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Re: [PATCH -tip 3/3] perf-probe: Use the actual address as a hint for uprobes
- From: Namhyung Kim <namhyung at kernel dot org>
- To: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>
- Cc: Arnaldo Carvalho de Melo <acme at ghostprotocols dot net>, Ingo Molnar <mingo at kernel dot org>, Srikar Dronamraju <srikar at linux dot vnet dot ibm dot com>, David Ahern <dsahern at gmail dot com>, lkml <linux-kernel at vger dot kernel dot org>, "Steven Rostedt \(Red Hat\)" <rostedt at goodmis dot org>, Oleg Nesterov <oleg at redhat dot com>, "David A. Long" <dave dot long at linaro dot org>, systemtap at sourceware dot org, yrl dot pp-manager dot tt at hitachi dot com
- Date: Mon, 23 Dec 2013 16:46:49 +0900
- Subject: Re: [PATCH -tip 3/3] perf-probe: Use the actual address as a hint for uprobes
- Authentication-results: sourceware.org; auth=none
- References: <20131220100255 dot 7169 dot 19384 dot stgit at kbuild-fedora dot novalocal> <20131220100302 dot 7169 dot 96318 dot stgit at kbuild-fedora dot novalocal> <20131220180351 dot GC28878 at ghostprotocols dot net> <52B75F9E dot 7000802 at hitachi dot com>
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