llvm dev - Mar 2017 - [LLD] Can't create dynamic relocation R_X86_64_64 against local symbol in readonly segment

Hi,

the attached example works with bfd-ld and with gold, but not with lld:

$ cat rodatareloc.s
...
.align 16
    leaq    .JTab(%rip), %r10
    jmp     *(%r10, %rdx, 8)
...
.section .rodata
.JTab:
    .quad .L00, .L01, .L02, .L03, .L04, .L05, .L06, .L07
    .quad .L08, .L09, .L10, .L11, .L12, .L13, .L14, .L15, .L16

$ 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?

Best regards,
Martin
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On Thu, Mar 23, 2017 at 01:23:24PM +0100, Martin Richtarsky via llvm-dev
wrote:
> 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?
This is not a bug, lld is completely correct. You are explicitly asking
for a read-only section and the choosen relocation is position
dependent, so yes, it correctly bails out.

Joerg

Yes, this is correct and expected behavior. But why do you want to make it
work?

On Thu, Mar 23, 2017 at 7:14 AM, Joerg Sonnenberger via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Thu, Mar 23, 2017 at 01:23:24PM +0100, Martin Richtarsky via llvm-dev
> wrote:
> > 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?
>
> This is not a bug, lld is completely correct. You are explicitly asking
> for a read-only section and the choosen relocation is position
> dependent, so yes, it correctly bails out.
>
> Joerg
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Martin Richtarsky via llvm-dev <llvm-dev at lists.llvm.org> writes:
> Hi,
>
> the attached example works with bfd-ld and with gold, but not with lld:
>
> $ cat rodatareloc.s
> ...
> .align 16
>     leaq    .JTab(%rip), %r10
>     jmp     *(%r10, %rdx, 8)
> ...
> .section .rodata
> .JTab:
>     .quad .L00, .L01, .L02, .L03, .L04, .L05, .L06, .L07
>     .quad .L08, .L09, .L10, .L11, .L12, .L13, .L14, .L15, .L16
>
> $ 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.

Cheers,
Rafael

Rafael Avila de Espindola wrote:
>> $ 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?

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.

Thanks and Best regards,
Martin