Re: [PATCH -tip 3/3] perf-probe: Use the actual address as a hint for uprobes

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2013/12/24 16:54), Namhyung Kim wrote:
> Hi Masami,
> 
> On Mon, 23 Dec 2013 19:50:10 +0900, Masami Hiramatsu wrote:
>> (2013/12/23 16:46), Namhyung Kim wrote:
>>> On Mon, 23 Dec 2013 06:54:38 +0900, Masami Hiramatsu wrote:
>>>> (2013/12/21 3:03), Arnaldo Carvalho de Melo wrote:
>>>>> Em Fri, Dec 20, 2013 at 10:03:02AM +0000, Masami Hiramatsu escreveu:
>>>> BTW, I'm not sure why debuginfo and nm shows symbol address + 0x400000,
>>>> and why the perf's map/symbol can remove this offset. Could you tell me
>>>> how it works?
>>>> If I can get the offset (0x400000) from binary, I don't need this kind
>>>> of ugly hacks...
>>>
>>> AFAIK the actual symbol address is what nm (and debuginfo) shows.  But
>>> perf adjusts symbol address to have a relative address from the start of
>>> mapping (i.e. file offset) like below:
>>>
>>> 	sym.st_value -= shdr.sh_addr - shdr.sh_offset;
>>
>> Thanks! this is what I really need!

BTW, what I've found is that the perf's map has start, end and pgoffs
but those are not initialized when we load user-binary (see dso__load_sym).
I'm not sure why.

>>> This way, we can handle mmap and symbol address almost uniformly
>>> (i.e. ip = map->start + symbol->address).  But this requires the mmap
>>> event during perf record.  For perf probe, we might need to synthesize
>>> mapping info from the section/segment header since it doesn't have the
>>> mmap event.  Currently, the dso__new_map() just creates a map starts
>>> from 0.
>>
>> I think the uprobe requires only the relative address, doesn't that?
> 
> Yes, but fetching arguments is little different than a normal relative
> address, I think.

Is this for uprobe probing address? or fetching symbol(global variables)?
I'd like to support uprobes probing address first.

> An offset of an argument bases on the mapping address of text segment.
> This fits naturally for a shared library case - base address is 0.  So
> we can use the symbol address (st_value) directly.  But for executables,
> the base address of text segment is 0x400000 on x86-64 and data symbol
> is on 0x6XXXXX typically.  So in this case the offset given to uprobe
> should be "@+0x2XXXXX" (st_value - text_base).

Oh, I see. I'd better make a testcase for checking what the best
way to get such offsets.

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com