Re: [PATCH v6] MIPS: Reject branch absolute relocs for PIC for linking

["Maciej W. Rozycki" <macro@orcam.me.uk>]

On Thu, 22 Feb 2024, YunQiang Su wrote:

> > > +               howto = MIPS_ELF_RTYPE_TO_HOWTO (abfd, r_type, NEWABI_P (abfd));
> > > +               info->callbacks->einfo
> > > +                 /* xgettext:c-format */
> > > +                 (_("%X%H: relocation %s against `*ABS*' cannot be used"
> >
> > There's no "*ABS*" in the source and IMHO that'd look confusing
> > to innocent users.  How about "...against an absolute value"?
> > Or "...against an absolute value or absolute symbol"?  Perhaps
> > the latter is a bit too wordy, but also more complete.
> >
> 
> Good idea. I think that we should use "...against an absolute value".
> "absolute symbol"  is not included in this code, as it's r_symndx is
> not STN_UNDEF.

 Well code says otherwise:

+	      if (branch_reloc_p (r_type) && r_symndx == STN_UNDEF)

did you mean "the relocation's r_symndx is STN_UNDEF"?

 But it can be a relocation against an absolute symbol, for example if the 
symbol referred to by a relocation is supplied by another module in the 
link or a linker script and said symbol is absolute.  This case has to be 
covered in testing as well, as I infer from your code it won't be handled 
correctly.

 In fact this situation is not unique to branch relocations, because no 
PC-relative relocation against an absolute symbol or value can be resolved 
at link time for PIC/PIE links.  So you need to make your code generic for 
any relocations that have the `pc_relative' flag set in their howto unless 
we have a way to make a dynamic relocation out of such a relocation (which 
is none right now, as we still only have R_MIPS_REL32 as the sole dynamic 
relocation, except for TLS stuff).

 Extra test cases will be appreciated if you can come up with ones.

  Maciej