linux-kernel - Re: [PATCH 4/5] x86: Allow nesting of the debug stack IDT setting

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Message-ID: <4FC7DDF1.5070606@zytor.com>
Date:	Thu, 31 May 2012 14:09:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Dave Jones <davej@...hat.com>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/5] x86: Allow nesting of the debug stack IDT setting

On 05/31/2012 01:56 PM, Steven Rostedt wrote:
> On Thu, 2012-05-31 at 13:39 -0700, H. Peter Anvin wrote:
>> On 05/31/2012 01:35 PM, Steven Rostedt wrote:
>>>
>>> Thus only the debug stack does the stack TSS trick.
>>>
>>> Is this what you were looking for? (God, it just shows how much time
>>> I've been spending on this crap, as I was able to find all this by
>>> memory and not grepping for it :-p )
>>>
>>
>> That's why I asked instead of pulled it up myself.
>>
>> However, what is missing is what the permitted transitions are.  This
>> affects the options.
> 
> Only the debug stack changes. These happen in the two places I already
> mentioned.
> 
> One, the switch of the IDT in NMI (to convert the IST from 4 to 0, and
> just keep the same stack).
> 
> Two, is the trick in the paranoidzeroentry_ist code (adding the debug
> stack). That changes the DEBUG stack pointer.
> 

No, I'm asking what environments (alternate stacks) are permitted where.
 That is the important information.

The interaction of the above two things is part of the problem, of
course, and I think it can (and should) be avoided.

	-hpa

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