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From: Milian Wolff <mail@milianw.de>
To: elfutils-devel@sourceware.org
Cc: Mark Wielaard <mark@klomp.org>
Subject: Re: dwfl_module_addrinfo and @plt entries
Date: Wed, 05 Jul 2017 13:34:00 -0000	[thread overview]
Message-ID: <3866408.Fn4hcxxxTc@milian-kdab2> (raw)
In-Reply-To: <20170106191753.GB8669@stream>

On Friday, January 6, 2017 8:17:53 PM CEST Mark Wielaard wrote:
> On Fri, Jan 06, 2017 at 11:28:25AM +0100, Milian Wolff wrote:
> > On Wednesday, January 4, 2017 2:42:23 PM CET Mark Wielaard wrote:
> > > Longer answer. An address pointing into the PLT does
> > > really point to an ELF symbol.
> > 
> > You mean: does _not_
> > Right?
> 
> Yes, I meant "does not point".
> 
> > > If we have such a backend function then we could even
> > > do what BFD apparently does. Which is to then create a
> > > "fake" symbol with as name real_function@plt. But I am
> > > not sure such fake symbols are very useful (and will
> > > quickly become confusing since they aren't real ELF
> > > symbols).
> > 
> > So the objdump command I used is leveraging BFD internally to give me the
> > @plt names? I noticed that I also see @plt in perf, which is also
> > probably using BFD internally. That at least clarifies why it works in
> > some tools but not in when using dwfl.
> 
> binutils objdump certainly does.
> 
> > > Hope that helps. And maybe inspires someone (you?) to
> > > write up such a backend function and corresponding
> > > dwfl frontend function.
> > 
> > It does help, thanks. I'm interested in contributing such functionality,
> > but, sadly, I'm not sure when I'll get the time to actually do it.
> 
> Thanks, wish I had spare time myself :)

I have now looked into this issue again and have found a way to workaround 
this limitation outside of elfutils, by manually resolving the address in a 
.plt section to a symbol. See:

https://github.com/KDAB/perfparser/commit/
885f88f3d66904cd94af65f802232f6c6dc339f4

This seems to work in my limited tests (only on X86_64). Beside the 32bit/
64bit difference, it isn't really platform dependent, is it? Or was this what 
you had in mind when you said the elfutils code would be "architecture 
specific [and] we would need a backend function that translates an address 
pointing into the PLT into an actual function address"?

If my code is roughly OK, then I'll try to put it into a patch for elfutils 
and submit it there. If it's fundamentally broken, please tell me. I still 
plan to get this functionality upstream into elfutils.

Cheers

-- 
Milian Wolff
mail@milianw.de
http://milianw.de

  reply	other threads:[~2017-07-05 13:34 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-01-04  0:41 Milian Wolff
2017-01-04 13:42 ` Mark Wielaard
2017-01-06 10:28   ` Milian Wolff
2017-01-06 19:17     ` Mark Wielaard
2017-07-05 13:34       ` Milian Wolff [this message]
2017-07-07 11:03         ` Mark Wielaard
2017-07-10 11:06           ` Milian Wolff
2017-08-28 14:28             ` Milian Wolff

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