linux-kernel - Re: [PATCH V2 1/9] perf tools: Fix symbol annotation for relocated kernel

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Date:	Thu, 30 Jan 2014 11:21:12 +0200
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH V2 1/9] perf tools: Fix symbol annotation for relocated
 kernel

On 30/01/14 10:59, Ingo Molnar wrote:
> 
> * Adrian Hunter <adrian.hunter@...el.com> wrote:
> 
>> On 29/01/14 20:57, Arnaldo Carvalho de Melo wrote:
>>> Em Wed, Jan 29, 2014 at 04:14:36PM +0200, Adrian Hunter escreveu:
>>>> Kernel maps map memory addresses to file offsets.
>>>> For symbol annotation, objdump needs the object VMA
>>>> addresses.  For an unrelocated kernel, that is the
>>>> same as the memory address.
>>>>
>>>> The addresses passed to objdump for symbol annotation
>>>> did not take into account kernel relocation.  This
>>>> patch fixes that.
>>>
>>> Question: To fix the problem reported by Linus, i.e. the very minimal
>>> fix, we only need this patch, right?
>>
>> Yes but the other fixes are needed too.
> 
> So, for the specific case of kernel address layout randomization, how 
> does this fix Linus's bug with KASLR enabled? How does the code 
> recover the random, runtime offset of the relocated kernel, which 
> varies from boot to boot?

By comparing the address of a symbol ("_text" or "_stext")
in /proc/kallsyms (or perf.data - see below) with the same
symbol in vmlinux.

perf tools call this the ref_reloc_sym and stores it in
perf.data hidden in the synthesized kernel mmap record.
e.g.

0xd8 [0x50]: event: 1
.
. ... raw event: size 80 bytes
.  0000:  01 00 00 00 01 00 50 00 ff ff ff ff 00 00 00 00  ......P.........
.  0010:  00 00 00 17 00 00 00 00 ff ff ff a8 ff ff ff ff  ................
.  0020:  c8 01 00 98 ff ff ff ff 5b 6b 65 72 6e 65 6c 2e  ........[kernel.
.  0030:  6b 61 6c 6c 73 79 6d 73 5d 5f 73 74 65 78 74 00  kallsyms]_stext.
.  0040:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
.
0 0xd8 [0x50]: PERF_RECORD_MMAP -1/0: [0x17000000(0xffffffffa8ffffff) @ 0xffffffff980001c8]: x [kernel.kallsyms]_stext

That tells perf tools that _stext was 0xffffffff980001c8.
Compare to vmlinux:

$ objdump -t vmlinux | grep _stext
ffffffff810001c8 g       .text	0000000000000000 _stext

So the relocation is 0xffffffff980001c8 - 0xffffffff810001c8
= 0x17000000


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