dwfl_module_addrinfo and @plt entries

Milian Wolff mail@milianw.de
Fri Jan 6 10:28:00 GMT 2017

On Wednesday, January 4, 2017 2:42:23 PM CET Mark Wielaard wrote:
> On Wed, Jan 04, 2017 at 01:41:26AM +0100, Milian Wolff wrote:
> > how do I get symbol information for @plt entries?
> Short answer. You cannot with the dwfl_module_getsym/addr*
> functions. And we don't have accessors for "fake" symbols
> (yet). Sorry.

Thanks for the clarification Mark.

> Longer answer. An address pointing into the PLT does
> really point to an ELF symbol.

You mean: does _not_

> The PLT/GOT contains entries
> that are architecture specific "jump targets" that contain
> (self-modifying) code/data on first access. A PLT entry
> is code that sets up the correct (absolute) address of
> a function on first access. And on second access it fetches
> that function address and jumps to it. Since this is
> architecture specific we would need a backend function
> that translates an address pointing into the PLT into
> an actual function address. You would then be able to
> fetch the actual ELF symbol that address is associated
> with.
> 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.

> 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.


Milian Wolff

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