linux-kernel - Re: Fix ordering of ftrace/livepatch calls on module load and unload

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Message-ID: <20160208175809.GA27290@packer-debian-8-amd64.digitalocean.com>
Date:	Mon, 8 Feb 2016 12:58:10 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Fix ordering of ftrace/livepatch calls on module load and unload

+++ Josh Poimboeuf [08/02/16 11:48 -0600]:
>On Fri, Feb 05, 2016 at 10:08:15PM -0500, Jessica Yu wrote:
>> As explained here [1], livepatch modules are failing to initialize properly
>> because the ftrace coming module notifier (which calls
>> ftrace_module_enable()) runs *after* the livepatch module notifier (which
>> enables the patch(es)). Thus livepatch attempts to apply patches to
>> modules before ftrace_module_enable() is even called for the corresponding
>> module(s). As a result, patch modules break. Ftrace code must run before
>> livepatch on module load, and the reverse is true on module unload.
>>
>> For ftrace and livepatch, order of initialization (plus exit/cleanup code) is
>> important for loading and unloading modules, and using module notifiers to
>> perform this work is not ideal since it is not always clear what gets called
>> when. In this patchset, dependence on the module notifier call chain is removed
>> in favor of hard coding the corresponding function calls in the module loader.
>> This promotes better code visibility and ensures that ftrace and livepatch code
>> get called in the correct order on patch module load and unload.
>>
>> Tested the changes with a test livepatch module that patches 9p and nilfs2,
>> and verified that the issue described in [1] is fixed.
>
>Since Rusty agreed to your suggested changes for splitting up
>complete_formation() and for setting mod->state = MODULE_STATE_GOING
>before calling the going notifiers in the error path, can you do a v4
>with those changes?  They should probably be split up like:
>
>1. split up complete_formation()
>2. set MODULE_STATE_GOING before calling going notifiers in error path
>3. remove ftrace module notifier
>4. remove livepatch module notifier

Sure, I'll do that.

Jessica