Building GRUB on platforms without ELF support

Building GRUB on platforms without ELF support

[Christian Franke]

Building GRUB modules requires ELF support in gas and ld. For platforms 
where ELF is not the native format, ld may support ELF output. If not 
(like on Cywin) some conversion to ELF is necessary.

In general, GNU objcopy allows conversion between object file formats. 
Unfortunately, objcopy (and BFD itself) does not include any support for 
the conversion of relocation formats (even conversion between ELF 
variants do not work). In particular, when converting PE (a COFF 
variant) to ELF, objcopy does not abort but silently produces bad 
PC-relative relocation offsets. In my first Cygwin patch, there is a 
hack to fix this in the GRUB ELF loader.

For specific conversions, fixing this in objcopy itself is easy. But 
there is not much chance that such pragmatic patches will be accepted 
upstream.
(http://sourceware.org/ml/binutils/2007-10/threads.html#00302)

I have prepared a reduced (~680 LoC) version of objcopy with the PE->ELF 
fix added. To support build on non-ELF platforms, I would suggest to add 
this to the GRUB codebase. It can be later extended for other platforms 
if desired.

Thanks for any comment.

Christian

Re: Building GRUB on platforms without ELF support

[Robert Millan]

On Fri, Nov 16, 2007 at 11:58:14PM +0100, Christian Franke wrote:
> Building GRUB modules requires ELF support in gas and ld. For platforms 
> where ELF is not the native format, ld may support ELF output. If not 
> (like on Cywin) some conversion to ELF is necessary.
> 
> In general, GNU objcopy allows conversion between object file formats. 
> Unfortunately, objcopy (and BFD itself) does not include any support for 
> the conversion of relocation formats (even conversion between ELF 
> variants do not work). In particular, when converting PE (a COFF 
> variant) to ELF, objcopy does not abort but silently produces bad 
> PC-relative relocation offsets. In my first Cygwin patch, there is a 
> hack to fix this in the GRUB ELF loader.
> 
> For specific conversions, fixing this in objcopy itself is easy. But 
> there is not much chance that such pragmatic patches will be accepted 
> upstream.
> (http://sourceware.org/ml/binutils/2007-10/threads.html#00302)
> 
> I have prepared a reduced (~680 LoC) version of objcopy with the PE->ELF 
> fix added. To support build on non-ELF platforms, I would suggest to add 
> this to the GRUB codebase. It can be later extended for other platforms 
> if desired.

I'm not sure what the GRUB maintainers will think, but I'm not very inclined
to duplicate stuff that binutils already have.

How about building binutils with --enable-targets=i386-elf ?  Maybe the
Cygwin maintainers would even add it as default.  We could also have a
configure check that aborts build when -m elf_i386 is not supported (which
may also be a problem on pure-x86_64 environment!) and prompt user to
rebuild binutils.

-- 
Robert Millan

<GPLv2> I know my rights; I want my phone call!
<DRM> What use is a phone call, if you are unable to speak?
(as seen on /.)

Re: Building GRUB on platforms without ELF support

[Marco Gerards]

Robert Millan <rmh@aybabtu.com> writes:

> On Fri, Nov 16, 2007 at 11:58:14PM +0100, Christian Franke wrote:
>> Building GRUB modules requires ELF support in gas and ld. For platforms 
>> where ELF is not the native format, ld may support ELF output. If not 
>> (like on Cywin) some conversion to ELF is necessary.
>> 
>> In general, GNU objcopy allows conversion between object file formats. 
>> Unfortunately, objcopy (and BFD itself) does not include any support for 
>> the conversion of relocation formats (even conversion between ELF 
>> variants do not work). In particular, when converting PE (a COFF 
>> variant) to ELF, objcopy does not abort but silently produces bad 
>> PC-relative relocation offsets. In my first Cygwin patch, there is a 
>> hack to fix this in the GRUB ELF loader.
>> 
>> For specific conversions, fixing this in objcopy itself is easy. But 
>> there is not much chance that such pragmatic patches will be accepted 
>> upstream.
>> (http://sourceware.org/ml/binutils/2007-10/threads.html#00302)
>> 
>> I have prepared a reduced (~680 LoC) version of objcopy with the PE->ELF 
>> fix added. To support build on non-ELF platforms, I would suggest to add 
>> this to the GRUB codebase. It can be later extended for other platforms 
>> if desired.
>
> I'm not sure what the GRUB maintainers will think, but I'm not very inclined
> to duplicate stuff that binutils already have.

So am I.

