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

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Message-ID: <501A7E94.4030309@linutronix.de>
Date:	Thu, 02 Aug 2012 15:20:20 +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 08/02/2012 03:05 PM, Oleg Nesterov wrote:
> I have no idea what DEBUGCTLMSR_BTF means without X86_EFLAGS_TF
> though. And if gdb itself is TIF_SINGLESTEP'ed, it won't return
> to userspace without report/schedule.

In the Manuel it is only described what happens together with the TF
bit. So one might think nothing. Userland can not read the bit back
however leaking it does not look right.

> But, yes sure! this doesn't look right and this is the source of
> other problems, and this is why I started this thread.
>
>> If the tracee task runs
>
> In the scenario I tried to describe above, the tracee does _not_ run.
>
> gdb switches to _another_ X86_EFLAGS_TF task before the tracee is resumed.
>
>> From the link above,
>
> 	We have the GDB process and the (stopped) tracee T. And we have
> 	another task X
>          ^^^^^^^^^^^^^^

Yes that is correct. Let me try to figure out how to plumb that leak
and fix this before playing with it further.

>
> Oleg.

Sebastian
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