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

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Message-ID: <20120801134652.GA4707@redhat.com>
Date:	Wed, 1 Aug 2012 15:46:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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, Sebastian Andrzej Siewior wrote:
>
> 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).

and forbidden ;) See ptrace_attach()->same_thread_group().

>> 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).

Oleg.

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