Re: [PATCH v3] elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 06/07/2022 10:49, Fangrui Song wrote:
> On 2022-06-07, Szabolcs Nagy wrote:
> > > -  int protected = (*ref
> > > -		   && ELFW(ST_VISIBILITY) ((*ref)->st_other) == STV_PROTECTED);
> > > -  if (__glibc_unlikely (protected != 0))
> > > -    {
> > > -      /* It is very tricky.  We need to figure out what value to
> > > -	 return for the protected symbol.  */
> > > -      if (type_class == ELF_RTYPE_CLASS_PLT)
> > > -	{
> > > -	  if (current_value.s != NULL && current_value.m != undef_map)
> > > -	    {
> > > -	      current_value.s = *ref;
> > > -	      current_value.m = undef_map;
> > > -	    }
> > > -	}
> > > -      else
> > > -	{
> > > -	  struct sym_val protected_value = { NULL, NULL };
> > > -
> > > -	  for (scope = symbol_scope; *scope != NULL; i = 0, ++scope)
> > > -	    if (do_lookup_x (undef_name, new_hash, &old_hash, *ref,
> > > -			     &protected_value, *scope, i, version, flags,
> > > -			     skip_map,
> > > -			     (ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA
> > > -			      && ELFW(ST_TYPE) ((*ref)->st_info) == STT_OBJECT
> > > -			      && type_class == ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA)
> > > -			     ? ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA
> > > -			     : ELF_RTYPE_CLASS_PLT, NULL) != 0)
> > > -	      break;
> > > -
> > > -	  if (protected_value.s != NULL && protected_value.m != undef_map)
> > > -	    {
> > > -	      current_value.s = *ref;
> > > -	      current_value.m = undef_map;
> > > -	    }
> > > -	}
> > > -    }
> > > -
> > 
> > i think we should keep this part without the
> > ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA bit.
> 
> I have played a bit but do not find any difference (with some examples using
> "canonical PLT entries") if I simply remove the whole if statement.  Do you
> find anything I may have missed?

yes, i posted an example earlier that behaves differently.

object symbol defined in exe and dso, the dso one is protected
and has a GOT reloc for it.

with the extra logic the GOT is resolved to the definition in
the dso, without it the exe interposes the protected symbol.