Re: [PATCH v5 6/7] csu: Move static pie self relocation later [BZ #27072]

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

The 01/21/2021 11:07, Adhemerval Zanella wrote:
> 
> 
> On 20/01/2021 12:31, Szabolcs Nagy via Libc-alpha wrote:
> > IFUNC resolvers may depend on tunables and cpu feature setup so
> > move static pie self relocation after those.
> > 
> > It is hard to guarantee that the ealy startup code does not rely
> > on relocations so this is a bit fragile. It would be more robust
> > to handle RELATIVE relocs early and only IRELATIVE relocs later,
> > but the current relocation processing code cannot do that.
> 
> As a side note, how hard would that be? I really think we should aim
> to make the bootstrap less fragile and it would also make porting
> static-pie to other architecture easier.

the IRELATIVE relocs are in the DT_JMPREL area, while RELATIVE
relocs are outside of it. (this split is to allow simpler lazy
binding, i don't know how strictly this is enforced in elf, but
since ifunc is a gnu extension we can just say that this is abi.)

so i think you only have to change/copy elf/dynamic-link.h and
have a ELF_DYNAMIC_RELOCATE variant that processes the two sets
of relocations separately.

if you don't want to rely on DT_JMPREL then i think you have to
change elf/do-rel.h as well to have a elf_dynamic_do_Rel variant
that does IRELATIVE separately.

either case changing these looked more scary to me than the
current patches: likely we have to copy complex logic with slight
modification and there are a lot of things going on there. (but
that was before i learnt about the i386 hidden issue and hidden
weak refs and tunables_strdup.)

> > 
> > The early startup code up to relocation processing includes
> > 
> >   _dl_aux_init (auxvec);
> >   __libc_init_secure ();
> >   __tunables_init (__environ);
> >   ARCH_INIT_CPU_FEATURES ();
> >   _dl_relocate_static_pie ();
> > 
> > These are simple enough that RELATIVE relocs can be avoided.
> > 
> > The following steps include
> > 
> >   ARCH_SETUP_IREL ();
> >   ARCH_SETUP_TLS ();
> >   ARCH_APPLY_IREL ();
> > 
> > On some targets IRELATIVE processing relies on TLS setup on
> > others TLS setup relies on IRELATIVE relocs, so the right
> > position for _dl_relocate_static_pie is target dependent.
> > For now move self relocation as early as possible on targets
> > that support static PIE.
> > 
> > Fixes bug 27072.
> 
> LGTM, thanks.
> 
> Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

thanks.