> How about building binutils with --enable-targets=i386-elf ?  Maybe the
> Cygwin maintainers would even add it as default.  We could also have a
> configure check that aborts build when -m elf_i386 is not supported (which
> may also be a problem on pure-x86_64 environment!) and prompt user to
> rebuild binutils.

That would be a nice check to have.

--
Marco

Re: Building GRUB on platforms without ELF support

[Christian Franke]

Marco Gerards wrote:
> Robert Millan <rmh@aybabtu.com> writes:
>
>   
>> On Fri, Nov 16, 2007 at 11:58:14PM +0100, Christian Franke wrote:
>>     
>>> ...
>>> For specific conversions, fixing this in objcopy itself is easy. But 
>>> there is not much chance that such pragmatic patches will be accepted 
>>> upstream.
>>> (http://sourceware.org/ml/binutils/2007-10/threads.html#00302)
>>>
>>> I have prepared a reduced (~680 LoC) version of objcopy with the PE->ELF 
>>> fix added. To support build on non-ELF platforms, I would suggest to add 
>>> this to the GRUB codebase. It can be later extended for other platforms 
>>> if desired.
>>>       
>> I'm not sure what the GRUB maintainers will think, but I'm not very inclined
>> to duplicate stuff that binutils already have.
>>     
>
> So am I.
>
>   

Actually, binutils actually does not have this ("convert relocatable 
object files from format X to ELF") stuff.


>> How about building binutils with --enable-targets=i386-elf ?  Maybe the
>> Cygwin maintainers would even add it as default.  


It is not so easy:

Cygwin's libbfd, ld and objcopy support elf32-i386, gas does not.

But ld refuses to output elf32-i386 if pe-i386 is the input format. This 
restriction is hard-coded into ld because some pe-specific operations 
are hard-coded into bfd backend (This includes the hack which fixes the 
pc-relative relocation in the executable).

Enable elf32-i386 as additional output format for gas does not work. The 
compilation fails, multiple emulations are not supported on i386 platform.

But even building gas as a cross-assembler does not help. Some assembler 
directives, like symbol type specs, are output format specific. Output 
from Cygwin's gcc produces syntax errors.

Therefore, you would need also a gcc build as a cross-compiler.



Not really worth the effort, so I posted the patch for kern/i386/dl.c 
instead.


>> We could also have a
>> configure check that aborts build when -m elf_i386 is not supported (which
>> may also be a problem on pure-x86_64 environment!) and prompt user to
>> rebuild binutils.
>>     
>
>   

Better prompt "sorry", as rebuilding binutils probably won't work OOTB 
in many cases ... :-)

Christian

Re: Building GRUB on platforms without ELF support

[Robert Millan]

On Mon, Nov 19, 2007 at 09:29:19PM +0100, Christian Franke wrote:
> 
> It is not so easy:
> 
> Cygwin's libbfd, ld and objcopy support elf32-i386, gas does not.
> 
> But ld refuses to output elf32-i386 if pe-i386 is the input format. This 
> restriction is hard-coded into ld because some pe-specific operations 
> are hard-coded into bfd backend (This includes the hack which fixes the 
> pc-relative relocation in the executable).
> 
> Enable elf32-i386 as additional output format for gas does not work. The 
> compilation fails, multiple emulations are not supported on i386 platform.
> 
> But even building gas as a cross-assembler does not help. Some assembler 
> directives, like symbol type specs, are output format specific. Output 
> from Cygwin's gcc produces syntax errors.
> 
> Therefore, you would need also a gcc build as a cross-compiler.

Can't the whole toolchain include ELF support as an alternative target?  Kinda
like "-m32" on x86_64.  I don't think it's unreasonable to request that Cygwin
provides this (and it'd probably solve other problems somewhere else).

Have you tried asking the Cygwin maintainers what they think about that?

-- 
Robert Millan

<GPLv2> I know my rights; I want my phone call!
<DRM> What use is a phone call, if you are unable to speak?
(as seen on /.)

Re: Building GRUB on platforms without ELF support

[Christian Franke]

Robert Millan wrote:
> ...
> Can't the whole toolchain include ELF support as an alternative
> target?  Kinda like "-m32" on x86_64.  I don't think it's unreasonable
> to request that Cygwin provides this (and it'd probably solve other
> problems somewhere else).
> 
> Have you tried asking the Cygwin maintainers what they think about
> that? 

Besides GRUB2 build, ELF probably support provides no additional
benefit. There are still a lot of open issues to get gcc 4.x working on
Cygwin. Therefore it IMO does not make sense to request such additional
feature now.

I have submitted a patch to fix the PE->ELF relocation info conversion
in objcopy. It was rejected upstream.

Christian