[llvm-dev] [lld][ELF] Addends adjustment for relocatable object

[Peter Smith via llvm-dev]

Hello Simon,

Can I just check something with you? When I tried the example the
.data section symbols given in the llvm-readelf -r output are
different, one has an offset of 0 and one has an offset of 4 so the
addends don't need updating. In ld the section symbols both have the
same offset of 0 so the addends need updating. So I think the two
approaches are equivalent.

Have I misunderstood?

Peter

On 27 September 2016 at 08:28, Simon Atanasyan via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> Now in case of relocatable object generation LLD merges and copies
> REL/RELA sections as is and does not touch any addends. But it is
> incorrect. If we have a relocation which targets a section, we have to
> adjust the relocation's addend to take in account that the section
> might be merged with other ones.
>
> Here is the reproduction script:
>
> % cat t1.s
>   .data
>   .long 0
>   .text
> bar:
>   movl $1, .data
>
> % cat t2.s
>   .data
>   .long 0
>   .text
> foo:
>   movl $1, .data
>
> % as t1.s -o t1.o
> % as t2.s -o t2.o
>
> % lld -flavor gnu -r t1.o t2.o
> % llvm-readobj -r a.out
>     0x3 R_X86_64_32S .data 0x0
>     0xE R_X86_64_32S .data 0x0
>
> % ld -r t1.o t2.o
> % llvm-readobj -r a.out
>     0x3 R_X86_64_32S .data 0x0
>     0xE R_X86_64_32S .data 0x4
>
> Are there any plans to fix this problem? I think it is rather easy to
> fix it for RELA sections, but the case is more complicated for REL
> sections. IMHO an alternative solution would be not to merge sections
> while generate a relocatable object.
>
> --
> Simon Atanasyan
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