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

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Date:	Wed, 01 Aug 2012 16:21:16 +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 04:01 PM, Oleg Nesterov wrote:
> On 08/01, Sebastian Andrzej Siewior wrote:
>>
>> 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.
>
> It seems that you replied to the wrong email or I am confused ;)

No I think I replied to the correct one :)
enable_step() is the only place for ptrace/debugger which is touching
DEBUGCTLMSR_BTF. It always sets DEBUGCTLMSR_BTF and TIF_BLOCKSTEP in
sync so why should they both end up different? And once 
__switch_to_extra() notices that TIF_BLOCKSTEP from the previous task
is different from the next task is different, then the CPU flag has
to be changed.

> Let's ignore kprobes here.

done.

>
> Oleg.
>


Sebastian
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