linux-kernel - Re: [PATCH] [RFC V1]s390/perf: fix 'start' address of module's map

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Date:	Mon, 11 Jul 2016 16:11:55 +0800
From:	Songshan Gong <gongss@...ux.vnet.ibm.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	acme@...nel.org, jolsa@...nel.org, dsahern@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC V1]s390/perf: fix 'start' address of module's map

在 7/8/2016 11:18 PM, Jiri Olsa 写道:
> On Thu, Jul 07, 2016 at 09:49:36AM +0800, Song Shan Gong wrote:
>> At preset, when creating module's map, perf gets 'start' address by parsing
>> 'proc/modules', but it's module base address, isn't the start address of
>> '.text' section. In most archs, it's OK. But for s390, it places 'GOT' and
>> 'PLT' relocations before '.text' section. So there exists an offset between
>> module base address and '.text' section, which will incur wrong symbol
>> resolution for modules.
>>
>> Fix this bug by getting 'start' address of module's map from parsing
>> '/sys/module/[module name]/sections/.text', not from '/proc/modules'.
>
> cool, does this fix the 'perf test 1' for s390? that'd be great

I've checked, 'perf test 1' is still failed. But the test is for testing 
whether 'vmlinux symtab matches kallsyms'. For vmlinux, it's built when 
compiling linux-kernel, so there's no any symbol info about module. For 
kallsyms, when loading, it also splits kernel symbols with module 
symbols. But this patch is intended to fix wrong symbol resolution for 
modules, so I think there's no relationship between this patch and the 
testcase 'perf test 1'.

I also debuged the reason why 'perf test 1' fails, there are two reasons 
at least:
1. perf gets kernel start address by finding the first no-zero value of 
symbols {'_text', '_stext'} in /proc/kallsyms; for s390, it's always 
'_stext', but actually, '_stext' is not the first symbol in s390, there 
are other symbols before '_stext', for example 'iplstart'.
2. In addition, when loading by parsing /proc/kallsyms, if the kernel 
map start value is a non-zero value getting from '_stext', because 
'_text' is zero, it will not be included in kernel map; but for vmlinux, 
it always add '_text' to kernel map whether '_text' is zero or not. So 
if '_text' is zero, whether adding '_text' to kernel map, it's 
non-consistent for loading by /proc/kallsyms and vmlinux.

I'll try to fix this bug later.

Thanks for your comments.

> I'll send few coments shortly
>
> thanks,
> jirka
>

-- 
SongShan Gong