[uclibc-ng-devel] [PATCH] ARC: ldso: don't use _DYNAMIC at gotpc construct to refer to GOT

Vineet Gupta Vineet.Gupta1 at synopsys.com
Mon Jan 9 21:56:43 CET 2017


On 01/09/2017 09:31 AM, Vineet Gupta wrote:
> Historically we've used a special construct _DYNAMIC at gotpc to refer to
> base address of GOT This has been done with a special hack in binutils
> which tools guys now wish to get rid off.
>
> So rewrite the 2 callers of this construct to use the simpler and more
> obvious constructs. There's existing _GLOBAL_OFFSET_TABLE symbols
> provided by linker which can be used for same purposes.
>
> Signed-off-by: Vineet Gupta <vgupta at synopsys.com>

Please ignore this one - it is broken with current binutils !
Assembler is also converting _DYNAMIC at pcl into _GLOBAL_OFFSET_TABLE

v2 to follow !

-Vineet

> ---
>  ldso/ldso/arc/dl-sysdep.h | 35 ++++++++++++-----------------------
>  1 file changed, 12 insertions(+), 23 deletions(-)
>
> diff --git a/ldso/ldso/arc/dl-sysdep.h b/ldso/ldso/arc/dl-sysdep.h
> index c6086e68e5c6..361a9bb9c0c3 100644
> --- a/ldso/ldso/arc/dl-sysdep.h
> +++ b/ldso/ldso/arc/dl-sysdep.h
> @@ -119,37 +119,26 @@ extern unsigned __udivmodsi4(unsigned, unsigned) attribute_hidden;
>     | (((type) == R_ARC_COPY) * ELF_RTYPE_CLASS_COPY))
>  
>  /*
> - * Get the runtime address of GOT[0]
> + * Get build time address of .dynamic as setup in GOT[0]
> + * This is called very early in _dl_start() so it has not been relocated to
> + * runtime value
>   */
>  static __always_inline Elf32_Addr elf_machine_dynamic(void)
>  {
> -	Elf32_Addr dyn;
> -
> -	__asm__("ld %0,[pcl,_DYNAMIC at gotpc]\n\t" : "=r" (dyn));
> -	return dyn;
> +  /* Declaring this hidden ensures that a PC-relative reference is used.  */
> +  extern const Elf32_Addr _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
> +  return _GLOBAL_OFFSET_TABLE_[0];
>  }
>  
>  /* Return the run-time load address of the shared object.  */
>  static __always_inline Elf32_Addr elf_machine_load_address(void)
>  {
> -    /* To find the loadaddr we subtract the runtime addr of a non-local symbol
> -     * say _DYNAMIC from it's build-time addr.
> -     * N.B., gotpc loads get optimized by the linker if it finds the symbol
> -     * is resolved locally.
> -     * A more robust - and efficient - solution would be to use a symbol
> -     * set by the linker.  To make it actually save space, we'd have to
> -     * suppress the unwanted text relocation in the linked dso, though.
> -     * (I.e. in ldso.so.*, though it's just another dso as far as bfd/ld
> -     * are concerned.)
> -     */
> -	Elf32_Addr addr, tmp;
> -	__asm__ (
> -        "ld  %1, [pcl, _DYNAMIC at gotpc] ;build addr of _DYNAMIC"   "\n"
> -        "add %0, pcl, _DYNAMIC at pcl     ;runtime addr of _DYNAMIC" "\n"
> -        "sub %0, %0, %1                ;delta"                    "\n"
> -        : "=&r" (addr), "=r"(tmp)
> -    );
> -	return addr;
> +	Elf32_Addr build_addr, run_addr;
> +
> +	build_addr = elf_machine_dynamic();
> +	__asm__ ("add %0, pcl, _DYNAMIC at pcl	\n" : "=r" (run_addr));
> +
> +	return run_addr - build_addr;
>  }
>  
>  static __always_inline void



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