linux-kernel - Re: livepatch: Implement separate coming and going module notifiers

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Message-ID: <20160129200451.GB14026@packer-debian-8-amd64.digitalocean.com>
Date:	Fri, 29 Jan 2016 15:04:51 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Miroslav Benes <mbenes@...e.cz>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: livepatch: Implement separate coming and going module notifiers

+++ Josh Poimboeuf [29/01/16 11:30 -0600]:
>On Fri, Jan 29, 2016 at 05:30:46PM +0100, Miroslav Benes wrote:
>> Otherwise than that it looks good. I agree there are advantages to split
>> the notifiers. For example we can replace the coming one with the function
>> call somewhere in load_module() to improve error handling if the patching
>> fails while loading a module. This would be handy with a consistency model
>> in the future.
>
>Yeah, we'll need something like that eventually.  Though we'll need to
>make sure that ftrace_module_enable() is still called beforehand, after
>setting MODULE_STATE_COMING state, due to the race described in 5156dca.
>
>Something like:
>
>[note: klp_module_notify_coming() is replaced with klp_module_enable()]
>
>diff --git a/kernel/module.c b/kernel/module.c
>index 8358f46..aeabd81 100644
>--- a/kernel/module.c
>+++ b/kernel/module.c
>@@ -3371,6 +3371,13 @@ static int complete_formation(struct module *mod, struct load_info *info)
> 	mod->state = MODULE_STATE_COMING;
> 	mutex_unlock(&module_mutex);
>
>+	ftrace_module_enable(mod);
>+	err = klp_module_enable(mod);
>+	if (err) {
>+		ftrace_release_mod(mod);
>+		return err;
>+	}

If we go this route, should we should print a big warning ("Livepatch
couldn't patch loading module X") instead of aborting the module load
completely? 

>+
> 	blocking_notifier_call_chain(&module_notify_list,
> 				     MODULE_STATE_COMING, mod);
> 	return 0;
>diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>index eca592f..c42cf37 100644
>--- a/kernel/trace/ftrace.c
>+++ b/kernel/trace/ftrace.c
>@@ -5045,9 +5045,6 @@ static int ftrace_module_notify(struct notifier_block *self,
> 	struct module *mod = data;
>
> 	switch (val) {
>-	case MODULE_STATE_COMING:
>-		ftrace_module_enable(mod);
>-		break;
> 	case MODULE_STATE_GOING:
> 		ftrace_release_mod(mod);
> 		break;
>
>-- 
>Josh