linux-kernel - Re: [PATCH v2] perf tools: don't adjust symbols in vDSO

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Date:	Tue, 30 Jun 2015 12:57:04 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Will Deacon <will.deacon@....com>
CC:	"acme@...hat.com" <acme@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kristina Martsenko <Kristina.Martsenko@....com>,
	Vladimir Nikulichev <nvs@...icks.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2] perf tools: don't adjust symbols in vDSO

On 29/06/15 16:52, Will Deacon wrote:
> On Mon, Jun 29, 2015 at 02:23:51PM +0100, Adrian Hunter wrote:
>> On 29/06/15 15:30, Adrian Hunter wrote:
>>> On 29/06/15 12:02, Adrian Hunter wrote:
>>>> On 27/06/15 12:10, Will Deacon wrote:
>>>>> @@ -824,6 +823,14 @@ int dso__load_sym(struct dso *dso, struct map *map,
>>>>>  	sec = syms_ss->symtab;
>>>>>  	shdr = syms_ss->symshdr;
>>>>>  
>>>>> +	/*
>>>>> +	 * Older x86 kernels prelink the vDSO at a high address, so
>>>>> +	 * we need to reflect that in map->pgoff in order to talk to
>>>>> +	 * objdump.
>>>>> +	 */
>>>>> +	if (dso__is_vdso(dso))
>>>>> +		map->pgoff = shdr.sh_addr - shdr.sh_offset;
>>>>
>>>> In the case of perf tools, maps map memory addresses to file offsets.
>>>> That is used to read from the object file, so you can't change the map.
>>>
>>> So what about just this instead:
>>>
>>> 	if (dso__is_vdso(dso))
>>> 		map->reloc = shdr.sh_addr - shdr.sh_offset;
>>>
>>
>> No that's no good either :-(
> 
> Yeah... we're fighting against symbol lookup wanting ->map_ip to give the
> address of the ELF symbol but unwinding wanting ->map_ip to give a relative
> offset for file reads.
> 
> Also, the reloc is a bit weird and I think needs to be the other way around
> (i.e. we convert from a map-relative address to an objdump address by
> *subtracting* the reloc). Even with that change, I run into problems with
> annotate comparing ELF symbol addresses with map addresses. Urgh.

Adding or subtracting shouldn't matter.

This seems to work (without any other changes):

	if (dso__is_vdso(dso))
		map->reloc = map->start - shdr.sh_addr + shdr.sh_offset;

Haven't tested 32-bit yet though.

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