Re: [PATCH 3/3] csu: Move static pie self relocation later [BZ #27072]

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 01/07/2021 04:51, H.J. Lu wrote:
> On Thu, Jan 7, 2021 at 4:46 AM Carlos O'Donell <carlos@redhat.com> wrote:
> >
> > On 1/7/21 7:36 AM, H.J. Lu via Libc-alpha wrote:
> > > On Thu, Jan 7, 2021 at 3:04 AM Szabolcs Nagy via Libc-alpha
> > > <libc-alpha@sourceware.org> wrote:
> > >>
> > >> On targets where hidden symbol access does not need RELATIVE
> > >> relocs, move the static pie self relocation after tunables and
> > >> cpu features are set up.  This allows processing IRELATIVE
> > >> relocs with correct ifunc dispatch logic.
> > >>
> > >> Unfortunately it is hard to guarantee that there will be no
> > >> dynamic relocations in the early start up code, so this is a
> > >> bit fragile. Ideally the RELATIVE relocs would be processed as
> > >> early as possible and IRELATIVE relocs after cpu features are
> > >> setup, but in glibc it is hard to separate them into two steps.
> > >
> > > x86 linker places IRELATIVE relocations the last:
> > >
> > > https://sourceware.org/bugzilla/show_bug.cgi?id=13302
> > >
> > > Does your linker have this bug fixed?
> >
> > Agreed, this is something I asked about during the design of
> > IFUNCs and I was told by Ulrich and others that IRELATIVE has
> > to be placed in a group and after RELATIVE relocs.
> 
> Not just before RELATIVE.   IRELATIVE should be processed the
> last before all other relocations.   See PR 13302 for other cases.

this is about static pie: sorting relocs does not help, the
problem is not that ifunc resolvers have some relocs in them,
but that the target specific relocation processing is one
monolithic switch in a loop that handles all relocs in one go,
there is no api to request only a subset of relocs to be
processed (either by type or ordering in the list of relocs).

in __libc_start_main we need to do

  do_relative_relocs();
  setup_auxv_tunables_cpu_etc();
  do_irelative_relocs();

which cannot be done without major changes in all targets and
generic elf reloc handling code. so the second best is

  setup_auxv_tunables_cpu_etc(); // avoid relative relocs
  do_all_relocs();

on targets where this can be done (other targets do
not support static pie).