Re: [PATCH v2] livepatch: old_name.number scheme in livepatch sysfs directory

On Tue, Nov 03, 2015 at 05:09:48PM +0100, Miroslav Benes wrote:

On Tue, 3 Nov 2015, Josh Poimboeuf wrote:

quoted

On Tue, Nov 03, 2015 at 11:52:08AM +0100, Miroslav Benes wrote:

quoted

On Mon, 2 Nov 2015, Chris J Arges wrote:

[...]

quoted

+static int klp_get_func_pos_callback(void *data, const char *name,
+				      struct module *mod, unsigned long addr)
+{
+	struct klp_find_arg *args = data;
+
+	if ((mod && !args->objname) || (!mod && args->objname))
+		return 0;
+
+	if (strcmp(args->name, name))
+		return 0;
+
+	if (args->objname && strcmp(args->objname, mod->name))
+		return 0;
+
+	/* on address match, return 1 to break kallsyms_on_each_symbol loop */
+	if (args->addr == addr)
+		return 1;
+
+	/* if we don't match addr, count instance of named symbol */
+	args->count++;
+
+	return 0;
+}
+
+static int klp_get_func_pos(struct klp_object *obj, struct klp_func *func)
+{
+	struct klp_find_arg args = {
+		.objname = obj->name,
+		.name = func->old_name,
+		.addr = func->old_addr,
+		.count = 0,
+	};
+
+	mutex_lock(&module_mutex);
+	kallsyms_on_each_symbol(klp_get_func_pos_callback, &args);
+	mutex_unlock(&module_mutex);
+
+	return args.count;
+}
+
 static int klp_init_func(struct klp_object *obj, struct klp_func *func)
 {
 	INIT_LIST_HEAD(&func->stack_node);
 	func->state = KLP_DISABLED;
 
 	return kobject_init_and_add(&func->kobj, &klp_ktype_func,
-				    &obj->kobj, "%s", func->old_name);
+				    &obj->kobj, "%s,%d", func->old_name,
+				    klp_get_func_pos(obj, func));
 }

There is a problem which I missed before. klp_init_func() is called before 
klp_find_verify_func_addr() in klp_init_object(). This means that 
func->old_addr is either not verified yet or worse it is still 0. This 
means that klp_get_func_pos_callback() never returns 1 and is thus called 
on each symbol. So if you for example patched cmdline_proc_show the 
resulting directory in sysfs would be called cmdline_proc_show,1 because 
addr is never matched. Had old_addr been specified the name would have 
been probably correct, but not for sure.

This should be fixed as well.

Even worse, klp_init_func() can be called even if the object hasn't been
loaded.  In that case there's no way to know what the value of n is, and
therefore no way to reliably create the sysfs entry.

Ah, right.

quoted

Should we create "func,n" in klp_init_object_loaded() instead of
klp_init_func()?

So that the function entries in sysfs would be created only when the 
object is loaded? Well, why not, but in that case it could easily confuse 
the user.

Maybe, but I think it would be fine if we document it.  It should only
be relied on by tools, anyway.

Object entry would be empty for not loaded object. I would not 
dare to propose to remove such object entries. It would make things worse. 

Why would removing an empty object entry make things worse?

So maybe we could introduce an attribute in sysfs object entry which would 
say if the object is loaded or not. Or something like that.

Hm, I'm not sure I see how this would help.

Hm, we can easily mess this up :)

Agreed 100% :-)

-- 
Josh