[PATCH] D96627: [llvm-jitlink] Implement JITLoaderGDB ObjectLinkingLayer plugin for ELF x86-64

Tue Feb 16 23:11:08 PST 2021

MaskRay added a comment.

In D96627#2567468 <https://reviews.llvm.org/D96627#2567468>, @lhames wrote:

> In D96627#2566871 <https://reviews.llvm.org/D96627#2566871>, @sgraenitz wrote:
>
>>> If we constructed the debug object file from scratch (rather than mutating the existing one)
>>
>> While this sounds reasonable to me, it appears to add quite some effort. I guess we could grow our own writer that can emit in-memory ELF objects in some specific format, but there's a load of details that would need consideration sooner or later (recalculating offsets, compressed sections, etc.). Eventually, doing it right would probably mean something like `llvm-objcopy --only-keep-debug`. The implementation of it is buried with the tool implementation currently, so I can hardly "just use" it: https://reviews.llvm.org/rG5ad0103d8a04cb066dfae4fc20b0dfcd9413f4d4
>>
>> What do you think?
>>
>> (I will try a hack in the next days to see whether it's viable at all, but I'd would keep hopes low that we get there anytime soon.)
>
> I think MachOWriter and ELFWriter seem very useful outside llvm-objcopy. @MaskRay -- Do you think it would be reasonable to move those classes into a library? Maybe Object? Or BinaryFormat?

Hey. I am happy to share what I know about ELF, but I never look into jit in detail. (I should do that at some point..)

There is a feature request about "librarify llvm-objcopy": https://bugs.llvm.org/show_bug.cgi?id=41044
There was incomplete work in 2019. The latest attempt is https://lists.llvm.org/pipermail/llvm-dev/2021-January/147892.html (I haven't followed it).

As someone who implemented llvm-objcopy --only-keep-debug, I know that llvm-objcopy has two writers, one placing segments than sections, the other one (--only-keep-debug) placing sections then segments.
If you look at objcopy/llvm-objcopy --add-section and think it can do JIT job,,, you may get disappointed😓 The feature does not affect program headers (especially the most important PT_LOAD)...
so generally --add-section is probably only meaningful for relocatable object files (ET_REL) and non-SHF_ALLOC sections in executable and shared object files (ET_EXEC, ET_DYN).

With my current limited understanding about JIT, I think you may be able to take inspiration from some llvm-objcopy code, but using it as a library to do what you intend to do ... may be really difficult.

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