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Subject[PATCH v12 04/12] livepatch: Consolidate klp_free functions
Date
The code for freeing livepatch structures is a bit scattered and tricky:

+ direct calls to klp_free_*_limited() and kobject_put() are
used to release partially initialized objects

+ klp_free_patch() removes the patch from the public list
and releases all objects except for patch->kobj

+ object_put(&patch->kobj) and the related wait_for_completion()
are called directly outside klp_mutex; this code is duplicated;

Now, we are going to remove the registration stage to simplify the API
and the code. This would require handling more situations in
klp_enable_patch() error paths.

More importantly, we are going to add a feature called atomic replace.
It will need to dynamically create func and object structures. We will
want to reuse the existing init() and free() functions. This would
create even more error path scenarios.

This patch implements a more clever free functions:

+ checks kobj.state_initialized instead of @limit

+ initializes patch->list early so that the check for empty list
always works

+ The action(s) that has to be done outside klp_mutex are done
in separate klp_free_patch_end() function. It waits only
when patch->kobj was really released via the _begin() part.

Note that it is safe to put patch->kobj under klp_mutex. It calls
the release callback only when the reference count reaches zero.
Therefore it does not block any related sysfs operation that took
a reference and might eventually wait for klp_mutex.

Note that __klp_free_patch() is split because it will be later
used in a _nowait() variant. Also klp_free_patch_end() makes
sense because it will later get more complicated.

This patch does not change the existing behavior.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Jason Baron <jbaron@akamai.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
---
include/linux/livepatch.h | 2 ++
kernel/livepatch/core.c | 92 +++++++++++++++++++++++++++++------------------
2 files changed, 59 insertions(+), 35 deletions(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 1163742b27c0..22e0767d64b0 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -138,6 +138,7 @@ struct klp_object {
* @list: list node for global list of registered patches
* @kobj: kobject for sysfs resources
* @enabled: the patch is enabled (but operation may be incomplete)
+ * @wait_free: wait until the patch is freed
* @finish: for waiting till it is safe to remove the patch module
*/
struct klp_patch {
@@ -149,6 +150,7 @@ struct klp_patch {
struct list_head list;
struct kobject kobj;
bool enabled;
+ bool wait_free;
struct completion finish;
};

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index b3956cce239e..3ca404545150 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -465,17 +465,15 @@ static struct kobj_type klp_ktype_func = {
.sysfs_ops = &kobj_sysfs_ops,
};

-/*
- * Free all functions' kobjects in the array up to some limit. When limit is
- * NULL, all kobjects are freed.
- */
-static void klp_free_funcs_limited(struct klp_object *obj,
- struct klp_func *limit)
+static void klp_free_funcs(struct klp_object *obj)
{
struct klp_func *func;

- for (func = obj->funcs; func->old_name && func != limit; func++)
- kobject_put(&func->kobj);
+ klp_for_each_func(obj, func) {
+ /* Might be called from klp_init_patch() error path. */
+ if (func->kobj.state_initialized)
+ kobject_put(&func->kobj);
+ }
}

/* Clean up when a patched object is unloaded */
@@ -489,26 +487,59 @@ static void klp_free_object_loaded(struct klp_object *obj)
func->old_addr = 0;
}

-/*
- * Free all objects' kobjects in the array up to some limit. When limit is
- * NULL, all kobjects are freed.
- */
-static void klp_free_objects_limited(struct klp_patch *patch,
- struct klp_object *limit)
+static void klp_free_objects(struct klp_patch *patch)
{
struct klp_object *obj;

- for (obj = patch->objs; obj->funcs && obj != limit; obj++) {
- klp_free_funcs_limited(obj, NULL);
- kobject_put(&obj->kobj);
+ klp_for_each_object(patch, obj) {
+ klp_free_funcs(obj);
+
+ /* Might be called from klp_init_patch() error path. */
+ if (obj->kobj.state_initialized)
+ kobject_put(&obj->kobj);
}
}

-static void klp_free_patch(struct klp_patch *patch)
+static void __klp_free_patch(struct klp_patch *patch)
{
- klp_free_objects_limited(patch, NULL);
if (!list_empty(&patch->list))
list_del(&patch->list);
+
+ klp_free_objects(patch);
+
+ if (patch->kobj.state_initialized)
+ kobject_put(&patch->kobj);
+}
+
+/*
+ * Some operations are synchronized by klp_mutex, e.g. the access to
+ * klp_patches list. But the caller has to wait for patch->kobj release
+ * callback outside the lock. Otherwise, there might be a deadlock with
+ * sysfs operations waiting on klp_mutex.
+ *
+ * This function implements the free part that has to be called under
+ * klp_mutex.
+ */
+static void klp_free_patch_wait_prepare(struct klp_patch *patch)
+{
+ /* Can be called in error paths before patch->kobj is initialized. */
+ if (patch->kobj.state_initialized)
+ patch->wait_free = true;
+ else
+ patch->wait_free = false;
+
+ __klp_free_patch(patch);
+}
+
+/*
+ * This function implements the free part that must be called outside
+ * klp_mutex.
+ */
+static void klp_free_patch_wait(struct klp_patch *patch)
+{
+ /* Wait only when patch->kobj was initialized */
+ if (patch->wait_free)
+ wait_for_completion(&patch->finish);
}

static int klp_init_func(struct klp_object *obj, struct klp_func *func)
@@ -609,20 +640,12 @@ static int klp_init_object(struct klp_patch *patch, struct klp_object *obj)
klp_for_each_func(obj, func) {
ret = klp_init_func(obj, func);
if (ret)
- goto free;
+ return ret;
}

- if (klp_is_object_loaded(obj)) {
+ if (klp_is_object_loaded(obj))
ret = klp_init_object_loaded(patch, obj);
- if (ret)
- goto free;
- }

- return 0;
-
-free:
- klp_free_funcs_limited(obj, func);
- kobject_put(&obj->kobj);
return ret;
}

@@ -637,6 +660,7 @@ static int klp_init_patch(struct klp_patch *patch)
mutex_lock(&klp_mutex);

patch->enabled = false;
+ INIT_LIST_HEAD(&patch->list);
init_completion(&patch->finish);

ret = kobject_init_and_add(&patch->kobj, &klp_ktype_patch,
@@ -659,12 +683,11 @@ static int klp_init_patch(struct klp_patch *patch)
return 0;

free:
- klp_free_objects_limited(patch, obj);
+ klp_free_patch_wait_prepare(patch);

mutex_unlock(&klp_mutex);

- kobject_put(&patch->kobj);
- wait_for_completion(&patch->finish);
+ klp_free_patch_wait(patch);

return ret;
}
@@ -693,12 +716,11 @@ int klp_unregister_patch(struct klp_patch *patch)
goto err;
}

- klp_free_patch(patch);
+ klp_free_patch_wait_prepare(patch);

mutex_unlock(&klp_mutex);

- kobject_put(&patch->kobj);
- wait_for_completion(&patch->finish);
+ klp_free_patch_wait(patch);

return 0;
err:
--
2.13.7
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