: > 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.
The easy way would be by calling div64_s64 and div64_u64, which are 64 bit all the way through.
Any objections?
The inlined bits seem to be stubs calling the _rem variants of the functions, and discarding the remainder.
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