Porting the linker

Jean Christophe Beyler jean.christophe.beyler@gmail.com
Thu May 27 15:08:00 GMT 2010

Dear all, I've pretty much worked through most problems to achieve
what I want. However, I'm still stuck in redirecting the sections.

I now know when I should overflow a section but I can't make it work.

For example, I can determine that file f1.o should overflow its data
to data_overflow. However, I get
However, I get a linking error : "Bad value"

I think it's because that data_overflow segment already exists in f1.o
and isn't empty so the VMA/LMA get mixed up in this case.

However, I thought of adding a section in the linker script after
data_overflow to handle my "during linking" overflow.

So I would have :
Small memory:

Bigger memory:
data_overflow (given by the programmer)
data_overflow_lk (at link time)

I've been looking at doing this change as soon as possible, and I can
determine at the ldlang_add_file stage what I should be overflowing.
Therefore, I tried to do something with bfd_map_over_sections. But I
don't quite understand what I should change during that traversal.

Any ideas or code example that you could show me that would for
example simply do this (or another direction that I'm missing). My
example is in place, my should_overflow works and tells me if I should
overflow that input section to the _overflow_lk section but I can't
seem to do it!

Here's the code extract:

static void
should_overflow_it (bfd *abfd, asection *sec, void *data)
  lang_input_statement_type *entry = data;

  if (should_overflow (abfd, sec, entry))
      //Just link this section to output section name + _overflow_lk


ldlang_add_file (lang_input_statement_type *entry)
  bfd_map_over_sections (entry->the_bfd, mycheck, entry);
  bfd_map_over_sections (entry->the_bfd, section_already_linked, entry);

Thanks again,

On Mon, May 17, 2010 at 5:03 PM, Jean Christophe Beyler
<jean.christophe.beyler@gmail.com> wrote:
> I've continued my investigations and, doing what you said to do, I
> figured that I could :
> - Modify the output segments to point to the bigger segment versions
> of them with a function such as this one after the lang_place_orphans
> :
> void
> moveToBiggerSegment (char* segment_suffix)
> {
>    {
>        asection *s;
>        for (s = file->the_bfd->sections; s != NULL; s = s->next)
>        {
>            if (s->output_section)
>            {
>                asection *os = s->output_section;
>                if (os != NULL)
>                {
>                    /* Let's move these to bigger segment */
>                    static char *whatToMove[] = { ".bss", ".sbss",
> ".lbss", ".data"};
>                    char buf[256];
>                    unsigned int i;
>                    for (i=0; i < sizeof (whatToMove) / sizeof
> (whatToMove[0]); i++)
>                    {
>                        if (strcmp (os->name, whatToMove[i]) == 0)
>                        {
>                            sprintf (buf, "%s_%s", whatToMove[i],
> segment_suffix);
>                            lang_output_section_statement_type *new_sec = NULL;
>                            new_sec = lang_output_section_statement_lookup(buf);
>                            s->output_section = new_sec->bfd_section;
>                        }
>                    }
>                }
>            }
>        }
>    }
> }
> (a) However, for a reason I don't know, this doesn't work with archive
> files such as what is in newlib. I'll have to see why, I know it has
> to do with the moving of .data to .data_segment_suffix...
> Second, once I've done that, I've lost what variables have to go into
> the smaller zone so I don't know yet how to do that.
> The one idea I had was:
> For example, let's consider the .bss, instead of sending it to
> .bss_segment_suffix, I'll send it to : .bss_temp
> And define .bss_temp, that should allow me to then go to all the data
> and add them into .bss until I fill that in. Then I'll send the rest
> to the bigger zone.
> (b) However, I still don't know how to, once segments have been sized
> up, see every variable/symbol and which segment it goes into.
> Any ideas about (a) or (b) would be appreciated.
> Thanks again,
> Jean Christophe Beyler
> On Wed, Mar 10, 2010 at 9:11 AM, Nick Clifton <nickc@redhat.com> wrote:
>> Hi Jean Christophe,
>>>> Just a thought - it might be easier in the linker to fill the large text
>>>> and
>>>> data sections first and then, as late as possible, extract portions of
>>>> those
>>>> sections to go into the smaller text and data sections.
>>> That could be a solution, any hints as to how to do that ?
>> As a guess - how about putting the code in where the section garbage
>> collection occurs ?  (Search for "gc_section" in the bfd/ directory for lots
>> of examples).  Currently the garbage collection code is used to strip out
>> unneeded sections, but maybe it will also prove to be a suitable place to
>> move parts of sections around.
>> Cheers
>>  Nick

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