[llvm-dev] Why -pie option force LLD to output shared obj file type, not executable?

[Peter Smith via llvm-dev]

Hello Steven,

I'm not Rui, but I may be able to help. We recently ran into something
like this on AArch64 with pr39810 lld would not accept --pie and
--shared simultaneously.

In general even on ld.bfd use of the --pie linker option sets the file
type to ET_DYN, ld.bfd has a special case when the start of the text
section is non 0 (I think). This was described in
https://sourceware.org/ml/binutils/2013-12/msg00081.html . As far as I
know this was for non-position independent executables that had been
compiled -fPIE just to get the small code model.

In the AArch64 case the problem was that --shared was being used to
work around ld.bfd using ET_EXEC, unfortunately it seems like you need
the ET_EXEC behaviour? If so I don't think that there are any flags
that you can give to lld to change this.

My understanding is that the only difference in the file ld.bfd
generates is the e_type field in the ELF header. It would be trivial
to write a quick tool to change the field from ET_DYN to ET_EXEC.

Hope this is of some use.

Peter




On Wed, 23 Jan 2019 at 15:47, Shi, Steven via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello Rui,
>
> I’m enabling the LLD in the Uefi firmware edk2 build. I meet a problem about the -pie option and cannot output the executable type obj file correctly. I need your advice.
>
> The Uefi firmware executable binary is the position independent + small code mode in 64bits. So we always add the options “-Wl,-pie -mcmodel=small” in our clang build toolchain. These options work well with binutils LD, but cannot work with LLD. I see the LLD uses the below code to force the output obj file as shared obj type if link with -pie. Is there any way to let LLD output executable type obj with -pie option? I’m OK if I have to link twice, first output DYN and then covert to EXEC in some way. Could you give some link command examples on it?
>
>
>
> lld\ELF\Writer.cpp
>
> static uint16_t getELFType() {
>
>   if (Config->Pic)
>
>     return ET_DYN;
>
>   if (Config->Relocatable)
>
>     return ET_REL;
>
>   return ET_EXEC;
>
> }
>
>
>
>
>
> Thanks
>
> Steven
>
>
>
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