Re: RFC: Objcopy: Section alignment

[Jan Beulich <jbeulich@suse.com>]

On 21.03.2024 15:55, Nick Clifton wrote:
>   It was recently pointed out to me that objcopy's --section-alignment
>   option does not actually set the alignment of any sections.  It just
>   sets a field in the PE file format's optional header.  This is rather
>   confusing for the user.

Hmm. For one --file-alignment would then want similar treatment, but
afaict that might be more difficult to achieve (sections would need
moving around in the file). Plus I seem to vaguely recall that there
wants to be a certain correlation between section and file alignments
of individual sections, which you may break with such an adjustment.

>   Plus it turns out that even if a section's alignment is set via the
>   --set=section-alignment option, its LMA and VMA will not be changed to
>   match the new alignment.

Perhaps do just this change in a first step? Albeit ...

> @@ -4267,6 +4289,30 @@ setup_section (bfd *ibfd, sec_ptr isection, void *obfdarg)
>    if (!bfd_set_section_alignment (osection, alignment))
>      err = _("failed to set alignment");
>  
> +  /* If the VMA is not aligned and it was not set by the user
> +     then adjust it.  */
> +  if (osection->vma & ((1 << alignment) - 1)
> +      && change_section_address == 0
> +      && find_section_list (bfd_section_name (isection), false,
> +			    SECTION_CONTEXT_ALTER_VMA
> +			    | SECTION_CONTEXT_SET_VMA) == NULL)
> +    {
> +      osection->vma += (1 << alignment);
> +      osection->vma &= ~ ((1 << alignment) -1);
> +    }
> +
> +  /* If the LMA is not aligned and it was not set by the user
> +     then adjust it.  */
> +  if (osection->lma & ((1 << alignment) - 1)
> +      && change_section_address == 0
> +      && find_section_list (bfd_section_name (isection), false,
> +			    SECTION_CONTEXT_ALTER_LMA
> +			    | SECTION_CONTEXT_SET_LMA) == NULL)
> +    {
> +      osection->lma += (1 << alignment);
> +      osection->lma &= ~ ((1 << alignment) -1);
> +    }

... by doing this adjustment, aren't you at risk of creating overlapping
sections? Should --set-section-alignment= perhaps be limited to
relocatable files?

Jan