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

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Date:	Tue, 30 Jun 2015 11:47:01 +0100
From:	Will Deacon <will.deacon@....com>
To:	Adrian Hunter <adrian.hunter@...el.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 Tue, Jun 30, 2015 at 10:57:04AM +0100, Adrian Hunter wrote:
> 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.

I was referring to the subtraction in map__rip_2objdump, but you've
addressed that below.

> 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.

FWIW, this works on arm64 and x86_64 with my testing:

  Tested-by: Will Deacon <will.deacon@....com>

Cheers,

Will
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