Re: [PATCH 5/5] mm/damon/sysfs: read ops_id only once

On Tue, 14 Jul 2026 14:25:08 +0000 [email protected] wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] The patch implements an incomplete fix for a TOCTOU data race. It correctly uses READ_ONCE() for `ops_id` to prevent `min_region_sz` corruption, but leaves an identical unresolved race condition for `addr_unit` immediately below it.
> --
> 
> --- Patch [5]: [PATCH 5/5] mm/damon/sysfs: read ops_id only once ---
> commit f0f9d921acb64429b2f4e2895a8088cc3b5477d5
> Author: SJ Park <[email protected]>
> 
> mm/damon/sysfs: read ops_id only once
> 
> This commit addresses a time-of-check to time-of-use race condition in
> damon_sysfs_apply_inputs() by reading ops_id only once. This prevents a
> concurrent sysfs write from causing min_region_sz to be incorrectly
> configured.
> 
> Link: https://lore.kernel.org/[email protected]
> 
> > diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
> > index b5fe036f78015..60a1a9e4ada34 100644
> > --- a/mm/damon/sysfs.c
> > +++ b/mm/damon/sysfs.c
> > @@ -2094,14 +2094,16 @@ static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
> 
> [ ... ]
> 
> >  	ctx->addr_unit = sys_ctx->addr_unit;
> >  	/* addr_unit is respected by only DAMON_OPS_PADDR */
> > -	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
> > +	if (ops_id == DAMON_OPS_PADDR)
> >  		ctx->min_region_sz = max(
> >  				DAMON_MIN_REGION_SZ / sys_ctx->addr_unit, 1);
> 
> [Severity: Medium]
> Is there a similar race condition remaining for sys_ctx->addr_unit?
> 
> While this patch successfully fixes the race for ops_id, sys_ctx->addr_unit
> is read twice sequentially in this same block of code without protection.
> 
> A concurrent sysfs write to addr_unit between the assignment to
> ctx->addr_unit and the division in the max() calculation could cause DAMON
> to inconsistently calculate min_region_sz, degrading monitoring precision.
> 
> Should addr_unit also be read only once using READ_ONCE() here to fully
> prevent the race?

Good catch.  I also found I forgot adding Fixes: tag.  I'll fix this in the
next revision.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=5


Thanks,
SJ