linux-kernel - Re: [PATCH 1/2] livepatch: Implement separate coming and going module notifiers

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Date:	Fri, 29 Jan 2016 15:08:23 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...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: [PATCH 1/2] livepatch: Implement separate coming and going
 module notifiers

On Fri, 29 Jan 2016 13:47:15 -0600
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > 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.  
> 
> For live patching it actually does need to be called for every module.
> We need to check if any previously loaded patches have any modifications
> which affect the module.
> 

But only if you are using the live kernel patching facility. My point
is, if I never use live kernel patching (which 99.9% of Linux users do
no use), then the call will basically be a nop.

With ftrace, that's not the story. Even if you don't ever use the
facility (like 99.8% of Linux users do not use ;-) the function is
still not a nop. There's three calls needed for each module.

1) convert all the calls to mcount/fentry into a nop, and save
those locations in a table. They are all marked as disabled (not to be
used)

2) After module setup (where the notifiers are called), the locations
need to be enabled, otherwise ftrace would never work for that module.

3) On module exit, the locations must be removed, otherwise ftrace may
still try to write to non-existing code which could brick specific
network cards.

These are done if ftrace is configured regardless if it is every
actually used by a user.

Thus, my question is, what does live kernel patching need to do if I
never add a patch?

-- Steve