linux-kernel - Re: [PATCH V4 8/9] perf buildid-cache: add ability to add kcore to the cache

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Message-ID: <CAM9d7chHTUvm9xZ191P-RbCgSNEBUanJ2hgYCgYvhA_10o-uhw@mail.gmail.com>
Date:	Tue, 8 Oct 2013 17:34:36 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH V4 8/9] perf buildid-cache: add ability to add kcore to
 the cache

On Tue, Oct 8, 2013 at 5:05 PM, Adrian Hunter <adrian.hunter@...el.com> wrote:
> On 08/10/13 10:41, Namhyung Kim wrote:
>> On Mon,  7 Oct 2013 09:50:17 +0300, Adrian Hunter wrote:
>>> kcore can be used to view the running kernel object code.
>>> However, kcore changes as modules are loaded and unloaded,
>>> and when the kernel decides to modify its own code.
>>> Consequently it is useful to create a copy of kcore at a
>>> particular time.  Unlike vmlinux, kcore is not unique
>>> for a given build-id.  And in addition, the kallsyms
>>> and modules files are also needed.  The tool therefore
>>> creates a directory:
>>>
>>>      ~/.debug/[kernel.kcore]/<build-id>/<YYYYmmddHHMMSShh>
>>>
>>> which contains: kcore, kallsyms and modules.
>>
>> Hmm..  I think the problem is that the kallsyms and kcore also have
>> module information and the build-id of kernel can identify the core
>> kernel part only.  So why not splitting modules from kcore and kallsyms?
>>
>> As the modules have their own build-id, we can extract module info from
>> kcore and kallsyms and put them under ~/.debug/[module]/<build-id>.
>> While at it, we can even synthesize symbol table and inject it into the
>> module kcore and get rid of the module kallsyms file.
>>
>> This way, we can identify all binaries using build-id only, no?
>
> No.
>
> kcore doesn't just give the module object code - it gives it
> linked in to the kernel.  Unless you have the modules at the same
> addresses the linking doesn't match what you traced.

Ah.. right.  I missed that.

Thanks,
Namhyung
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