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

[Michal Hocko]

On Wed 30-04-14 12:04:04, Maxim Patlasov wrote:
> Hi Rik!
> 
> On 04/29/2014 11:19 PM, Rik van Riel 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.
> 
> The patch looks correct, but I'm afraid it can hide an actual bug in a
> caller of pos_ratio_polynom(). The latter is not intended for setpoint >
> limit. All callers take pains to ensure that setpoint <= limit. Look, for
> example, at global_dirty_limits():

The bug might trigger even if setpoint < limit because the result is
trucated to s32 and I guess this is what is going on here?
Is (limit - setpoint + 1) > 4G possible?

> 
> >     if (background >= dirty)
> >        background = dirty / 2;
> 
> If you ever encountered "limit - setpoint + 1" equal zero, it may be worthy
> to investigate how you came to setpoint greater than limit.
> 
> Thanks,
> Maxim
> 
> >
> >Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> >Cc: stable@xxxxxxxxxxxxxxx
> >---
> >  mm/page-writeback.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> >diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >index ef41349..2682516 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 int divisor;
> >  	long long pos_ratio;
> >  	long x;
> >+	divisor = limit - setpoint;
> >+	if (!divisor)
> >+		divisor = 1;
> >+
> >  	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> >-		    limit - setpoint + 1);
> >+		    divisor);
> >  	pos_ratio = x;
> >  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> >  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> 

-- 
Michal Hocko
SUSE Labs
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