Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

Michal Hocko
2014-04-30T14:49:12Z

On Wed 30-04-14 10:31:29, Rik van Riel wrote:
> On 04/30/2014 09:48 AM, Michal Hocko wrote:
> >On Wed 30-04-14 09:30:35, Rik van Riel wrote:
> >[...]
> >>Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
> >>
> >>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.
> >>
> >>Signed-off-by: Rik van Riel <riel@redhat.com>
> >>---
> >> mm/page-writeback.c | 13 +++++++++++--
> >> 1 file changed, 11 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >>index ef41349..f98a297 100644
> >>--- a/mm/page-writeback.c
> >>+++ b/mm/page-writeback.c
> >>@@ -597,11 +597,16 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
> >> unsigned long dirty,
> >> unsigned long limit)
> >> {
> >>+ unsigned long divisor;
> >> long long pos_ratio;
> >> long x;
> >>
> >>+ divisor = limit - setpoint;
> >>+ if (!divisor)
> >>+ divisor = 1; /* Avoid div-by-zero */
> >>+
> >
> >This is still prone to u64 -> s32 issue, isn't it?
> >What was the original problem anyway? Was it really setpoint > limit or
> >rather the overflow?
>
> Thinking about it some more, is it possible that
> limit and/or setpoint are larger than 32 bits, but
> the difference between them is not?
>
> In that case, truncating both to 32 bits before
> doing the subtraction would be troublesome, and
> it would be better to do a cast in the comparison:
>
> if (!(s32)divisor)
> divisor = 1;

How is that any different than defining divisor as 32b directly?

--
Michal Hocko
SUSE Labs