Re: how to add segment at address 0?

[Vladimir Kondratiev <vladimir dot kondratiev at intel dot com>]

Nick,
Thanks,
it is cool, but when I compile it and run, it segfaults. GDB do indeed 
see variable at address 0.
Looks like problem is with section .note.ABI-tag; this section present 
anyway and at the same address, but if I compile without ld script, 
there is also NOTE segment in PHDRS, .note.ABI-tag resides in this segment.
How should I define this section?
If I just add line
 note       PT_NOTE ;
to PHDRS, this segment do not get address assigned. For "good" file it 
looks like

Program Header:
   PHDR off    0x00000034 vaddr 0x08048034 paddr 0x08048034 align 2**2
        filesz 0x000000c0 memsz 0x000000c0 flags r-x
 INTERP off    0x000000f4 vaddr 0x080480f4 paddr 0x080480f4 align 2**0
        filesz 0x00000013 memsz 0x00000013 flags r--
   LOAD off    0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
        filesz 0x00000426 memsz 0x00000426 flags r-x
   LOAD off    0x00000428 vaddr 0x08049428 paddr 0x08049428 align 2**12
        filesz 0x0000010c memsz 0x00000110 flags rw-
DYNAMIC off    0x0000043c vaddr 0x0804943c paddr 0x0804943c align 2**2
        filesz 0x000000c8 memsz 0x000000c8 flags rw-
   NOTE off    0x00000108 vaddr 0x08048108 paddr 0x08048108 align 2**2
        filesz 0x00000020 memsz 0x00000020 flags r--

I use gcc 3.2.1 and binutils:
[]$ ld --version
GNU ld version 2.13.90.0.16 20021126

I attached my default ld script (whole output of ld --verbose) for 
reference, it is also a bit differ from yours; sure I did applied your 
modifications to my default ld script when tested it.

Vladimir.

Nick Clifton wrote:

> Hi Vladimir,
>
> > I need to allocate some data at address 0. Otherwise, it is ordinal
> > "C" program. All I need is one section starting at address 0.
> >
> > I tried to tweak LD script to add one more segment. Here I faced with
> > problem:
> >
> > I printed "default" LD script using "--verbose" flag and started to
> > modify it. But, since I need to add segment, I need to use PHDRS. LD
> > "default" script have no PHDRS directive. On myself, I did not
> > succeeded to write proper PHDRS and set up sections to use proper
> > segments.
> >
> > How should I do it? Let's say, I need to write "C" program where
> > section ".zero_based" will start at address 0.
>
> The linker manual can tell you all you need to know.
>
> However you may find the following example helpful.
>
>   % cat data.c
>   int data_at_zero = 1;
>
>   % cat text.c
>   int data_not_at_zero = 2;
>   extern int data_at_zero;
>
>   int main (void) { data_not_at_zero = data_at_zero; return 0; }
>
>   % gcc data.c text.c -Wl,-T -Wl,zero.ld
>   % objdump -h a.out | head
>
>   a.out:     file format elf32-i386
>  
>   Sections:
>   Idx Name          Size      VMA       LMA       File off  Algn
>     0 .data_at_zero 00000004  00000000  00000000  00001000  2**2
>                     CONTENTS, ALLOC, LOAD, DATA
>     1 .interp       00000013  080480f4  080480f4  000000f4  2**0
>                     CONTENTS, ALLOC, LOAD, READONLY, DATA
>     2 .note.ABI-tag 00000020  08048108  08048108  00000108  2**2
>
> I have attached zero.ld which was made from the default linker script
> and adding a section like this:
>
>   . = 0; .data_at_zero : { data.o(.data) } :data_zero
>
> and a set of PHDRS like this:
>
>   PHDRS
>   {
>     headers    PT_PHDR PHDRS ;
>     interp     PT_INTERP ;
>     text       PT_LOAD FILEHDR PHDRS ;
>     data       PT_LOAD ;
>     data_zero  PT_LOAD ;
>     dynamic    PT_DYNAMIC ;
>   }
>
> Cheers
>         Nick

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