linux-kernel - Re: [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool whitelist

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Date:	Wed, 13 Jan 2016 11:55:03 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool
 whitelist


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > Well, I can't say that I'm crazy about all those new tools adding markers to 
> > unrelated kernel code.
> > 
> > Can't you teach stacktool to ignore the whole machine_real_restart() function 
> > simply?
> 
> Well, these STACKTOOL_IGNORE whitelist markers are only needed in a handful of 
> places, and only for code that does very weird things.  Yes, they're a bit ugly, 
> but IMO they also communicate valuable information: "be careful, this code does 
> something very weird."

How common are these markers? Like with lockdep, it all depends on magnitude:

 - If it's less than 10 I'd say it's OK.

 - If it's dozens then it's ho-hum.

 - If certain types of annotations can go over 100, then they are unacceptable.

all such in-code overhead has to be balanced against the utility of the tooling.

> As for whether to put the whitelist info in the code vs hard-coding it in 
> stacktool, I think it's clearer and less "magical" to put them directly in the 
> code.

That's true - but I think Boris tried to ask something slightly different: can 
stacktool be taught to detect weird signatures automatically, and to ignore them 
automatically?

Stuff like 16-bit code sure wounds 'weird' and the tool could detect that?

Thanks,

	Ingo