Re: [PATCH] perf util: take pgoff into account when reporting elf to libdwfl

Milian Wolff
2018-10-30T20:36:51Z

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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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