Re: [RFC PATCH v1.3 09/19] mm/damon/core: disallow probe_hits overflow on attrs only monitoring

[SJ Park <sj@kernel.org>]

On Fri, 10 Jul 2026 00:42:35 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Missing validation in `damon_valid_probe_params()` fails to prevent a 32-bit integer overflow of the weighted sum of probe hits.
> --
> 
> --- Patch [9]: [RFC PATCH v1.3 09/19] mm/damon/core: disallow probe_hits overflow on attrs only monitoring ---
> commit c184df8a3c8632af7a06675128fa4e375050fa8c
> Author: SJ Park <sj@kernel.org>
> Subject: mm/damon/core: disallow probe_hits overflow on attrs only monitoring
> 
> This commit adds validation logic to prevent an 8-bit overflow of
> probe_hits when the aggregation interval is much larger than the
> sampling interval. It checks the parameters in a new function,
> damon_valid_probe_params().
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 7d49420ea26c4..4f1425e56950b 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -1332,6 +1332,19 @@ static void damos_set_filters_default_reject(struct damos *s)
> >  		damos_filters_default_reject(&s->ops_filters);
> >  }
> >  
> > +static bool damon_valid_probe_params(struct damon_ctx *ctx)
> > +{
> > +	unsigned long sample_interval;
> > +
> > +	if (!damon_has_probe_weights(ctx))
> > +		return true;
> > +
> > +	sample_interval = ctx->attrs.sample_interval ? : 1;
> > +	if (ctx->attrs.aggr_interval / sample_interval > U8_MAX)
> > +		return false;
> > +	return true;
> > +}
> 
> [Severity: Medium]
> Since we are adding validation for the 8-bit probe_hits overflow here,
> should we also validate that the 32-bit weighted sum of probe hits doesn't
> overflow?
> 
> If a DAMON context is configured with probes where probe->weight is large
> enough, probe->weight * max_probe_hits (up to 255) could exceed UINT_MAX, or
> their cumulative sum could overflow. This would result in an incorrect score
> being returned by damon_probe_hits_wsum(), causing improper region merges in
> damon_merge_regions_of() and degrading monitoring accuracy.
> 
> While damon_has_probe_weights() currently returns false making this path
> unreachable, would it be safer to add the weighted sum overflow check now to
> prevent issues when probe weights are fully implemented?

That's exactly what the next patch of this series is doing.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260710002349.111414-1-sj@kernel.org?part=9


Thanks,
SJ