linux-kernel - Re: [PATCHSET 00/13] tracing/uprobes: Add support for more fetch methods (v6)

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Message-ID: <87siv2uk1q.fsf@sejong.aot.lge.com>
Date:	Tue, 12 Nov 2013 17:00:01 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Hyeoncheol Lee <cheol.lee@....com>,
	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"zhangwei\(Jovi\)" <jovi.zhangwei@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCHSET 00/13] tracing/uprobes: Add support for more fetch methods (v6)

Hi Oleg and Masami,

On Sat, 9 Nov 2013 16:23:13 +0100, Oleg Nesterov wrote:
> On 11/09, Masami Hiramatsu wrote:
>>
>> In that case, I suggest you to use "@+addr" for the relative address,
>> since that is an offset, isn't that? :)
>
> Agreed, @+addr looks better!

Looks good to me too.

>
>> BTW, it seems that @addr syntax is hard to use for uprobes, because
>> current uprobes is based on a binary, not a process, we cannot specify
>> which process is probed when we define it.
>
> Yes, exactly. That is why we suggest that user-space should pass the
> ip-relative address (actually offset). This should hopefully solve all
> problems with relocations.

Let me clarify what I understand.

For @addr syntax: kernel does no translation and uses given address

For @+addr syntax: user-space uses relative symbol address from a loaded
                   base address and kernel calculates the base address
                   using "current->utask->vaddr - tu->offset".

Is that right?

Thanks,
Namhyung
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