[PATCH 4/8] livepatch: Add an extra flag to distinguish registered patches

[Petr Mladek]

The initial implementation of the atomic replace feature keeps the replaced
patches on the stack. But people would like to remove the replaced patches
from different reasons that will be described in the following patch.

This patch is just a small preparation step. We will need to keep
the replaced patches registered even when they are not longer on the stack.
It is because they are typically unregistered by the module exit script.

Therefore we need to detect the registered patches another way. We could
not use kobj.state_initialized because it is racy. The kobject is destroyed
by an asynchronous call and could not be synchronized using klp_mutex.

This patch solves the problem by adding a flag into struct klp_patch.
It is manipulated under klp_mutex and therefore it is safe. It is easy
to understand and it is enough in most situations.

The function klp_is_patch_registered() is not longer needed. Though
it was renamed to klp_is_patch_on_stack() and used in __klp_enable_patch()
as a new sanity check.

This patch does not change the existing behavior.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Jessica Yu <jeyu@xxxxxxxxxx>
Cc: Jiri Kosina <jikos@xxxxxxxxxx>
Cc: Miroslav Benes <mbenes@xxxxxxx>
Cc: Jason Baron <jbaron@xxxxxxxxxx>
---
 include/linux/livepatch.h |  2 ++
 kernel/livepatch/core.c   | 24 ++++++++++++++++++------
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index f28af280f9e0..d6e6d8176995 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -150,6 +150,7 @@ struct klp_object {
  * @list:	list node for global list of registered patches
  * @kobj:	kobject for sysfs resources
  * @obj_list:	dynamic list of the object entries
+ * @registered: reliable way to check registration status
  * @enabled:	the patch is enabled (but operation may be incomplete)
  * @finish:	for waiting till it is safe to remove the patch module
  */
@@ -163,6 +164,7 @@ struct klp_patch {
 	struct list_head list;
 	struct kobject kobj;
 	struct list_head obj_list;
+	bool registered;
 	bool enabled;
 	struct completion finish;
 };
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 18c400bd9a33..70c67a834e9a 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -45,6 +45,11 @@
  */
 DEFINE_MUTEX(klp_mutex);
 
+/*
+ * Stack of patches. It defines the order in which the patches can be enabled.
+ * Only patches on this stack might be enabled. New patches are added when
+ * registered. They are removed when they are unregistered.
+ */
 static LIST_HEAD(klp_patches);
 
 static struct kobject *klp_root_kobj;
@@ -97,7 +102,7 @@ static void klp_find_object_module(struct klp_object *obj)
 	mutex_unlock(&module_mutex);
 }
 
-static bool klp_is_patch_registered(struct klp_patch *patch)
+static bool klp_is_patch_on_stack(struct klp_patch *patch)
 {
 	struct klp_patch *mypatch;
 
@@ -378,7 +383,7 @@ int klp_disable_patch(struct klp_patch *patch)
 
 	mutex_lock(&klp_mutex);
 
-	if (!klp_is_patch_registered(patch)) {
+	if (!patch->registered) {
 		ret = -EINVAL;
 		goto err;
 	}
@@ -407,7 +412,11 @@ static int __klp_enable_patch(struct klp_patch *patch)
 	if (WARN_ON(patch->enabled))
 		return -EINVAL;
 
-	/* enforce stacking: only the first disabled patch can be enabled */
+	/* Enforce stacking. */
+	if (!klp_is_patch_on_stack(patch))
+		return -EINVAL;
+
+	/* Only the first disabled patch can be enabled. */
 	if (patch->list.prev != &klp_patches &&
 	    !list_prev_entry(patch, list)->enabled)
 		return -EBUSY;
@@ -478,7 +487,7 @@ int klp_enable_patch(struct klp_patch *patch)
 
 	mutex_lock(&klp_mutex);
 
-	if (!klp_is_patch_registered(patch)) {
+	if (!patch->registered) {
 		ret = -EINVAL;
 		goto err;
 	}
@@ -519,7 +528,7 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
 
 	mutex_lock(&klp_mutex);
 
-	if (!klp_is_patch_registered(patch)) {
+	if (!patch->registered) {
 		/*
 		 * Module with the patch could either disappear meanwhile or is
 		 * not properly initialized yet.
@@ -528,6 +537,7 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
 		goto err;
 	}
 
+
 	if (patch->enabled == enabled) {
 		/* already in requested state */
 		ret = -EINVAL;
@@ -1004,6 +1014,7 @@ static int klp_init_patch(struct klp_patch *patch)
 	}
 
 	list_add_tail(&patch->list, &klp_patches);
+	patch->registered = true;
 
 	mutex_unlock(&klp_mutex);
 
@@ -1034,7 +1045,7 @@ int klp_unregister_patch(struct klp_patch *patch)
 
 	mutex_lock(&klp_mutex);
 
-	if (!klp_is_patch_registered(patch)) {
+	if (!patch->registered) {
 		ret = -EINVAL;
 		goto err;
 	}
@@ -1045,6 +1056,7 @@ int klp_unregister_patch(struct klp_patch *patch)
 	}
 
 	klp_free_patch(patch);
+	patch->registered = false;
 
 	mutex_unlock(&klp_mutex);
 
-- 
2.13.6