linux-kernel - Re: Q: user_enable_single_step() && update

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Date:	Wed, 01 Aug 2012 15:54:48 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Roland McGrath <roland@...k.frob.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	ananth@...ibm.com, a.p.zijlstra@...llo.nl, mingo@...hat.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: Q: user_enable_single_step() && update_debugctlmsr()

On 08/01/2012 03:46 PM, Oleg Nesterov wrote:

>>> But, worse, isn't it wrong? Suppose that debugger switches to
>>> another TIF_SINGLESTEP&&   !TIF_BLOCKSTEP task, in this case
>>> we "leak" DEBUGCTLMSR_BTF, no?
>>
>> __switch_to_xtra() should notice the difference in the TIF_BLOCKSTEP
>> flag and disable it.
>
> And how it can notice the difference if there is no difference?
>
> (unless, of course debugger is TIF_BLOCKSTEP'ed).

Yes. enable_step() sets DEBUGCTLMSR_BTF along with TIF_BLOCKSTEP. 
kprobes checks the same flag before touching DEBUGCTLMSR_BTF.

>
> Oleg.
>
Sebastian
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