linux-kernel - Re: [PATCH] x86: fix typo(?) in step.c

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Date:	Thu, 6 Mar 2008 12:34:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Roland McGrath <roland@...hat.com>
Cc:	Jan Beulich <jbeulich@...ell.com>, tglx@...utronix.de,
	hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: fix typo(?) in step.c

* Roland McGrath <roland@...hat.com> wrote:

> The block-step test only tested that PTRACE_SINGLEBLOCK worked right. 
> I just souped it up to also test that PTRACE_SINGLESTEP still works 
> immediately afterwards.  This still does not show any problem from 
> this bug.  The case that would be broken by it is rather more arcane.  
> I haven't worked out the test case that fails with the bogon.

my interpretation of the bug would be that we fail to mask out the 
block-step MSR bit [because we mask out bit 25 instead of bit 1], and 
hence the bug would cause that MSR bit to stay enabled in other tasks 
too.

So in theory the bug should manifest itself as block-step mode never 
clearing itself, once activated. (but this would never leak into other 
tasks because we've got the thread.debugctlmsr abstraction that protects 
them)

	Ingo
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