RFC: -Bsymbolic, hidden and protected

RFC: -Bsymbolic, hidden and protected

[H. J. Lu]

Loading an ELF DSO may require many relocations and accesing global
symbols within the DSO have to go through PLT/GOT, even if they are
defined within the DSO. -Bsymbloc avoids relocatins against global
symbols defined within the DSO. But it is too late for compiler to
take the full advantage of it and we can't do it selectly for
individual symbol. Hidden visibility will optimize everything, but it
makes global symbols local to the DSO. You can't use it on exported
symbols. Protected visibility has almost everything, but requires
special handling at run-time.

I was wondering if we could allow version script to overwrite
hidden symbols. That is if a symbol is global in version script, we
export it even if it is marked hidden. It may improve run-time
performance.


H.J.

Re: RFC: -Bsymbolic, hidden and protected

[Alan Modra]

On Wed, Mar 02, 2005 at 08:14:59PM -0800, H. J. Lu wrote:
> Protected visibility has almost everything, but requires
> special handling at run-time.
> 
> I was wondering if we could allow version script to overwrite
> hidden symbols. That is if a symbol is global in version script, we
> export it even if it is marked hidden. It may improve run-time
> performance.

How is this going to be different from using protected symbols?

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

Re: RFC: -Bsymbolic, hidden and protected

[H. J. Lu]

On Thu, Mar 03, 2005 at 11:19:13PM +1030, Alan Modra wrote:
> On Wed, Mar 02, 2005 at 08:14:59PM -0800, H. J. Lu wrote:
> > Protected visibility has almost everything, but requires
> > special handling at run-time.
> > 
> > I was wondering if we could allow version script to overwrite
> > hidden symbols. That is if a symbol is global in version script, we
> > export it even if it is marked hidden. It may improve run-time
> > performance.
> 
> How is this going to be different from using protected symbols?

No run-time special handling accociated with protected symbols, which
may take extra lookup.


H.J.

Re: RFC: -Bsymbolic, hidden and protected

[H. J. Lu]

On Thu, Mar 03, 2005 at 07:55:04AM -0800, H. J. Lu wrote:
> On Thu, Mar 03, 2005 at 11:19:13PM +1030, Alan Modra wrote:
> > On Wed, Mar 02, 2005 at 08:14:59PM -0800, H. J. Lu wrote:
> > > Protected visibility has almost everything, but requires
> > > special handling at run-time.
> > > 
> > > I was wondering if we could allow version script to overwrite
> > > hidden symbols. That is if a symbol is global in version script, we
> > > export it even if it is marked hidden. It may improve run-time
> > > performance.
> > 
> > How is this going to be different from using protected symbols?
> 
> No run-time special handling accociated with protected symbols, which
> may take extra lookup.

Also compiler/linker can't optimize protected function pointers.


H.J.

Re: RFC: -Bsymbolic, hidden and protected

[Alan Modra]

On Thu, Mar 03, 2005 at 08:55:10AM -0800, H. J. Lu wrote:
> On Thu, Mar 03, 2005 at 07:55:04AM -0800, H. J. Lu wrote:
> > On Thu, Mar 03, 2005 at 11:19:13PM +1030, Alan Modra wrote:
> > > On Wed, Mar 02, 2005 at 08:14:59PM -0800, H. J. Lu wrote:
> > > > Protected visibility has almost everything, but requires
> > > > special handling at run-time.
> > > > 
> > > > I was wondering if we could allow version script to overwrite
> > > > hidden symbols. That is if a symbol is global in version script, we
> > > > export it even if it is marked hidden. It may improve run-time
> > > > performance.
> > > 
> > > How is this going to be different from using protected symbols?
> > 
> > No run-time special handling accociated with protected symbols, which
> > may take extra lookup.
> 
> Also compiler/linker can't optimize protected function pointers.

Which means that if you export hidden function symbols from shared
libraries by means of a version script, you will break function pointer
comparisons.

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

Re: RFC: -Bsymbolic, hidden and protected

[H. J. Lu]

On Fri, Mar 04, 2005 at 08:24:44AM +1030, Alan Modra wrote:
> On Thu, Mar 03, 2005 at 08:55:10AM -0800, H. J. Lu wrote:
> > On Thu, Mar 03, 2005 at 07:55:04AM -0800, H. J. Lu wrote:
> > > On Thu, Mar 03, 2005 at 11:19:13PM +1030, Alan Modra wrote:
> > > > On Wed, Mar 02, 2005 at 08:14:59PM -0800, H. J. Lu wrote:
> > > > > Protected visibility has almost everything, but requires
> > > > > special handling at run-time.
> > > > > 
> > > > > I was wondering if we could allow version script to overwrite
> > > > > hidden symbols. That is if a symbol is global in version script, we
> > > > > export it even if it is marked hidden. It may improve run-time
> > > > > performance.
> > > > 
> > > > How is this going to be different from using protected symbols?
> > > 
> > > No run-time special handling accociated with protected symbols, which
> > > may take extra lookup.
> > 
> > Also compiler/linker can't optimize protected function pointers.
> 
> Which means that if you export hidden function symbols from shared
> libraries by means of a version script, you will break function pointer
> comparisons.

