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

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Date:	Wed, 01 Aug 2012 15:32:37 +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 03:01 PM, Oleg Nesterov wrote:
> Lets ignore uprobes which needs the changes anyway. This is
> only used by ptrace and the task is stopped. So, unless I missed
> something obvious, this update_debugctlmsr() is simply unneeded,
> __switch_to/__switch_to_xtra should notice _TIF_BLOCKSTEP and do
> update_debugctlmsr(DEBUGCTLMSR_BTF).

It looks like it unless a processes ptraces itself (which does not make
much sense anyway).

> 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.

> IOW, it seems to me we could safely remove update_debugctlmsr()
> arch/x86/kernel/step.c. However, if we want to re-use this code
> in uprobes, then we probably need to add "if (child == current)".
It looks that way.

>
> Or I am totally confused. Help!
>
> Oleg.

Sebastian
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