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

[Chris Down]

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 at chrisdown.name>
---
 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");
 #endif
 
-- 
2.54.0