Messages in this thread | | | From | Namhyung Kim <> | Subject | Re: [PATCH -tip 3/3] perf-probe: Use the actual address as a hint for uprobes | Date | Mon, 23 Dec 2013 16:46:49 +0900 |
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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;
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.
Thanks, Namhyung
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