-Ur option documentation

-Ur option documentation

[Florian Weimer]

The manual says this:

'-Ur'
     For anything other than C++ programs, this option is equivalent to
     '-r': it generates relocatable output--i.e., an output file that
     can in turn serve as input to 'ld'.  When linking C++ programs,
     '-Ur' _does_ resolve references to constructors, unlike '-r'.  It
     does not work to use '-Ur' on files that were themselves linked
     with '-Ur'; once the constructor table has been built, it cannot be
     added to.  Use '-Ur' only for the last partial link, and '-r' for
     the others.

For ELF, this refers to the legacy init function, right?  So it's not
really about C++ in that case.  And the restriction is gone if the
target uses ELF SHT_INIT_ARRAY sections.

Thanks,
Florian

Re: -Ur option documentation

[Andreas Schwab]

On Mai 05 2023, Florian Weimer via Binutils wrote:

> The manual says this:
>
> '-Ur'
>      For anything other than C++ programs, this option is equivalent to
>      '-r': it generates relocatable output--i.e., an output file that
>      can in turn serve as input to 'ld'.  When linking C++ programs,
>      '-Ur' _does_ resolve references to constructors, unlike '-r'.  It
>      does not work to use '-Ur' on files that were themselves linked
>      with '-Ur'; once the constructor table has been built, it cannot be
>      added to.  Use '-Ur' only for the last partial link, and '-r' for
>      the others.
>
> For ELF, this refers to the legacy init function, right?

No, it refers to the a.out N_SETx symbols.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Re: -Ur option documentation

[Florian Weimer]

* Andreas Schwab:

> On Mai 05 2023, Florian Weimer via Binutils wrote:
>
>> The manual says this:
>>
>> '-Ur'
>>      For anything other than C++ programs, this option is equivalent to
>>      '-r': it generates relocatable output--i.e., an output file that
>>      can in turn serve as input to 'ld'.  When linking C++ programs,
>>      '-Ur' _does_ resolve references to constructors, unlike '-r'.  It
>>      does not work to use '-Ur' on files that were themselves linked
>>      with '-Ur'; once the constructor table has been built, it cannot be
>>      added to.  Use '-Ur' only for the last partial link, and '-r' for
>>      the others.
>>
>> For ELF, this refers to the legacy init function, right?
>
> No, it refers to the a.out N_SETx symbols.

So this doesn't matter to ELF at all?  Would it be possible to update
the documentation accordingly?

Thanks,
Florian

Re: -Ur option documentation

[Simon Richter]

Hi,

On 5/8/23 19:47, Florian Weimer via Binutils wrote:

>>> '-Ur'

>>> For ELF, this refers to the legacy init function, right?
>> No, it refers to the a.out N_SETx symbols.

> So this doesn't matter to ELF at all?  Would it be possible to update
> the documentation accordingly?

I remember that we used -Ur excessively when linking programs and 
libraries for uClinux.

With -Ur, a different linker script is used that looks a lot like the 
script used for a final link, i.e. it builds init and fini arrays and 
ctor and dtor blocks, creates the usual generated symbols (_stext, 
_etext, _sdata, _edata, _sbss, _ebss, ...) and merges everything into a 
single section.

The linker then resolves internal relocations, and normally only 
relocations against the start address of the output section remain, 
which are output into the resulting relocatable object, which can be 
easily converted to a binary plus a table of offsets where the absolute 
address must be added.

Nasty hack, but it works.

    Simon

Re: -Ur option documentation

[Fangrui Song]

On Mon, May 8, 2023 at 3:47 AM Florian Weimer via Binutils
<binutils@sourceware.org> wrote:
>
> * Andreas Schwab:
>
> > On Mai 05 2023, Florian Weimer via Binutils wrote:
> >
> >> The manual says this:
> >>
> >> '-Ur'
> >>      For anything other than C++ programs, this option is equivalent to
> >>      '-r': it generates relocatable output--i.e., an output file that
> >>      can in turn serve as input to 'ld'.  When linking C++ programs,
> >>      '-Ur' _does_ resolve references to constructors, unlike '-r'.  It
> >>      does not work to use '-Ur' on files that were themselves linked
> >>      with '-Ur'; once the constructor table has been built, it cannot be
> >>      added to.  Use '-Ur' only for the last partial link, and '-r' for
> >>      the others.
> >>
> >> For ELF, this refers to the legacy init function, right?
> >
> > No, it refers to the a.out N_SETx symbols.
>
> So this doesn't matter to ELF at all?  Would it be possible to update
> the documentation accordingly?
>
> Thanks,
> Florian
>

Anecdotally, -Ur may be so obscure that nearly no project uses. ld.lld
doesn't implement this option and I haven't heard of any distribution
using ld.lld having broken packages due to -Ur...

