[llvm-dev] llvm-link has no --allow-multiple-definition

[Eli Friedman via llvm-dev]

If you want to load a pass plugin into lld, you need to write something like " -Wl,-mllvm=-load=plugin.so".  Granted, that doesn't actually work on trunk at the moment; posted https://reviews.llvm.org/D75879 .

Once you actually have the plugin loaded into the linker, EP_FullLinkTimeOptimizationLast should work.

-Eli

> -----Original Message-----
> From: Marc <mh at mh-sec.de>
> Sent: Monday, March 9, 2020 2:26 PM
> To: Eli Friedman <efriedma at quicinc.com>
> Cc: llvm-dev <llvm-dev at lists.llvm.org>
> Subject: [EXT] Re: [llvm-dev] llvm-link has no --allow-multiple-definition
>
> Hi Eli,
>
> I know the page, and the only mentioning about LTO or link time
> optimization I found is the option
> PassManagerBuilder::EP_FullLinkTimeOptimizationLast - and nobody knows
> how this works (and asked about it on this list too before with no
> reply), and the author who wrote that part in the llvm code at Redhat
> did not reply to a question how to actually use it. And there is no
> example code or any code using this option anywhere on the Internet I
> could find.
>
> And loading such a plugin via -Xclang -load -Xclang plugin.so with
> EP_FullLinkTimeOptimizationLast doesnt work.
>
> So if you can shed any light about how to write a plugin that allows
> running a pass within ld - I would be very greatful! (and a large
> community of people fuzzing code :)
>
> Regards,
> Marc
>
>
> On 09.03.20 20:24, Eli Friedman wrote:
> > Despite the name, llvm-link isn't really a proper linker.  If you need symbols to
> get resolved exactly the same way an ELF linker would resolve them, I would
> recommend using lld.
> >
> > It's possible to write a pass plugin that hooks into the LTO pipeline.
> http://llvm.org/docs/WritingAnLLVMPass.html describes this.
> >
> > -Eli
> >
> >> -----Original Message-----
> >> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Marc via
> llvm-
> >> dev
> >> Sent: Sunday, March 8, 2020 10:01 AM
> >> To: llvm-dev <llvm-dev at lists.llvm.org>
> >> Subject: [EXT] [llvm-dev] llvm-link has no --allow-multiple-definition
> >>
> >> Hello,
> >>
> >> I am developing an LTO instrumentation of programs for fuzzing (for
> >> afl++) and I run into the following problem:
> >>
> >> a lot of open source program have multiple definitons of the same
> >> functions. In normal compile mode this is not a problem, however with
> >> LTO this only works with the linker option:
> >>  -Wl,--allow-multiple-definition
> >>
> >> Now to be able to instrument (it is an "opt" pass") at link time I need
> >> to merge all bitcode files together, and this works great - unless I run
> >> into this error when I link them to llvm-link:
> >>
> >> error: Linking globals named 'process_arg': symbol multiply defined!
> >>
> >> (this is for the bogofilter tools, but happens with other open source
> >> program too).
> >>
> >> My issue is that neither is --allow-multiple-definition an llvm-link
> >> command line option, nor can I find an equivalent from the --help output.
> >>
> >> is there a plan to add this (IMHO) important feature? Thank you!
> >>
> >> Regards,
> >> Marc
> >>
> >> --
> >> Marc Heuse
> >> www.mh-sec.de
> >>
> >> PGP: AF3D 1D4C D810 F0BB 977D  3807 C7EE D0A0 6BE9 F573
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Marc Heuse
> Mobil: +49 177 9611560
> Fax: +49 30 37309726
> www.mh-sec.de
>
> Marc Heuse - IT-Security Consulting
> Winsstr. 68
> 10405 Berlin
>
> Ust.-Ident.-Nr.: DE244222388
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