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

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Message-ID: <20160120060943.GA16574@treble.redhat.com>
Date:	Wed, 20 Jan 2016 00:09:43 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.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 19, 2016 at 09:50:31PM -0800, H. Peter Anvin wrote:
> On 01/19/16 21:42, Josh Poimboeuf wrote:
> >>>>
> >>>>   Apparently(?) xen treats the ud2 special when it's followed by "78 65
> >>>>   6e".  This is confusing for stacktool because ud2 is normally a dead
> >>>>   end, and it thinks the instructions after it will never run.
> >>>>   
> 
> Cute.  UD2 followed by three ASCII characters.  I guess that becomes
> really fun when using UD2 + a string for error reporting.  Of course, no
> software does that...

Ah, I completely missed the fact that they're ASCII.  And it spells
"xen" of course :-)

-- 
Josh