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Re: dwfl_module_addrinfo and @plt entries


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


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