[PATCH] elf: Properly set sh_offset for .tbss sections

[Alan Modra]

On Mon, Apr 28, 2025 at 08:08:56AM +0200, Jan Beulich wrote:
> On 28.04.2025 04:47, H.J. Lu wrote:
> > Set sh_offset for .tbss sections to their nominal offset after aligning.
> > They are not loaded from disk so the value doesn't really matter, except
> > when the .tbss section is the first one in a PT_TLS segment.  In that
> > case, it sets the p_offset for the PT_TLS segment, which according to
> > the ELF gABI ought to satisfy p_offset % p_align == p_vaddr % p_align.
> > 
> > bfd/
> > 
> > PR ld/32896
> > * elf.c (assign_file_positions_for_load_sections): Properly set
> > sh_offset for .tbss sections.
> 
> Why does the order of checks (PT_LOAD then SHT_NOBITS) need changing here?
> While pre-existing, I'm also having a hard time seeing why among SHT_NOBITS
> sections SHF_TLS ones would need special casing here. While I agree that
> the usual requirements have to be met by PT_TLS, the entire change feels
> like it's papering over an issue elsewhere (presumably in the laying out of
> load segments).

The patch is OK.  Please apply.

Jan, there is a lot of history to this code.  sh_offset for SHT_NOBITS
is somewhat arbitrary since those sections are not loaded from the
image.  p_offset for a program header with p_filesz zero is likewise
arbitrary, except that the gABI makes no exception regarding alignment
modulo p_align for such headers.

The problem is that various other tools break (or have been broken in
the past) when we make what seems like logical changes to our code,
such as the one you point out, that all SHT_NOBITS sections ought to
be treated the same here.  See pr12921 and pr25662.

-- 
Alan Modra