Re: -Ur option documentation

[Florian Weimer]

* Fangrui Song:

> Anecdotally, -Ur may be so obscure that nearly no project uses. ld.lld
> doesn't implement this option and I haven't heard of any distribution
> using ld.lld having broken packages due to -Ur...

Certainly a possibility.  (I only saw it by accident in the manual, I
don't have a use case for it.)

Thanks,
Florian

Re: -Ur option documentation

[Nick Clifton]

Hi Guys,

OK, how about this rewording of the option's description:

   For programs that do not use constructors or destructors,
   or ELF based systems that use the newer SHT_INIT_ARRY
   section type this option is equivalent to -r: it generates
   relocatable output -- i.e., an output file that can in turn
   serve as input to ld.  For qualifying binaries however the
   -Ur option is different in that it *does* resolve
   references to constructors, unlike -r.

   It does not work to use -Ur on files that were themselves
   linked with -Ur; once the constructor table has been built,
   it cannot be added to.  Use -Ur only for the last partial
   link, and -r for the others.

Cheers
   Nick

Re: -Ur option documentation

[Florian Weimer]

* Nick Clifton:

> Hi Guys,
>
> OK, how about this rewording of the option's description:
>
>   For programs that do not use constructors or destructors,
>   or ELF based systems that use the newer SHT_INIT_ARRY
>   section type this option is equivalent to -r: it generates
>   relocatable output -- i.e., an output file that can in turn
>   serve as input to ld.  For qualifying binaries however the
>   -Ur option is different in that it *does* resolve
>   references to constructors, unlike -r.
>
>   It does not work to use -Ur on files that were themselves
>   linked with -Ur; once the constructor table has been built,
>   it cannot be added to.  Use -Ur only for the last partial
>   link, and -r for the others.

Andreas said that DT_INIT should still work with -Ur, so maybe you don't
need the SHT_INIT_ARRY clarification?

Thanks,
Florian

Re: -Ur option documentation

[Nick Clifton]

Hi Florian,

> Andreas said that DT_INIT should still work with -Ur, so maybe you don't
> need the SHT_INIT_ARRY clarification?

So:

     For programs that do not use constructors or destructors,
     or for ELF based systems this option is equivalent to -r:
     it generates relocatable output -- i.e., an output file
     that can in turn serve as input to ld.  For other binaries
     however the -Ur option is similar to -r but it also resolves
     references to constructors and destructors.

     It does not work to use -Ur on files that were themselves
     linked with -Ur; once the constructor table has been built,
     it cannot be added to.  Use -Ur only for the last partial
     link, and -r for the others.

Cheers
   Nick

Re: -Ur option documentation

[Florian Weimer]

* Nick Clifton:

> Hi Florian,
>
>> Andreas said that DT_INIT should still work with -Ur, so maybe you don't
>> need the SHT_INIT_ARRY clarification?
>
> So:
>
>     For programs that do not use constructors or destructors,
>     or for ELF based systems this option is equivalent to -r:
>     it generates relocatable output -- i.e., an output file
>     that can in turn serve as input to ld.  For other binaries
>     however the -Ur option is similar to -r but it also resolves
>     references to constructors and destructors.
>
>     It does not work to use -Ur on files that were themselves
>     linked with -Ur; once the constructor table has been built,
>     it cannot be added to.  Use -Ur only for the last partial
>     link, and -r for the others.

Maybe “For those systems where -r and -Ur behave differently, it does
not work to use -Ur …”?

Thanks,
Florian

Re: -Ur option documentation

[Nick Clifton]

Hi Florian,

>>      It does not work to use -Ur on files that were themselves
>>      linked with -Ur; once the constructor table has been built,
>>      it cannot be added to.  Use -Ur only for the last partial
>>      link, and -r for the others.
> 
> Maybe “For those systems where -r and -Ur behave differently, it does
> not work to use -Ur …”?

That is reasonable.  I have pushed a patch to reword that paragraph
as you have suggested.

Cheers
   Nick