linux-kernel - Re: [PATCH 0/2] perf: symbol offset breakage with separated debug

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Message-ID: <20100803135420.GV29507@ghostprotocols.net>
Date:	Tue, 3 Aug 2010 10:54:20 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Dave Martin <dave.martin@...aro.org>
Cc:	linux-kernel@...r.kernel.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	kernel-team@...ts.ubuntu.com, Will Deacon <Will.Deacon@....com>,
	Linaro Dev <linaro-dev@...ts.linaro.org>
Subject: Re: [PATCH 0/2] perf: symbol offset breakage with separated debug

Em Tue, Aug 03, 2010 at 12:48:34PM +0100, Dave Martin escreveu:
> Hi all,
> 
> I've hit some problems in trying to get perf record/report to
> work correctly on systems with separated debug images (Ubuntu,
> in particular).  I worked on some patches to ensure that
> separate debug images do actually get loaded, when present --
> these commits have now been merged in linux-2.6-tip/master.
> (See below for a list of the commits.)
> 
> Now that these are in place, I'm observing a new problem which
> can mess up symbol locations -- though I think there might be a
> practical workaround, I can't see a trivial solution, so I'm
> keen to hear if anyone has any ideas.
> 
> 
> The problem:
> 
> perf makes some incorrect assumptions, which mean that the
> symbol locations seen by perf report and friends can be
> incorrect.
> 
> My analysis:
> 
> 	a) perf represents symbols as offsets from the start of
> 		the mmap'd code region which contains each
> 		symbol.

The symbol library in perf uses separate map_ip and unmap_ip to cope
with those, covering things like prelinked binaries, the kernel with its
absolute addresses, kexec cases where we capture a reference relocation
symbol so that we can adjust later when using the unrelocated vmlinux,
etc.
 
> 	b) For shared libs (ET_DYN objects), the desired offset
> 		is (usually) equal to the VMA of each given

VMA here stands for Virtual Memory Address, not Virtual Memory Area
where a map is loaded, right? I guess so, but trying to get to common
ground.

> 		symbol in the image.  Perf assumes that these
> 		offsets are always equal, and this normally
> 		works.
> 
> 		Atypical link configurations could cause this

Which ones? We need to detect those and act accordingly, doing
adjustments at load time or by using a separate map_ip/unmap_ip pair of
functions in struct map.

I'll continue reading your patches, the first one probably fixes a bug,
but wonder if you looked at the struct map->{map_ip,unmap_ip} routines
in tools/perf/util/map.h.

- Arnaldo
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