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

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Message-ID: <87shuvv3g5.fsf@concordia.ellerman.id.au>
Date:	Wed, 27 Jul 2016 19:24:26 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>, anton@...ba.org
Cc:	Song Shan Gong <gongss@...ux.vnet.ibm.com>, jolsa@...nel.org,
	dsahern@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] s390/perf: fix 'start' address of module's map

Arnaldo Carvalho de Melo <acme@...nel.org> writes:

> Em Tue, Jul 26, 2016 at 10:14:18PM +0200, Christian Borntraeger escreveu:
>> On 07/26/2016 09:50 PM, Arnaldo Carvalho de Melo wrote:
>> > Em Thu, Jul 21, 2016 at 11:10:51AM +0800, Song Shan Gong escreveu:
>> >> 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.
>> > 
>> > I'll apply this as it fixes the problem for you and we need to get fixes
>> > in ASAP to get this into 4.8, but why can't we just use your method for
>> > all arches and get rid of this arch__ hook? I.e. if I look here in my
>> > x86_64 notebook I see:
>> > 
>> > [acme@...et linux]$ cat /sys/module/tun/sections/.text 
>> > 0xffffffffc0af2000
>> > [acme@...et linux]$ grep tun /proc/modules 
>> > tun 28672 4 vhost_net, Live 0xffffffffc0af2000
>> > [acme@...et linux]$ 
>> > 
>> > So I could as well use what is in /sys/module/tun/sections/.text instead
>> > of reading it from /proc/modules and, in s390, reading it from
>> > /sys/module/tun/sections/.text.
>> > 
>> > Do you see any problem with using this approach for _all_ arches?
>> 
>> I think it should work well for _all_ arches but it will probably be
>> hard to test this without help. 
>
> Well, we could check for the cases we don't know, i.e. read from both
> and warn about cases where it is different, except for s390 where we now
> which is the right one to pick.
>
>> I wouldn't be surprised if other architectures than s390 actually have
>> the same issue, so doing this for everybody might atually fix this somewhere
>> else.
>
> Would be nice to get info from other arch people, Michael, how this goes
> on ppc?

It doesn't look like this is a problem on powerpc - at least I haven't
heard of it.

Looking at a system I have here, for all modules (26) the value in
/proc/modules matches the .text section in /sys.

So I think using /sys should be fine for us.

cheers