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

From: Namhyung Kim
Date: Tue Dec 24 2013 - 03:46:28 EST

On Tue, 24 Dec 2013 17:27:45 +0900, Masami Hiramatsu wrote:
> (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.

It's only set from a mmap event either sent from kernel or synthesized
using /proc/<pid>/maps. We cannot know the load address of a library
until it gets loaded but for an executable, we could use the address of
ELF segments/sections.

>>>> 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.

It's for argument fetching. For probing, you can simply use a relative

>> 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.

Okay, please share the result then. :)

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