linux-kernel - Re: [BUG] perf report: fails to symbolize when vaddr is non zero for shared objects

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Message-ID: <CABPqkBTG++sr8G8bceLc+6LXHydCT2OZCm8+fR+QHQrSd6fDVQ@mail.gmail.com>
Date:	Wed, 17 Jun 2015 11:54:12 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [BUG] perf report: fails to symbolize when vaddr is non zero for
 shared objects

On Tue, Jun 16, 2015 at 7:34 AM, Arnaldo Carvalho de Melo
<acme@...hat.com> wrote:
> Em Tue, Jun 16, 2015 at 12:03:01AM -0700, Stephane Eranian escreveu:
>> Hi,
>>
>> It has been brought to my attention that on systems where the text
>> of shared libs is not loaded with a zero virtual address, perf report
>> fails to symbolize
>> correctly samples. This is true of older versions of perf and also the latest
>> in tip.git.
>>
>> I looked at symbol-elf.c and I did not see a place where the vaddr was taken
>> into account from the program headers in the case of ET_DYN. I see it for
>> ET_EXE, though.
>>
>> $ readelf -e lib.so
>>   Type:                              DYN (Shared object file)
>>   ....
>>   Type          Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
>>  LOAD           0x000000 0x0000d000 0x0000d000 0x73657c 0x73657c R E 0x1000
>>
>> If you get samples in the shared lib, they will be off, possibly
>> attributed to the wrong
>> functions.
>>
>> Could this be fixed quickly?
>
> You tell me, do you have a patch to propose? :-)
>
Well, need to make sure I understand the code flow in dso__load_sym()
and callers.
You have elf_read_maps() which seems to be reading the phdr but, it would need
to be called systematically for ET_DYN. I will experiment with this.
Worst case, we
need to add another function to read the phdr and use the p_vaddr.
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