[PATCH v4 2/3] Revert "livepatch: Remove reliable stacktrace check in klp_try_switch_task()"

[Miroslav Benes]

This reverts commit 1d98a69e5cef3aeb68bcefab0e67e342d6bb4dad. Commit
31adf2308f33 ("livepatch: Convert error about unsupported reliable
stacktrace into a warning") weakened the enforcement for architectures
to have reliable stack traces support. The system only warns now about
it.

It only makes sense to reintroduce the compile time checking in
klp_try_switch_task() again and bail out early.

Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
---
 kernel/livepatch/transition.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 8e8c79d4b204..7e7ef04689d1 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -293,6 +293,13 @@ static bool klp_try_switch_task(struct task_struct *task)
 	if (task->patch_state == klp_target_state)
 		return true;
 
+	/*
+	 * For arches which don't have reliable stack traces, we have to rely
+	 * on other methods (e.g., switching tasks at kernel exit).
+	 */
+	if (!klp_have_reliable_stack())
+		return false;
+
 	/*
 	 * Now try to check the stack for any to-be-patched or to-be-unpatched
 	 * functions.  If all goes well, switch the task to the target patch
-- 
2.21.0