Oddity in the linker code on AIX

Oddity in the linker code on AIX

[Eric Botcazou]

Hi,

We're toying a bit with Binutils on AIX and I stumbled on the following 
construct in coff_compute_section_file_positions:

#ifdef ALIGN_SECTIONS_IN_FILE
      if ((abfd->flags & EXEC_P) != 0)
	{
	  /* Make sure this section is aligned on the right boundary - by
	     padding the previous section up if necessary.  */

	  old_sofar = sofar;
#ifdef RS6000COFF_C
	  /* AIX loader checks the text section alignment of (vma - filepos)
	     So even though the filepos may be aligned wrt the o_algntext, for
	     AIX executables, this check fails. This shows up when a native
	     AIX executable is stripped with gnu strip because the default vma
	     of native is 0x10000150 but default for gnu is 0x10000140.  Gnu
	     stripped gnu excutable passes this check because the filepos is
	     0x0140.  This problem also show up with 64 bit shared objects. The
	     data section must also be aligned.  */
	  if (!strcmp (current->name, _TEXT)
	      || !strcmp (current->name, _DATA))
	    {
	      bfd_vma pad;
	      bfd_vma align;

	      sofar = BFD_ALIGN (sofar, 1 << current->alignment_power);

	      align = 1 << current->alignment_power;
	      pad = abs (current->vma - sofar) % align;

	      if (pad)
		{
		  pad = align - pad;
		  sofar += pad;
		}
	    }
	  else
#else
	    {
	      sofar = BFD_ALIGN (sofar, 1 << current->alignment_power);
	    }
#endif
	  if (previous != (asection *) NULL)
	    previous->size += sofar - old_sofar;
	}

#endif


Note the mix between preprocessor conditional and C conditional constructs.  
As a result, the code eventually compiled on AIX is:

      if ((abfd->flags & EXEC_P) != 0)
	{
	  /* Make sure this section is aligned on the right boundary - by
	     padding the previous section up if necessary.  */

	  old_sofar = sofar;

	  /* AIX loader checks the text section alignment of (vma - filepos)
	     So even though the filepos may be aligned wrt the o_algntext, for
	     AIX executables, this check fails. This shows up when a native
	     AIX executable is stripped with gnu strip because the default vma
	     of native is 0x10000150 but default for gnu is 0x10000140.  Gnu
	     stripped gnu excutable passes this check because the filepos is
	     0x0140.  This problem also show up with 64 bit shared objects. The
	     data section must also be aligned.  */
	  if (!strcmp (current->name, _TEXT)
	      || !strcmp (current->name, _DATA))
	    {
	      bfd_vma pad;
	      bfd_vma align;

	      sofar = BFD_ALIGN (sofar, 1 << current->alignment_power);

	      align = 1 << current->alignment_power;
	      pad = abs (current->vma - sofar) % align;

	      if (pad)
		{
		  pad = align - pad;
		  sofar += pad;
		}
	    }
	  else

	  if (previous != (asection *) NULL)
	    previous->size += sofar - old_sofar;
	}

which looks a little suspicious to me.


Am I right in thinking that the author got a bit confused here?

Thanks in advance.

-- 
Eric Botcazou

Re: Oddity in the linker code on AIX

[Ian Lance Taylor]

Eric Botcazou <ebotcazou@adacore.com> writes:

> #ifdef ALIGN_SECTIONS_IN_FILE
>       if ((abfd->flags & EXEC_P) != 0)
> 	{
> 	  /* Make sure this section is aligned on the right boundary - by
> 	     padding the previous section up if necessary.  */
> 
> 	  old_sofar = sofar;
> #ifdef RS6000COFF_C
> 	  /* AIX loader checks the text section alignment of (vma - filepos)
> 	     So even though the filepos may be aligned wrt the o_algntext, for
> 	     AIX executables, this check fails. This shows up when a native
> 	     AIX executable is stripped with gnu strip because the default vma
> 	     of native is 0x10000150 but default for gnu is 0x10000140.  Gnu
> 	     stripped gnu excutable passes this check because the filepos is
> 	     0x0140.  This problem also show up with 64 bit shared objects. The
> 	     data section must also be aligned.  */
> 	  if (!strcmp (current->name, _TEXT)
> 	      || !strcmp (current->name, _DATA))
> 	    {
> 	      bfd_vma pad;
> 	      bfd_vma align;
> 
> 	      sofar = BFD_ALIGN (sofar, 1 << current->alignment_power);
> 
> 	      align = 1 << current->alignment_power;
> 	      pad = abs (current->vma - sofar) % align;
> 
> 	      if (pad)
> 		{
> 		  pad = align - pad;
> 		  sofar += pad;
> 		}
> 	    }
> 	  else
> #else
> 	    {
> 	      sofar = BFD_ALIGN (sofar, 1 << current->alignment_power);
> 	    }
> #endif

It's pretty clear from looking at the original patch introducing this
code:

2001-12-20  Tom Rix  <trix@redhat.com>

	* coffcode.h (coff_compute_section_file_positions): Add special AIX
	loader alignment of text section.

that the #else above should be a #endif, and the #endif should be
removed.

Ian

Re: Oddity in the linker code on AIX

[Eric Botcazou]

> It's pretty clear from looking at the original patch introducing this
> code:
>
> 2001-12-20  Tom Rix  <trix@redhat.com>
>
> 	* coffcode.h (coff_compute_section_file_positions): Add special AIX
> 	loader alignment of text section.
>
> that the #else above should be a #endif, and the #endif should be
> removed.

OK, thank you for confirming.  I'm not sure mainline builds on AIX for the 
time being so I may need a little time to post a patch.

-- 
Eric Botcazou