[PATCH 3/3] elf: Assume disjointed .rela.dyn and .rela.plt for loader

[H.J. Lu]

On Mon, Nov 1, 2021 at 9:44 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 01/11/2021 09:50, Adhemerval Zanella wrote:
> > The linker might add another section between the two relocation section
> > for the loader as well.  For instance on arm, lld does:
> >
> >   [ 7] .rel.dyn          REL             000007f0 0007f0 000088 08   A        1   0  4
> >   [ 8] .ARM.exidx        ARM_EXIDX       00000878 000878 0000b0 00  AL1       2   0  4
> >   [ 9] .rel.plt          REL             00000928 000928 000028 08  AI        1  17  4
> >
> > This patch removes the ELF_DURING_STARTUP optimization and assume
> > both section might no be linear.
> >
> > Checked on x86_64, i686, aarch64, armhf, powerpc64le, powerpc64,
> > and powerpc (to check if this breaks on different architectures).
>
> Following up the discussion on glibc call, below it an update of the
> patch description.
>
> ---
>
> The patch removes the the ELF_DURING_STARTUP optimization and assume
> both .rel.dyn and .rel.plt might no be linear.  This allows some
> code simplification since relocation will be handled independently
> where it done on bootstrap.
>
> I could not see any performance implications, at least on x86_64.
> Running 10000 time the command
>
>   LD_DEBUG=statistics ./elf/ld.so ./libc.so
>
> And filtering the "total startup time in dynamic loader" result,
> the geometric mean I see are:
>
>                   patched       master
>   Ryzen 7 5900x     24140        24952
>   i7-4510U          45957        45982
>
> (the results do show some variation, I did not make any statistical
> analysis).
>
> It also allows build arm with lld, since it inserts ".ARM.exidx"
> between ".rel.dyn" and ".rel.plt" for the loader.
>
> Checked on x86_64-linux-gnu and arm-linux-gnueabihf.
>
> > ---
> >  elf/dynamic-link.h | 32 +++++++++-----------------------
> >  1 file changed, 9 insertions(+), 23 deletions(-)
> >
> > diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
> > index ac4cc70dea..f619615e5c 100644
> > --- a/elf/dynamic-link.h
> > +++ b/elf/dynamic-link.h
> > @@ -65,12 +65,6 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
> >
> >  #ifdef RESOLVE_MAP
> >
> > -# if defined RTLD_BOOTSTRAP || defined STATIC_PIE_BOOTSTRAP
> > -#  define ELF_DURING_STARTUP (1)
> > -# else
> > -#  define ELF_DURING_STARTUP (0)
> > -# endif
> > -
> >  /* Get the definitions of `elf_dynamic_do_rel' and `elf_dynamic_do_rela'.
> >     These functions are almost identical, so we use cpp magic to avoid
> >     duplicating their code.  It cannot be done in a more general function
> > @@ -106,9 +100,8 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
> >                                                                             \
> >       if (ranges[0].start + ranges[0].size == (start + size))               \
> >         ranges[0].size -= size;                                             \
> > -     if (ELF_DURING_STARTUP                                                \
> > -         || (!(do_lazy)                                                    \
> > -             && (ranges[0].start + ranges[0].size) == start))              \
> > +     if (!(do_lazy)                                                        \
> > +         && (ranges[0].start + ranges[0].size) == start)                   \
> >         {                                                                   \
> >           /* Combine processing the sections.  */                           \
> >           ranges[0].size += size;                                           \
> > @@ -121,20 +114,13 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
> >         }                                                                   \
> >        }                                                                            \
> >                                                                             \
> > -    if (ELF_DURING_STARTUP)                                                \
> > -      elf_dynamic_do_##reloc ((map), scope, ranges[0].start, ranges[0].size,  \
> > -                           ranges[0].nrelative, 0, skip_ifunc);  \
> > -    else                                                                   \
> > -      {                                                                            \
> > -     int ranges_index;                                                     \
> > -     for (ranges_index = 0; ranges_index < 2; ++ranges_index)              \
> > -       elf_dynamic_do_##reloc ((map), scope,                               \
> > -                               ranges[ranges_index].start,                 \
> > -                               ranges[ranges_index].size,                  \
> > -                               ranges[ranges_index].nrelative,             \
> > -                               ranges[ranges_index].lazy,                  \
> > -                               skip_ifunc);                \
> > -      }                                                                            \
> > +      for (int ranges_index = 0; ranges_index < 2; ++ranges_index)         \
> > +        elf_dynamic_do_##reloc ((map), scope,                                      \
> > +                             ranges[ranges_index].start,                   \
> > +                             ranges[ranges_index].size,                    \
> > +                             ranges[ranges_index].nrelative,               \
> > +                             ranges[ranges_index].lazy,                    \
> > +                             skip_ifunc);                                  \
> >    } while (0)
> >
> >  # if ELF_MACHINE_NO_REL || ELF_MACHINE_NO_RELA
> >

Please submit the v2 patch with the updated commit log.

Thanks.

-- 
H.J.