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

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Date:	Mon, 29 Jun 2015 14:52:39 +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 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.

Will
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