Re: [PATCH 2/2] sched_ext: Maintain an online cid mask in the scheduler arena

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] sched_ext: bit corruption in online_cmask on 32-bit big-endian architectures
--

commit caa3d27a5f1591e49ffdf405f84e61f1f37a10f5
Author: Tejun Heo <tj@kernel.org>

sched_ext: Maintain an online cid mask in the scheduler arena

This commit adds a kernel-maintained online cid mask in the scheduler's
arena for child schedulers. It populates this mask after the cid mapping
is finalized and tracks online/offline states based on hotplug notifications.

> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 10af28a9f2c0c..7fbef7f11d3ae 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
[ ... ]
> @@ -3663,8 +3663,20 @@ static void handle_hotplug(struct rq *rq, bool online)
>  		s16 *tbl = rcu_dereference_check(scx_cpu_to_cid_tbl,
>  						 lockdep_is_cpus_held());
>  
> -		if (tbl)
> +		if (tbl) {
> +			struct scx_sched *pos;
> +
>  			cpu_or_cid = tbl[cpu];
> +
> +			guard(raw_spinlock_irqsave)(&scx_sched_lock);
> +			list_for_each_entry(pos, &scx_sched_all, all) {
> +				struct scx_cmask *mask = pos->online_cmask;
> +
> +				if (mask)
> +					__assign_bit(cpu_or_cid, (unsigned long *)mask->bits,
> +						     online);

[Severity: Medium]
Does casting mask->bits to an unsigned long pointer corrupt the bitmask
layout on 32-bit big-endian architectures?

Since mask->bits is declared as a u64 array in struct scx_cmask, BPF
programs expect a standard 64-bit integer bit layout. However,
__assign_bit() operates on native words. On 32-bit big-endian systems,
the most significant 32 bits are at offset 0, meaning __assign_bit(0)
modifies bit 32 of the underlying u64 instead of bit 0.

This would cause BPF programs to observe bit cpu_or_cid + 32 toggled
instead of cpu_or_cid, which could lead to dispatching tasks to offline
CPUs or starving tasks on online CPUs.

> +			}
> +		}
>  	}
>  
>  	if (online && SCX_HAS_OP(sch, cpu_online))

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260915082725.3881071-1-tj@kernel.org?part=2