Re: [PATCH 2/3] sched_ext: Add scx_cmask_test() and scx_cmask_for_each_cid()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] scx_cmask_for_each_cid yields out-of-bounds phantom CIDs due to an overestimated loop bound and stale padding words.
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commit edd3ffa1eb6aa37c70fee16c5770f542f1484a83
Author: Tejun Heo <tj@kernel.org>
Subject: sched_ext: Add scx_cmask_test() and scx_cmask_for_each_cid()

Add single-bit test and iterator over set cids in an scx_cmask.

> diff --git a/kernel/sched/ext_cid.h b/kernel/sched/ext_cid.h
> index abea22ba2cc25..d4c3ebba74162 100644
> --- a/kernel/sched/ext_cid.h
> +++ b/kernel/sched/ext_cid.h

[ ... ]

> @@ -218,11 +218,42 @@ static inline void scx_cmask_reframe(struct scx_cmask *m, u32 base, u32 nr_cids)
>  	m->nr_cids = nr_cids;
>  }
>  
> -static inline void __scx_cmask_set(struct scx_cmask *m, u32 cid)
> +static inline void __scx_cmask_set(u32 cid, struct scx_cmask *m)
>  {
> -	if (!__scx_cmask_contains(m, cid))
> +	if (!__scx_cmask_contains(cid, m))
>  		return;
> -	*__scx_cmask_word(m, cid) |= BIT_U64(cid & 63);
> +	*__scx_cmask_word(cid, m) |= BIT_U64(cid & 63);
>  }
>  
> +/**
> + * scx_cmask_test - test whether @cid is set in @m
> + * @cid: cid to test
> + * @m: cmask to test
> + *
> + * Return %false if @cid is outside @m's active range. Otherwise return the
> + * bit's value. Read via READ_ONCE so callers can race set/clear writers.
> + */
> +static inline bool scx_cmask_test(u32 cid, const struct scx_cmask *m)
> +{
> +	if (!__scx_cmask_contains(cid, m))
> +		return false;
> +	return READ_ONCE(*__scx_cmask_word(cid, m)) & BIT_U64(cid & 63);
> +}
> +
> +/**
> + * scx_cmask_for_each_cid - iterate set cids in @m
> + * @cid: s32 loop var that receives each set cid in turn
> + * @m: cmask to iterate
> + *
> + * Visits set bits within @m's active range in ascending order. Relies on the
> + * head/tail-zero invariant of bits[], so no per-cid range check is needed.
> + */
> +#define scx_cmask_for_each_cid(cid, m)						\
> +	for (u64 __bs = (m)->base & ~63u, __wi = 0,				\
> +		     __nw = SCX_CMASK_NR_WORDS((m)->nr_cids);			\
> +	     __wi < __nw; __wi++)						\
> +		for (u64 __w = READ_ONCE((m)->bits[__wi]);			\
> +		     __w && ((cid) = __bs + __wi * 64 + __ffs64(__w), true);	\
> +		     __w &= __w - 1)
> +
>  #endif /* _KERNEL_SCHED_EXT_CID_H */

[Severity: High]
Does this iterator yield phantom CIDs outside of the mask's active range?

Since SCX_CMASK_NR_WORDS() overestimates worst-case capacity (e.g. for
nr_cids=1, it evaluates to 2), __nw will be larger than necessary. The
iterator lacks a strict cid < base + nr_cids bound check to compensate.

When scx_cmask_reframe() shrinks a mask, it clears only m->bits[0] and
m->bits[last_word]:

kernel/sched/ext_cid.h:scx_cmask_reframe() {
        if (nr_cids) {
                u32 last_word = ((base & 63) + nr_cids - 1) / 64;

                m->bits[0] = 0;
                m->bits[last_word] = 0;
        }
        ...
}

Any words beyond last_word remain uncleared. If a mask is reframed to a
smaller size (like nr_cids=1), SCX_CMASK_NR_WORDS(1) is 2, so the loop bound
__nw is 2. The iterator will process the stale, uncleared bits in m->bits[1],
yielding CIDs outside the active range.

Similarly, if nr_cids is 0, scx_cmask_reframe() skips clearing entirely, but
SCX_CMASK_NR_WORDS(0) is 1. The iterator will traverse m->bits[0] and yield
CIDs for an empty mask.

Is a strict boundary check inside the loop needed?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260603172153.3081861-1-tj@kernel.org?part=2