Re: [patch] handle unaligned arm abs relocs

[Carlos O'Donell <carlos_odonell@mentor.com>]

On 12/12/2011 7:20 PM, Mike Frysinger wrote:
> background can be found here:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51456
> 
> patch is below
> -mike
> 
> 2011-12-12  Mike Frysinger  <vapier@gentoo.org>
> 
> 	* dl-machine.h (elf_machine_rel, R_ARM_ABS32): Declare "reloc_value".
> 	Replace reloc_addr addition with memcpy's.
> 
> diff --git a/sysdeps/arm/dl-machine.h b/sysdeps/arm/dl-machine.h
> index 5ceeaa9..fe64800 100644
> --- a/sysdeps/arm/dl-machine.h
> +++ b/sysdeps/arm/dl-machine.h
> @@ -413,6 +413,7 @@ elf_machine_rel (struct link_map *map, const Elf32_Rel 
> *reloc,
>  	  break;
>  	case R_ARM_ABS32:
>  	  {
> +	    Elf32_Addr reloc_value;
>  # ifndef RTLD_BOOTSTRAP
>  	   /* This is defined in rtld.c, but nowhere in the static
>  	      libc.a; make the reference weak so static programs can
> @@ -431,7 +432,10 @@ elf_machine_rel (struct link_map *map, const Elf32_Rel 
> *reloc,
>  		 used while loading those libraries.  */
>  	      value -= map->l_addr + refsym->st_value;
>  # endif
> -	    *reloc_addr += value;
> +	    /* Support relocations on mis-aligned offsets.  */
> +	    memcpy (&reloc_value, reloc_addr_arg, sizeof (reloc_value));
> +	    reloc_value += value;
> +	    memcpy (reloc_addr_arg, &reloc_value, sizeof (reloc_value));

I don't believe that memcpy is safe this early in the loader.

You might be lucky and get an inlined memcpy or builtin, but you
might not and if you go through the PLT you'll fault since
it's not yet setup.

On PARISC we do a byte copy of unaligned relocation.

e.g.
      /* .eh_frame can have unaligned relocs.  */
      if ((unsigned long) reloc_addr_arg & 3)
        {
          char *rel_addr = (char *) reloc_addr_arg;
          rel_addr[0] = value >> 24;
          rel_addr[1] = value >> 16;
          rel_addr[2] = value >> 8;
          rel_addr[3] = value;
          return;
        }

>  	    break;
>  	  }
>  	case R_ARM_TLS_DESC:

Cheers,
Carlos.
-- 
Carlos O'Donell
Mentor Graphics / CodeSourcery
carlos@codesourcery.com
+1 (613) 963 1026