linux-kernel - Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

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Message-ID: <1280915205.1923.882.camel@laptop>
Date:	Wed, 04 Aug 2010 11:46:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Avi Kivity <avi@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>, akpm@...l.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

On Fri, 2010-07-16 at 11:05 -0700, H. Peter Anvin wrote:
> 
> I really hope noone ever gets the idea of touching user space from an
> NMI handler, though, and expecting it to work... 

Perf actually already does that to unwind user-space stacks... ;-)

See arch/x86/kernel/cpu/perf_event.c:copy_from_user_nmi() and its users.

What we do is a manual page table walk (using __get_user_pages_fast) and
simply bail when the page is not available.

That said, I think that the thing that started the whole
per-cpu-per-context temp stack-frame storage story also means that that
function is now broken and can lead to kmap_atomic corruption.

I really should brush up that stack based kmap_atomic thing, last time I
got stuck on FRV wanting things.

Linus should I refresh that whole series and give a FRV a slow but
working implementation and then let David Howells sort out things if he
cares about that?

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