[PATCH v2] elf: Assume disjointed .rela.dyn and .rela.plt for loader

[H.J. Lu]

On Mon, Nov 1, 2021 at 1:23 PM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> The patch removes the the ELF_DURING_STARTUP optimization and assume
> both .rel.dyn and .rel.plt might not be subsequent.  This allows some
> code simplification since relocation will be handled independently
> where it is done on bootstrap.
>
> At least on x86_64_64, I can not measure any performance implications.
> 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 is:
>
>                   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
> --
> 2.32.0
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.