Making x86_64 PLT elision configurable

[Nick Clifton <nickc at redhat dot com>]

Hi H.J, Hi Jan,

  I would like to add a run time option, with a configurable default,
  which would stop the linker from optimizing away x86_64 PLT entries.
  The reason being that certain tools like ltrace and rtld-audit rely
  upon global symbols having PLT entries that they can intercept.  When
  these entries are optimized away, the tools stop working.

  I get that having these PLT entries might slow the program down, but I
  think that a user should be able to choose whether the speed of the
  linked binary is more important that being able to use the tools on
  it.  And toolchain builders ought to be able to decide the default
  behaviour for their linker.

  So I have two questions for you:

  * Do you have any objections to this idea ?

  * Are you happy for me to create a patch, or would you rather
    make one yourselves ?

Cheers
  Nick

PS. Just to be clear, I am starting with the x86_64, but I would expect
    that whatever option we create would be architecture neutral and
    apply to all targets that want to optimise away PLT entries.
    
PPS. Specifically I am talking about changing this piece of code in
    bfd/elfxx-x86_64.c:elf_x86_allocate_dynrelocs():
    
  if (htab->plt_got != NULL
      && h->type != STT_GNU_IFUNC
      && !h->pointer_equality_needed
      && h->plt.refcount > 0
      && h->got.refcount > 0)
    {
      /* Don't use the regular PLT if there are both GOT and GOTPLT
	 reloctions.  */
      h->plt.offset = (bfd_vma) -1;

      /* Use the GOT PLT.  */
      eh->plt_got.refcount = 1;
    }

  to something like:

  if (info->allow_plt_elision
      && htab->plt_got != NULL
      && h->type != STT_GNU_IFUNC
      && !h->pointer_equality_needed
      && h->plt.refcount > 0
      && h->got.refcount > 0)
    {
     ...