[PATCH v3] livepatch: Don't block removal of patches that are safe to unload

[Chengming Zhou <zhouchengming@bytedance.com>]

module_put() is not called for a patch with "forced" flag. It should
block the removal of the livepatch module when the code might still
be in use after forced transition.

klp_force_transition() currently sets "forced" flag for all patches on
the list.

In fact, any patch can be safely unloaded when it passed through
the consistency model in KLP_UNPATCHED transition.

By other words, the "forced" flag must be set only for livepatches
that are being removed. In particular, set the "forced" flag:

  + only for klp_transition_patch when the transition to KLP_UNPATCHED
    state was forced.

  + all replaced patches when the transition to KLP_PATCHED state was
    forced and the patch was replacing the existing patches.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
Changes in v3:
 - rewrite more clear commit message by Petr.

Changes in v2:
 - interact nicely with the atomic replace feature noted by Miroslav.
---
 kernel/livepatch/transition.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 5683ac0d2566..7f25a5ae89f6 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -641,6 +641,18 @@ void klp_force_transition(void)
 	for_each_possible_cpu(cpu)
 		klp_update_patch_state(idle_task(cpu));
 
-	klp_for_each_patch(patch)
-		patch->forced = true;
+	/*
+	 * Only need to set forced flag for the transition patch
+	 * when force transition to KLP_UNPATCHED state, but
+	 * have to set forced flag for all replaced patches
+	 * when force atomic replace transition.
+	 */
+	if (klp_target_state == KLP_UNPATCHED)
+		klp_transition_patch->forced = true;
+	else if (klp_transition_patch->replace) {
+		klp_for_each_patch(patch) {
+			if (patch != klp_transition_patch)
+				patch->forced = true;
+		}
+	}
 }
-- 
2.20.1