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

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Date:	Fri, 29 Jan 2016 14:51:08 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Miroslav Benes <mbenes@...e.cz>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: livepatch: Implement separate coming and going module notifiers

+++ Steven Rostedt [29/01/16 14:29 -0500]:
>On Fri, 29 Jan 2016 20:25:15 +0100 (CET)
>Miroslav Benes <mbenes@...e.cz> wrote:
>
>> It is possible to achieve the same goal even with the notifiers. They are
>> processed synchronously in complete_formation(). So we can put our klp
>> hook after that, right? Or better, put it to load_module() after
>> complete_formation() call. There is an error handling code even today
>> (that is, parse_args() or mod_sysfs_setup() can fail). Moreover, we'll
>> have a hook there with Jessica's relocation rework patch set.
>
>The problem with notifiers is that you don't know what is being called.
>A function call directly in the code, where it will always be needed if
>configured in, is a reasonable need to not use a notifier.

That's a pretty good reason. This bug popped up in the first place
due to a lack of clarity on which notifier gets called when.

>Although, I have to admit, if live kernel patching is configured in,
>it's not always needed to be called here, does it? With ftrace, the
>call has to be done when ftrace is configured in regardless if tracing
>is used or not.

Livepatch has to check if any of its patches affect the coming module,
so hardcoded or not it will still need to loop through the klp_patches
list like it does in the current notifiers.

Jessica