Re: [PATCH] workqueue: validate cpumask_first() result in llc_populate_cpu_shard_id()

[Tejun Heo]

Hello,

On Fri, Apr 10, 2026 at 01:49:50AM -0700, Breno Leitao wrote:
> In llc_populate_cpu_shard_id(), cpumask_first(sibling_cpus) is used to
> find the leader CPU, and the result is then used to index into
> cpu_shard_id[]. Add a bounds check with WARN_ON_ONCE to guard against
> unexpected values before using it as an array index.
> 
> Store the result in a local variable to make the code clearer, as also
> to avoid calling cpumask_first() twice.
> 
> Fixes: 5920d046f7ae3 ("workqueue: add WQ_AFFN_CACHE_SHARD affinity scope")
...
> @@ -8318,7 +8319,11 @@ static void __init llc_populate_cpu_shard_id(const struct cpumask *pod_cpus,
>  			 * The siblings' shard MUST be the same as the leader.
>  			 * never split threads in the same core.
>  			 */
> -			cpu_shard_id[c] = cpu_shard_id[cpumask_first(sibling_cpus)];
> +			leader = cpumask_first(sibling_cpus);
> +
> +			if (WARN_ON_ONCE(leader >= nr_cpu_ids))
> +				continue;
> +			cpu_shard_id[c] = cpu_shard_id[leader];

sibling_cpus can't be empty, right? This is mostly to shut up the reported
compiler warning? If so, can you please note that in a ocmment and the
description?

Thanks.

-- 
tejun