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

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Date:	Tue, 12 Jan 2016 20:37:10 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	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

On Tue, Jan 12, 2016 at 12:56:06PM -0600, Josh Poimboeuf wrote:
> I'd say those are all weird things that C code should _normally_ not do
> (especially emitting fake instructions!).

The kernel does weird things, that's fine.

> Generally I agree (but I don't know what other tools you're talking
> about which require adding clutter).

kasan and kmemleak, for example.

> As I said there's hopefully only a handful of code locations which
> need the STACKTOOL_IGNORE stuff.

Can you get rid of them too?

> For example, how can it possibly grok a fake xen instruction without
> some kind of a whitelist, either hard-coded in the tool or annotated
> some other way?

I'd much prefer a whitelist which the tool parses, loads, etc, if you
don't want to hardcode it, to annotating kernel code.

> For example, how can it possibly grok a fake xen instruction without
> some kind of a whitelist, either hard-coded in the tool or annotated
> some other way?

Pointer to the place? I could take a look when I get a chance.

> Saying nothing at all in order to prevent false positives would also
> by definition allow some false negatives, which would make stacktool
> useless for its intended purpose of enabling reliable stack traces.

I'd take output from the tool anyday of the week which says something
like: "Looka here, this looks funny, you might want to do something
about it." than imposing annotations on code.

It might even move people into rewriting the code into tool-compliant
version.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.