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From:   Milian Wolff <milian.wolff@kdab.com>
To:     Arnaldo Carvalho de Melo <acme@kernel.org>
Cc:     jolsa@kernel.org, Linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org
Subject: Re: [PATCH] perf util: take pgoff into account when reporting elf to
 libdwfl
Date:   Tue, 30 Oct 2018 21:36:39 +0100
Message-ID: <2799419.aVeE0VMyCs@agathebauer>
Organization: KDAB
In-Reply-To: <20181029174014.GG21857@kernel.org>
References: <20181029141644.3907-1-milian.wolff@kdab.com>
 <3361238.noN1Jb7VD2@milian-kdab2> <20181029174014.GG21857@kernel.org>
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On Montag, 29. Oktober 2018 18:40:14 CET Arnaldo Carvalho de Melo wrote:
> Em Mon, Oct 29, 2018 at 04:26:27PM +0100, Milian Wolff escreveu:
> > On Monday, October 29, 2018 3:16:44 PM CET Milian Wolff wrote:
> > > Libdwfl parses an ELF file itself and creates mappings for the
> > > individual sections. Perf on the other hand sees raw mmap events which
> > > represent individual sections. When we encounter an address pointing
> > > into a mapping with pgoff != 0, we must take that into account and
> > > report the file at the non-offset base address.
> > 
> > > This fixes unwinding with libdwfl in some cases. E.g. for a file like:
> > <snip>
> > 
> > > Note that the backtrace is still stopping too early, when
> > > compared to the nice results obtained via libunwind. It's
> > > unclear so far what the reason for that is.
> > 
> > The remaining issue is due to a bug in elfutils:
> > 
> > https://sourceware.org/ml/elfutils-devel/2018-q4/msg00089.html
> > 
> > With both patches applied, libunwind and elfutils produce the same output
> > for the above scenario.
> 
> I'm updating the patch to remove:
> 
> "It's unclear so far what the reason for that is."
> 
> Adding:
> 
> "See https://sourceware.org/ml/elfutils-devel/2018-q4/msg00089.html for
> a patch fixing that."
> 
> Ok?

Yes, thanks. I figured the fix for elfutils out after I submitted the perf 
patch.

> Or are you saying that that "unclear" part applies to both libunwind
> and elfutils?

No, libunwind worked fine without these patches for this specific case.

Cheers

-- 
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts
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