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

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Date:	Wed, 06 Nov 2013 17:48:54 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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)

On Tue, 5 Nov 2013 18:45:35 +0100, Oleg Nesterov wrote:
> On 11/05, Namhyung Kim wrote:
>>
>> This is what I have for now:
>>
>> static void __user *get_user_vaddr(struct pt_regs *regs, unsigned long addr,
>> 				   struct trace_uprobe *tu)
>> {
>> 	unsigned long base_addr;
>> 	unsigned long vaddr;
>>
>> 	base_addr = instruction_pointer(regs) - tu->offset;
>> 	vaddr = base_addr + addr;
>>
>> 	return (void __force __user *) vaddr;
>> }
>>
>> When I tested it, it was able to fetch global and bss data from both of
>> executable and library properly.
>
> Heh ;) I didn't expect you will agree with this suggestion. But if you
> think it can work - great!

It seems to work for me well except the cross-fetch.

But I'm not sure it'll work for every cases.  It would be great if some
elf gurus come up and give some feedbacks.

Masami?

>
> Let me clarify just in case. Yes, _personally_ I think we should try
> to avoid the vma games, and it looks better to me this way. But I won't
> argue if you change your mind, I understand this approach has its own
> disadvantages.
>
> As for "-= tu->offset"... Can't we avoid it? User-space needs to calculate
> the "@" argument anyway, why it can't also substruct this offset?

Hmm.. it makes sense too. :)

>
> Or perhaps we can change parse_probe_arg("@") to update "param" ? Yes,
> in this case it needs another argument, not sure...
>
>> But it still doesn't work for uretprobes
>> as you said before.
>
> This looks simple,
>
> 	+	if (is_ret_probe(tu)) {
> 	+		saved_ip = instruction_pointer(regs);
> 	+		instruction_pointer_set(func);
> 	+	}
> 		store_trace_args(...);
> 	+	if (is_ret_probe(tu))
> 	+		instruction_pointer_set(saved_ip);
>
> although not pretty.

So for normal non-uretprobes, func == instruction_pointer(), right?

If so, just passing func as you suggested looks better than this.

Thanks,
Namhyung
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