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 address. > >> 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. :) Thanks, Namhyung -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
