livepatch: Remove Nop structures when unused

Replaced patches are removed from the stack when the transition is
finished. It means that Nop structures will never be needed again
and can be removed. Why should we care?

  + Nop structures give the impression that the function is patched
    even though the ftrace handler has no effect.

  + Ftrace handlers do not come for free. They cause slowdown that might
    be visible in some workloads. The ftrace-related slowdown might
    actually be the reason why the function is no longer patched in
    the new cumulative patch. One would expect that cumulative patch
    would help solve these problems as well.

  + Cumulative patches are supposed to replace any earlier version of
    the patch. The amount of NOPs depends on which version was replaced.
    This multiplies the amount of scenarios that might happen.

    One might say that NOPs are innocent. But there are even optimized
    NOP instructions for different processors, for example, see
    arch/x86/kernel/alternative.c. And klp_ftrace_handler() is much
    more complicated.

  + It sounds natural to clean up a mess that is no longer needed.
    It could only be worse if we do not do it.

This patch allows to unpatch and free the dynamic structures independently
when the transition finishes.

The free part is a bit tricky because kobject free callbacks are called
asynchronously. We could not wait for them easily. Fortunately, we do
not have to. Any further access can be avoided by removing them from
the dynamic lists.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 45 insertions, 3 deletions

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index ecb7660f1d8b..113645ee86b6 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -611,11 +611,16 @@ static struct kobj_type klp_ktype_func = {
         .sysfs_ops = &kobj_sysfs_ops,
 };
 
-static void klp_free_funcs(struct klp_object *obj)
+static void __klp_free_funcs(struct klp_object *obj, bool nops_only)
 {
         struct klp_func *func, *tmp_func;
 
         klp_for_each_func_safe(obj, func, tmp_func) {
+                if (nops_only && !func->nop)
+                        continue;
+
+                list_del(&func->node);
+
                 /* Might be called from klp_init_patch() error path. */
                 if (func->kobj_added) {
                         kobject_put(&func->kobj);
@@ -640,12 +645,17 @@ static void klp_free_object_loaded(struct klp_object *obj)
         }
 }
 
-static void klp_free_objects(struct klp_patch *patch)
+static void __klp_free_objects(struct klp_patch *patch, bool nops_only)
 {
         struct klp_object *obj, *tmp_obj;
 
         klp_for_each_object_safe(patch, obj, tmp_obj) {
-                klp_free_funcs(obj);
+                __klp_free_funcs(obj, nops_only);
+
+                if (nops_only && !obj->dynamic)
+                        continue;
+
+                list_del(&obj->node);
 
                 /* Might be called from klp_init_patch() error path. */
                 if (obj->kobj_added) {
@@ -656,6 +666,16 @@ static void klp_free_objects(struct klp_patch *patch)
         }
 }
 
+static void klp_free_objects(struct klp_patch *patch)
+{
+        __klp_free_objects(patch, false);
+}
+
+static void klp_free_objects_dynamic(struct klp_patch *patch)
+{
+        __klp_free_objects(patch, true);
+}
+
 /*
  * This function implements the free operations that can be called safely
  * under klp_mutex.
@@ -1085,6 +1105,28 @@ void klp_discard_replaced_patches(struct klp_patch *new_patch)
 }
 
 /*
+ * This function removes the dynamically allocated 'nop' functions.
+ *
+ * We could be pretty aggressive. NOPs do not change the existing
+ * behavior except for adding unnecessary delay by the ftrace handler.
+ *
+ * It is safe even when the transition was forced. The ftrace handler
+ * will see a valid ops->func_stack entry thanks to RCU.
+ *
+ * We could even free the NOPs structures. They must be the last entry
+ * in ops->func_stack. Therefore unregister_ftrace_function() is called.
+ * It does the same as klp_synchronize_transition() to make sure that
+ * nobody is inside the ftrace handler once the operation finishes.
+ *
+ * IMPORTANT: It must be called right after removing the replaced patches!
+ */
+void klp_discard_nops(struct klp_patch *new_patch)
+{
+        klp_unpatch_objects_dynamic(klp_transition_patch);
+        klp_free_objects_dynamic(klp_transition_patch);
+}
+
+/*
  * Remove parts of patches that touch a given kernel module. The list of
  * patches processed might be limited. When limit is NULL, all patches
  * will be handled.