Re: [PATCH 1/3] sched/core: Skip the shadow stack end check when SCS is disabled

[Peter Zijlstra]

On Tue, Oct 27, 2026 at 02:36:09PM +0800, Chris Down wrote:
> With CONFIG_DYNAMIC_SCS, shadow call stacks are only used when the boot
> CPU lacks pointer authentication. scs_release() already checks
> scs_is_enabled() before it looks at the shadow stack since commit
> 9beccca09840 ("scs: add support for dynamic shadow call stacks"), but
> schedule_debug() does not.
> 
> With CONFIG_SCHED_STACK_END_CHECK=y it therefore checks init_task's
> unused shadow stack on every context switch. That is harmless right now,
> but the check is redundant and costs a load from an unrelated cache line
> in __schedule().
> 
> So, let's guard on scs_is_enabled(). It is a static branch, and it folds
> to true when CONFIG_DYNAMIC_SCS is not set.
> 
> Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d7c3b43158de..fc269fd10192 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6152,7 +6152,7 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
>  	if (task_stack_end_corrupted(prev))
>  		panic("corrupted stack end detected inside scheduler\n");
>  
> -	if (task_scs_end_corrupted(prev))
> +	if (scs_is_enabled() && task_scs_end_corrupted(prev))
>  		panic("corrupted shadow stack detected inside scheduler\n");

How about you push that scs_is_enabled() into task_scs_end_corrupted(),
this makes less noise here, and also keeps the next patch saner, no?