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

Mon Mar 16 13:34:45 PDT 2020

Hi,

On 12/03/2020 01:21, Eli Friedman via llvm-dev wrote:

>  I don't think that many people are using pass plugins in the first place;
> 

I think that this group is actually not that small (e.g. downstream 
projects, academia, LLVM beginners), but possibly not as active on 
llvm-dev as others.

-Andrzej

> 
>> -----Original Message-----
>> From: Marc <mh at mh-sec.de>
>> Sent: Tuesday, March 10, 2020 12:00 AM
>> 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,
>>
>> that sounds like the -mllvm option would be a good a mention in the
>> WritingAnLLVMPass document ;-)
>>
>> Two further questions for this and I am keeping this on the list so
>> other people have pointers to this if they struggle with the same thing:
>>
>> 1. Does this require to use lld? (I am sure it is, just to clarify, for
>> the very low chance that it would also work with gnu ld and the llvm
>> gold plugin performce this magic)
>>
>> 1a. Follow up question if this required lld - does lld has feature
>> parity (and same parameter names) with e.g. gcc-9 ld?
>>
>> 2. When you say it does not work with trunk at the moment - did it work
>> previously? e.g. with the llvm 9 release? Or did it never work so far
>> and is in the progress of being fixed?
>>
>> Thanks a lot!
>>
>> Regards,
>> Marc
>>
>>
>> On 09.03.20 23:41, Eli Friedman wrote:
>>> 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
>>>> PGP: AF3D 1D4C D810 F0BB 977D  3807 C7EE D0A0 6BE9 F573
>>
>> --
>> 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
>> PGP: AF3D 1D4C D810 F0BB 977D  3807 C7EE D0A0 6BE9 F573
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