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

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Message-ID: <20100716193053.GD7338@basil.fritz.box>
Date:	Fri, 16 Jul 2010 21:30:53 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Avi Kivity <avi@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

On Fri, Jul 16, 2010 at 11:25:19AM -0700, Linus Torvalds wrote:
> On Fri, Jul 16, 2010 at 11:15 AM, Avi Kivity <avi@...hat.com> wrote:
> >
> > I think the concern here is about an NMI handler's code running in vmalloc
> > space, or is it something else?
> 
> I think the concern was also potentially doing things like backtraces
> etc that may need access to the module data structures (I think the
> ELF headers end up all being in vmalloc space too, for example).
> 
> The whole debugging thing is also an issue. Now, I obviously am not a
> big fan of remote debuggers, but everybody tells me I'm wrong. And
> putting a breakpoint on NMI is certainly not insane if you are doing
> debugging in the first place. So it's not necessarily always about the
> page faults.

We already have infrastructure for kprobes to prevent breakpoints
on critical code (the __kprobes section). In principle kgdb/kdb
could be taught about honoring those too.

That wouldn't help for truly external JTAG debuggers, but I would assume
those generally can (should) handle any contexts anyways.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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