[PATCH v2 1/3] scs: Skip the shadow stack end check when SCS is disabled

[Chris Down <chris@chrisdown.name>]

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().

Make task_scs_end_corrupted() check scs_is_enabled() itself. It is a
static branch, and it folds to true when CONFIG_DYNAMIC_SCS is not set.

Signed-off-by: Chris Down <chris@chrisdown.name>
---
 include/linux/scs.h | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/include/linux/scs.h b/include/linux/scs.h
index 4ab5bdc898cf..29ec2b8cf7bf 100644
--- a/include/linux/scs.h
+++ b/include/linux/scs.h
@@ -45,14 +45,6 @@ static inline unsigned long *__scs_magic(void *s)
 	return (unsigned long *)(s + SCS_SIZE) - 1;
 }
 
-static inline bool task_scs_end_corrupted(struct task_struct *tsk)
-{
-	unsigned long *magic = __scs_magic(task_scs(tsk));
-	unsigned long sz = task_scs_sp(tsk) - task_scs(tsk);
-
-	return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
-}
-
 DECLARE_STATIC_KEY_FALSE(dynamic_scs_enabled);
 
 static inline bool scs_is_dynamic(void)
@@ -69,6 +61,17 @@ static inline bool scs_is_enabled(void)
 	return scs_is_dynamic();
 }
 
+static inline bool task_scs_end_corrupted(struct task_struct *tsk)
+{
+	unsigned long *magic = __scs_magic(task_scs(tsk));
+	unsigned long sz = task_scs_sp(tsk) - task_scs(tsk);
+
+	if (!scs_is_enabled())
+		return false;
+
+	return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
+}
+
 #else /* CONFIG_SHADOW_CALL_STACK */
 
 static inline void *scs_alloc(int node) { return NULL; }
-- 
2.56.1