livepatch: add (un)patch callbacks

Provide livepatch modules a klp_object (un)patching notification
mechanism.  Pre and post-(un)patch callbacks allow livepatch modules to
setup or synchronize changes that would be difficult to support in only
patched-or-unpatched code contexts.

Callbacks can be registered for target module or vmlinux klp_objects,
but each implementation is klp_object specific.

  - Pre-(un)patch callbacks run before any (un)patching transition
    starts.

  - Post-(un)patch callbacks run once an object has been (un)patched and
    the klp_patch fully transitioned to its target state.

Example use cases include modification of global data and registration
of newly available services/handlers.

See Documentation/livepatch/callbacks.txt for details and
samples/livepatch/ for examples.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/livepatch/core.c

@@ -54,11 +54,6 @@ static bool klp_is_module(struct klp_object *obj)
 	return obj->name;
 }
 
-static bool klp_is_object_loaded(struct klp_object *obj)
-{
-	return !obj->name || obj->mod;
-}
-
 /* sets obj->mod if object is not vmlinux and module is found */
 static void klp_find_object_module(struct klp_object *obj)
 {
@@ -285,6 +280,8 @@ static int klp_write_object_relocations(struct module *pmod,
 
 static int __klp_disable_patch(struct klp_patch *patch)
 {
+	struct klp_object *obj;
+
 	if (klp_transition_patch)
 		return -EBUSY;
 
@@ -295,6 +292,10 @@ static int __klp_disable_patch(struct klp_patch *patch)
 
 	klp_init_transition(patch, KLP_UNPATCHED);
 
+	klp_for_each_object(patch, obj)
+		if (patch->enabled && obj->patched)
+			klp_pre_unpatch_callback(obj);
+
 	/*
 	 * Enforce the order of the func->transition writes in
 	 * klp_init_transition() and the TIF_PATCH_PENDING writes in
@@ -388,13 +389,18 @@ static int __klp_enable_patch(struct klp_patch *patch)
 		if (!klp_is_object_loaded(obj))
 			continue;
 
+		ret = klp_pre_patch_callback(obj);
+		if (ret) {
+			pr_warn("pre-patch callback failed for object '%s'\n",
+				klp_is_module(obj) ? obj->name : "vmlinux");
+			goto err;
+		}
+
 		ret = klp_patch_object(obj);
 		if (ret) {
-			pr_warn("failed to enable patch '%s'\n",
-				patch->mod->name);
-
-			klp_cancel_transition();
-			return ret;
+			pr_warn("failed to patch object '%s'\n",
+				klp_is_module(obj) ? obj->name : "vmlinux");
+			goto err;
 		}
 	}
 
@@ -403,6 +409,11 @@ static int __klp_enable_patch(struct klp_patch *patch)
 	patch->enabled = true;
 
 	return 0;
+err:
+	pr_warn("failed to enable patch '%s'\n", patch->mod->name);
+
+	klp_cancel_transition();
+	return ret;
 }
 
 /**
@@ -871,13 +882,27 @@ int klp_module_coming(struct module *mod)
 			pr_notice("applying patch '%s' to loading module '%s'\n",
 				  patch->mod->name, obj->mod->name);
 
+			ret = klp_pre_patch_callback(obj);
+			if (ret) {
+				pr_warn("pre-patch callback failed for object '%s'\n",
+					obj->name);
+				goto err;
+			}
+
 			ret = klp_patch_object(obj);
 			if (ret) {
 				pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
 					patch->mod->name, obj->mod->name, ret);
+
+				if (patch != klp_transition_patch)
+					klp_post_unpatch_callback(obj);
+
 				goto err;
 			}
 
+			if (patch != klp_transition_patch)
+				klp_post_patch_callback(obj);
+
 			break;
 		}
 	}
@@ -927,9 +952,15 @@ void klp_module_going(struct module *mod)
 			 * is in transition.
 			 */
 			if (patch->enabled || patch == klp_transition_patch) {
+
+				if (patch != klp_transition_patch)
+					klp_pre_unpatch_callback(obj);
+
 				pr_notice("reverting patch '%s' on unloading module '%s'\n",
 					  patch->mod->name, obj->mod->name);
 				klp_unpatch_object(obj);
+
+				klp_post_unpatch_callback(obj);
 			}
 
 			klp_free_object_loaded(obj);

kernel/livepatch/core.h

@@ -1,6 +1,44 @@
 #ifndef _LIVEPATCH_CORE_H
 #define _LIVEPATCH_CORE_H
 
+#include <linux/livepatch.h>
+
 extern struct mutex klp_mutex;
 
+static inline bool klp_is_object_loaded(struct klp_object *obj)
+{
+	return !obj->name || obj->mod;
+}
+
+static inline int klp_pre_patch_callback(struct klp_object *obj)
+{
+	int ret;
+
+	ret = (obj->callbacks.pre_patch) ?
+		(*obj->callbacks.pre_patch)(obj) : 0;
+
+	obj->callbacks.post_unpatch_enabled = !ret;
+
+	return ret;
+}
+
+static inline void klp_post_patch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.post_patch)
+		(*obj->callbacks.post_patch)(obj);
+}
+
+static inline void klp_pre_unpatch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.pre_unpatch)
+		(*obj->callbacks.pre_unpatch)(obj);
+}
+
+static inline void klp_post_unpatch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.post_unpatch_enabled &&
+	    obj->callbacks.post_unpatch)
+		(*obj->callbacks.post_unpatch)(obj);
+}
+
 #endif /* _LIVEPATCH_CORE_H */

kernel/livepatch/patch.c

@@ -28,6 +28,7 @@
 #include <linux/slab.h>
 #include <linux/bug.h>
 #include <linux/printk.h>
+#include "core.h"
 #include "patch.h"
 #include "transition.h"
 

kernel/livepatch/transition.c

@@ -109,9 +109,6 @@ static void klp_complete_transition(void)
 		}
 	}
 
-	if (klp_target_state == KLP_UNPATCHED && !immediate_func)
-		module_put(klp_transition_patch->mod);
-
 	/* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 	if (klp_target_state == KLP_PATCHED)
 		klp_synchronize_transition();
@@ -130,6 +127,24 @@ static void klp_complete_transition(void)
 	}
 
 done:
+	klp_for_each_object(klp_transition_patch, obj) {
+		if (!klp_is_object_loaded(obj))
+			continue;
+		if (klp_target_state == KLP_PATCHED)
+			klp_post_patch_callback(obj);
+		else if (klp_target_state == KLP_UNPATCHED)
+			klp_post_unpatch_callback(obj);
+	}
+
+	/*
+	 * See complementary comment in __klp_enable_patch() for why we
+	 * keep the module reference for immediate patches.
+	 */
+	if (!klp_transition_patch->immediate && !immediate_func &&
+	    klp_target_state == KLP_UNPATCHED) {
+		module_put(klp_transition_patch->mod);
+	}
+
 	klp_target_state = KLP_UNDEFINED;
 	klp_transition_patch = NULL;
 }