llvm dev - Aug 2020 - Seeking help on LLD and extending ELF

Hi all,
I am experimenting with extending ELF. I want to add a custom ELF section,
something similar to a 'SyntheticSection'. However, I'm not able to
find documentation / doxygen for LLD code. Are there any existing examples of
adding custom ELF sections at link time in LLVM framework? Is it possible? Any
pointers?
Thanks in advance!Cheers!


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Hello,

There isn't any existing guide that I'm aware of. What you'll need
to do in the code will be heavily dependent on what you need the custom ELF
section to do. For example if the contents of the section are largely determined
at compile/assembly time and the linker just collates and relocates them
probably won't need much code in LLD. If the sections need custom processing
or even generation at link time then a SyntheticSection is a good model.

My advice would be to look at how LLD handles something like EHFrameSection or
MergeSyntheticSection as these handle data coming in from the
compiler/assembler, so they aren't 100% linker generated, yet they have
custom processing outside of just resolving relocations. SyntheticSections like
the PLT/GOT are entirely linker generated as a result of relocations.

In LLD typically you'll find most of the code changes in:
InputFiles.cpp (Do I need to do anything at file load time to handle my custom
section)
Writer.cpp (The majority of the linker "back-end", where
SyntheticSections are created)
SyntheticSections.cpp (The custom code for the SyntheticSections lives here)

For the vast majority of the time you'll be looking in lld/ELF and if you
are making target specific changes in one of the Arch subdirectories.

You'll also find llvm/include/llvm/BinaryFormat/ELF useful if you are adding
custom section flags and types. In general it is a good idea to add support for
any new section type to llvm-readobj, llvm-objdump and elf2yaml, yaml2elf to
make writing tests easier.

I don't have much more time to go into detail. In general if you get stuck,
ask here and we'll do our best to help. The more specific the question the
better.

Peter

p.s.
There have been a few LLD presentations at the LLVM developer meetings and at
other conferences like FOSDEM. I'm sure a quick web-search will find them.

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Dobby
HouseElf via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 11 August 2020 21:03
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Seeking help on LLD and extending ELF

Hi all,

I am experimenting with extending ELF. I want to add a custom ELF section,
something similar to a 'SyntheticSection'. However, I'm not able to
find documentation / doxygen for LLD code.
Are there any existing examples of adding custom ELF sections at link time in
LLVM framework? Is it possible?
Any pointers?

Thanks in advance!
Cheers!

Thank you, Peter for the detailed answer and all the pointers. I'll look up
LLD presentations.
Best regards!
    On Wednesday, August 12, 2020, 01:10:52 AM PDT, Peter Smith <peter.smith
at arm.com> wrote:
 
 Hello,

There isn't any existing guide that I'm aware of. What you'll need
to do in the code will be heavily dependent on what you need the custom ELF
section to do. For example if the contents of the section are largely determined
at compile/assembly time and the linker just collates and relocates them
probably won't need much code in LLD. If the sections need custom processing
or even generation at link time then a SyntheticSection is a good model.

My advice would be to look at how LLD handles something like EHFrameSection or
MergeSyntheticSection as these handle data coming in from the
compiler/assembler, so they aren't 100% linker generated, yet they have
custom processing outside of just resolving relocations. SyntheticSections like
the PLT/GOT are entirely linker generated as a result of relocations.

In LLD typically you'll find most of the code changes in:
InputFiles.cpp (Do I need to do anything at file load time to handle my custom
section)
Writer.cpp (The majority of the linker "back-end", where
SyntheticSections are created)
SyntheticSections.cpp (The custom code for the SyntheticSections lives here)

For the vast majority of the time you'll be looking in lld/ELF and if you
are making target specific changes in one of the Arch subdirectories.

You'll also find llvm/include/llvm/BinaryFormat/ELF useful if you are adding
custom section flags and types. In general it is a good idea to add support for
any new section type to llvm-readobj, llvm-objdump and elf2yaml, yaml2elf to
make writing tests easier.

I don't have much more time to go into detail. In general if you get stuck,
ask here and we'll do our best to help. The more specific the question the
better.

Peter

p.s.
There have been a few LLD presentations at the LLVM developer meetings and at
other conferences like FOSDEM. I'm sure a quick web-search will find them.

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Dobby
HouseElf via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 11 August 2020 21:03
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Seeking help on LLD and extending ELF

Hi all,

I am experimenting with extending ELF. I want to add a custom ELF section,
something similar to a 'SyntheticSection'. However, I'm not able to
find documentation / doxygen for LLD code.
Are there any existing examples of adding custom ELF sections at link time in
LLVM framework? Is it possible?
Any pointers?

Thanks in advance!
Cheers!


  
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