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

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

Your .rodata contains a vector of addresses, and we don't know the actual
addresses until load-time, because your .so file can be loaded to anywhere
in your address space. Thus, we cannot fix .rodata contents at link-time.
So your .so needs load-time modifications to the .rodata section. (Does
this answer your question?)

I think our default choice of forbidding relocations against read-only
sections is reasonable, because most programs don't need text relocations,
and if that happens, it is with a high probability a mistake instead of an
intentional behavior.

On Thu, Mar 23, 2017 at 8:47 AM, Martin Richtarsky via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170323/517dc893/attachment.html>

> 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.

On Mar 23, 2017 10:05 AM, "Rui Ueyama via llvm-dev" <llvm-dev at
lists.llvm.org>
wrote:

Your .rodata contains a vector of addresses, and we don't know the actual
addresses until load-time, because your .so file can be loaded to anywhere
in your address space. Thus, we cannot fix .rodata contents at link-time.
So your .so needs load-time modifications to the .rodata section. (Does
this answer your question?)

I think our default choice of forbidding relocations against read-only
sections is reasonable, because most programs don't need text relocations,


In this case it is rodata though which is a bit more reasonable. If this is
really common then we can probably limit our stricter default only to
executable sections.

and if that happens, it is with a high probability a mistake instead of an
intentional behavior.


Our diagnostic should probably mention to use -z notext though in the case
a user needs it. For example, a user might be grateful that we found their
bug, but they want to file it in their tracker and get back to it later
because their focus at the moment is just to try out LLD on their project
(we want them to be able to do that without yak shaving).

It might also be good to distill this discussion into a doc page and have
our diagnostic emit a note with a link to it, for users that are less
familiar and would benefit from a longer explanation that is too long for a
diagnostic. (The rust compiler does something like this for its diagnostics
and I found it pretty neat)

-- Sean Silva



On Thu, Mar 23, 2017 at 8:47 AM, Martin Richtarsky via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170324/deb7cc7e/attachment.html>