Re: [PATCH] i386: Avoid avoid rely on linker optimization to avoid relocation

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

On Thu, Nov 17, 2022 at 4:43 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> lld does not implement all the linker optimization to avoid the GOT
> relocation as done by binutils (bfd/elf32-i386.c:elf_i386_convert_load_reloc).
> The current 'movl main@GOT(%ebx), %eax' will then create a GOT
> relocation when building with lld, which make static-pie status to
> not being able to start the provided main function.
>
> The change uses a __wrap_main local symbol, which in turn calls main
> (similar as used by aarch64 and s390x).
>
> Checked on i686-linux-gnu with binutils and lld.
> ---
>  sysdeps/i386/start.S | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/sysdeps/i386/start.S b/sysdeps/i386/start.S
> index 4ec04bdfd7..d593c4de00 100644
> --- a/sysdeps/i386/start.S
> +++ b/sysdeps/i386/start.S
> @@ -98,11 +98,10 @@ ENTRY (_start)
>         pushl main@GOT(%ebx)
>  # else
>         /* Avoid relocation in static PIE since _start is called before
> -          it is relocated.  Don't use "leal main@GOTOFF(%ebx), %eax"
> -          since main may be in a shared object.  Linker will convert
> -          "movl main@GOT(%ebx), %eax" to "leal main@GOTOFF(%ebx), %eax"
> +          it is relocated.  This also avoid rely on linker optimization to
> +          transform 'movl main@GOT(%ebx), %eax' to 'leal main@GOTOFF(%ebx)'
>            if main is defined locally.  */
> -       movl main@GOT(%ebx), %eax
> +       leal __wrap_main@GOTOFF(%ebx), %eax
>         pushl %eax
>  # endif
>
> @@ -130,6 +129,12 @@ ENTRY (_start)
>  1:     movl    (%esp), %ebx
>         ret
>  #endif
> +
> +#if defined PIC && !defined SHARED
> +__wrap_main:
> +       _CET_ENDBR
> +       jmp     main

Shouldn't it be "jmp main@PLT"?

> +#endif
>  END (_start)
>
>  /* To fulfill the System V/i386 ABI we need this symbol.  Yuck, it's so
> --
> 2.34.1
>


-- 
H.J.