[PATCH v4 08/10] csu: Move static pie self relocation later [BZ #27072]

[Szabolcs Nagy]

The 01/19/2021 06:48, H.J. Lu wrote:
> On Tue, Jan 19, 2021 at 6:37 AM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> >
> >
> > On 19/01/2021 11:35, Szabolcs Nagy wrote:
> > > The 01/19/2021 11:07, Adhemerval Zanella wrote:
> > >> On 18/01/2021 13:25, 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.
> > >>>
> > >>> The early startup code before relocation processing includes
> > >>>
> > >>>   _dl_aux_init (auxvec);
> > >>>   __libc_init_secure ();
> > >>>   __tunables_init (__environ);
> > >>>   ARCH_INIT_CPU_FEATURES ();
> > >>>
> > >>> These are simple enough that RELATIVE relocs can be avoided.
> > >>>
> > >>> __ehdr_start may require RELATIVE relocation so it was moved
> > >>> later, fortunately ehdr and phdr are not used in the early code.
> > >>>
> > >>> Fixes bug 27072.
> > >>
> > >> LGTM, thanks.
> > >>
> > >> Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
> > >
> > >
> > > sigh, this is an old version of this patch, i made a
> > > mistake putting the series together.
> > >
> > > the problem is that _dl_phdr is used in ARCH_SETUP_TLS
> > > (to get the tls program headers) so the __ehdr_start
> > > magic should be before that (this only matters if auxv
> > > lacks AT_PHDR for some reason, which should not happen
> > > normally on linux, so testing won't show the problem)
> >
> > By normally do you mean it might happen on a specific kernel version
> > or is it architecture specific?

i guess __ehdr_start symbol can be useful and with it
glibc does not have to depend on auxv (which an elf
loader like valgrind/qemu-user may get wrong)

however it is only used as a fallback and on linux
AT_PHDR is always expected to be present. (i don't
know if this ever triggers)

> 
> I think we can leave __ehdr_start ASIS since it doesn't need RELATIVE
> relocation.  I verified it by adding -Wl,-z,report-relative-reloc when building
> elf/sln on x86.

it needs relative reloc on aarch64: it can be an undefined weak
symbol and that must be 0. a pc relative address computation
cannot give 0 (unless linker does some instruction rewriting,
but on aarch64 the address computation is multiple instructions
that can be spread far apart). so yeah it needs a GOT entry and
that will be either 0 or needs a RELATIVE reloc.