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

From: Namhyung Kim
Date: Tue Dec 24 2013 - 02:54:53 EST

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!
>> 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.

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).

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