linux-kernel - Re: [PATCH v2 2/5] x86/uprobes: implement x86 specific arch_uprobe_*

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Message-ID: <20120814142736.GA8123@redhat.com>
Date:	Tue, 14 Aug 2012 16:27:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Roland McGrath <roland@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	stan_shebs@...tor.com
Subject: Re: [PATCH v2 2/5] x86/uprobes: implement x86 specific
	arch_uprobe_*_step

On 08/14, Sebastian Andrzej Siewior wrote:
>
> On 08/13/2012 03:24 PM, Oleg Nesterov wrote:
>>
>> this patch still adds restore_flags into arch_uprobe_task.
>
> Yes, but

OOPS. Yes, we need a new member in ->utask now to record the state
of TIF_SINGLESTEP (X86_EFLAGS_TF actually).

I meant that, since the patch still uses TIF_SINGLESTEP,
arch_uprobe_disable_step() can check it but somehow I forgot that
since arch_uprobe_enable_step() still does user_enable_single_step()
TIF_SINGLESTEP is always set.

>>>   static void prepare_fixups(struct arch_uprobe *auprobe, struct insn *insn)
>>>   {
>>> -	bool fix_ip = true, fix_call = false;	/* defaults */
>>> +	bool fix_ip = true, fix_call = false, fix_tf = false;	/* defaults */
>>>   	int reg;
>>>
>>>   	insn_get_opcode(insn);	/* should be a nop */
>>>
>>>   	switch (OPCODE1(insn)) {
>>> +	case 0x9d:
>>> +		/* popf */
>>> +		fix_tf = true;
>>> +		break;
>>>   	case 0xc3:		/* ret/lret */
>>>   	case 0xcb:
>>>   	case 0xc2:
>>> @@ -277,6 +284,8 @@ static void prepare_fixups(struct arch_uprobe *auprobe, struct insn *insn)
>>>   		auprobe->fixups |= UPROBE_FIX_IP;
>>>   	if (fix_call)
>>>   		auprobe->fixups |= UPROBE_FIX_CALL;
>>> +	if (fix_tf)
>>> +		auprobe->fixups |= UPROBE_TF_CHANGES;
>>>   }
>>
>> I won't insist, but do we really need fix_tf? "case 0x9d" could simply
>> add UPROBE_TF_CHANGES.
>
> if it is not 0x9d (in most cases) we need to decide on per-process
> basis (not per-breakpoint) whether the task has gdb watching it or not.

Yes, yes, I see, thanks.

But this doesn't explain why do we need to add the new variable, fix_tf.

	case 0x9d:
		auprobe->fixups |= UPROBE_TF_CHANGES;
		break;

seems enough.

Oleg.

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