Date:	Sat, 21 Feb 2015 21:10:33 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Ingo Molnar <mingo@...nel.org>
cc:	Vojtech Pavlik <vojtech@...e.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: live patching design (was: Re: [PATCH 1/3] sched: add
 sched_task_call())

On Sat, 21 Feb 2015, Ingo Molnar wrote:

> > I see the difference, but I am afraid you are simplifying 
> > the situation a litle bit too much.
> > 
> > There will always be properties of patches that will make 
> > them unapplicable in a "live patching" way by design. 
> > Think of data structure layout changes (*).
> 
> Yes.

The (*) note described that it's actually in theory possible, but the 
price to pay is very high.

So it's possible to write a patch that does this, and have it use a 
different consistency model, which would take care of data structure 
layout change (and the fact that this particular consistency model is very 
expensive, would be a clear fact).

Your "simple one-method-to-rule-them-all aproach" is not sufficient for 
this.

Even such a stretched case doesn't justify existency of consistency models 
for you?

> > Or think of kernel that has some 3rd party vendor module loaded, and 
> > this module spawning a ktrehad that is not capable of parking itself.
> 
> The kernel will refuse to live patch until the module is fixed. It is a 
> win by any measure.

Depends on your point of view. If you are an administrator of the 
vulnerable system, you have no to very little clue why patching your 
system is not possible (and what you should to to have it fixed in the 
coming days, before your system is taken down by attackers, for example), 
and would be really sad to be forced to run with unpatched system.

I see your point that this would be a good lever we'd have to 3rd party 
vendors, but OTOH we'd be taking OS users as hostages in some sense.

> > Or think of patching __notrace__ functions.
> 
> Why would __notrace__ functions be a problem in the 'simple' method? 
> Live patching with this method will work even if ftrace is not built in, 
> we can just patch out the function in the simplest possible fashion, 
> because we do it atomically and don't have to worry about per task 
> 'transition state' - like kGraft does.
> 
> It's a massive simplification and there's no need to rely on ftrace's 
> mcount trick. (Sorry Steve!)

This would indeed be possible iff we take the "global synchronizing point" 
aproach you are proposing. Still technicalities to be solved (what happens 
if you are patching that already has ftrace on it, etc), but probably 
nothing principal.

> > So it's not black and white, it's really a rather philosophical 
> > question where to draw the line (and make sure that everyone is aware 
> > of where the line is and what it means).
> 
> Out of the three examples you mentioned, two are actually an advantage 
> to begin with - so I'd suggest you stop condescending me ...

Ugh, if you feel my e-mails like attempts to condescend you, I am really 
surprised; I thought we are discussing technical details. If you feel 
otherwise, we should probably just stop discussing then.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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