[llvm-dev] [LLD] Can't create dynamic relocation R_X86_64_64 against local symbol in readonly segment

[Mark Kettenis via llvm-dev]

> Date: Thu, 23 Mar 2017 16:47:11 +0100
> From: Martin Richtarsky via llvm-dev <llvm-dev at lists.llvm.org>
> 
> >> $ gcc -o rodatareloc.s.o -c rodatareloc.s
> >> $ lld -o rodatareloc.so -shared rodatareloc.s.o
> >>
> >> ld: error: rodatareloc.s.o:(.rodata+0x0): can't create dynamic
> >> relocation
> >> R_X86_64_64 against local symbol in readonly segment defined in
> >> rodatareloc.s.o
> >>
> >>
> >> Changing the section from .rodata to .data fixes it, but I guess this
> >> should be supported also for .rodata. Should I open a bug?
> >
> > I think this is just a difference in defaults. If you pass "-z notext"
> > to lld it should work.
> 
> Thanks, this helps! Any reason why the defaults are different to gold and
> presumably bfd-ld?

Yes.  Releocations in .rodata are undesirable.  This way the
programming errors or compiler bugs that cause them get caught.

> I don't understand much of what is happening behind the scenes, but on
> loading time, couldn't the rodata section just be mapped read-only, and
> still the relocation be applied to the text section, pointing to wherever
> rodata was mapped? No modification should be necessary to the rodata
> section.

The dynamic linker can patch these up.  But doing so kills the
"shared" aspect of the object as the pages in question will no longer
be shared between processes.