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

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Message-ID: <20150629145508.GA2307@redhat.com>
Date:	Mon, 29 Jun 2015 11:55:08 -0300
From:	"acme@...hat.com" <acme@...hat.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Adrian Hunter <adrian.hunter@...el.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

Em Mon, Jun 29, 2015 at 02:52:39PM +0100, Will Deacon escreveu:
> 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.

Its been a long time that I looked at this, but perhaps these can help:

/* rip/ip <-> addr suitable for passing to `objdump --start-address=` */
u64 map__rip_2objdump(struct map *map, u64 rip);

/* objdump address -> memory address */
u64 map__objdump_2mem(struct map *map, u64 ip);

The comments in the changeset that introduced it have more info:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7a2b6209863626cf8362e5ff4653491558f91e67

- Arnaldo

- Arnaldo
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