Yes. If you don't do any function pointer comparisons on exported
functions in DSO, you should be OK.


H.J.

PATCH: Force a hidden symbol global

[H. J. Lu]

On Thu, Mar 03, 2005 at 05:37:20PM -0800, H. J. Lu wrote:
> On Fri, Mar 04, 2005 at 08:24:44AM +1030, Alan Modra wrote:
> > On Thu, Mar 03, 2005 at 08:55:10AM -0800, H. J. Lu wrote:
> > > On Thu, Mar 03, 2005 at 07:55:04AM -0800, H. J. Lu wrote:
> > > > On Thu, Mar 03, 2005 at 11:19:13PM +1030, Alan Modra wrote:
> > > > > On Wed, Mar 02, 2005 at 08:14:59PM -0800, H. J. Lu wrote:
> > > > > > Protected visibility has almost everything, but requires
> > > > > > special handling at run-time.
> > > > > > 
> > > > > > I was wondering if we could allow version script to overwrite
> > > > > > hidden symbols. That is if a symbol is global in version script, we
> > > > > > export it even if it is marked hidden. It may improve run-time
> > > > > > performance.
> > > > > 
> > > > > How is this going to be different from using protected symbols?
> > > > 
> > > > No run-time special handling accociated with protected symbols, which
> > > > may take extra lookup.
> > > 
> > > Also compiler/linker can't optimize protected function pointers.
> > 
> > Which means that if you export hidden function symbols from shared
> > libraries by means of a version script, you will break function pointer
> > comparisons.
> 
> Yes. If you don't do any function pointer comparisons on exported
> functions in DSO, you should be OK.
> 

This patch will allow forcing a hidden symbol as global. It can be
used for performance and languange specific features.


H.J.
----
2005-04-21  H.J. Lu  <hongjiu.lu@intel.com>

	* elf-bfd.h (elf_link_hash_entry): Add forced_global.

	* elflink.c (bfd_elf_link_record_dynamic_symbol): Don't check
	symbol visibilty when we force a forced local symbol to global.
	(_bfd_elf_link_renumber_dynsyms): Move forced local symbols
	just before global ones.
	(_bfd_elf_link_assign_sym_version): Set the forced_global field
	if a forced local symbol is marked as global explicitly.
	(elf_link_output_extsym): Handle forced_global.

--- bfd/elf-bfd.h.global	2005-04-04 10:35:34.000000000 -0700
+++ bfd/elf-bfd.h	2005-04-21 10:00:36.000000000 -0700
@@ -155,6 +155,8 @@ struct elf_link_hash_entry
   unsigned int hidden : 1;
   /* Symbol was forced to local scope due to a version script file.  */
   unsigned int forced_local : 1;
+  /* Symbol was forced to global scope due to a version script file.  */
+  unsigned int forced_global : 1;
   /* Symbol was marked during garbage collection.  */
   unsigned int mark : 1;
   /* Symbol is referenced by a non-GOT/non-PLT relocation.  This is
--- bfd/elflink.c.global	2005-04-19 10:42:57.000000000 -0700
+++ bfd/elflink.c	2005-04-21 13:55:00.000000000 -0700
@@ -377,22 +377,24 @@ bfd_elf_link_record_dynamic_symbol (stru
       /* XXX: The ABI draft says the linker must turn hidden and
 	 internal symbols into STB_LOCAL symbols when producing the
 	 DSO. However, if ld.so honors st_other in the dynamic table,
-	 this would not be necessary.  */
-      switch (ELF_ST_VISIBILITY (h->other))
-	{
-	case STV_INTERNAL:
-	case STV_HIDDEN:
-	  if (h->root.type != bfd_link_hash_undefined
-	      && h->root.type != bfd_link_hash_undefweak)
-	    {
-	      h->forced_local = 1;
-	      if (!elf_hash_table (info)->is_relocatable_executable)
-		return TRUE;
-	    }
+	 this would not be necessary.  Don't check symbol visibilty
+	 when we force a forced local symbol to global.  */
+      if (!h->forced_global)
+	switch (ELF_ST_VISIBILITY (h->other))
+	  {
+	  case STV_INTERNAL:
+	  case STV_HIDDEN:
+	    if (h->root.type != bfd_link_hash_undefined
+		&& h->root.type != bfd_link_hash_undefweak)
+	      {
+		h->forced_local = 1;
+		if (!elf_hash_table (info)->is_relocatable_executable)
+		  return TRUE;
+	      }
 
-	default:
-	  break;
-	}
+	  default:
+	    break;
+	  }
 
       h->dynindx = elf_hash_table (info)->dynsymcount;
       ++elf_hash_table (info)->dynsymcount;
@@ -702,9 +704,9 @@ _bfd_elf_link_omit_section_dynsym (bfd *
 }
 
 /* Assign dynsym indices.  In a shared library we generate a section
-   symbol for each output section, which come first.  Next come symbols
-   which have been forced to local binding.  Then all of the back-end
-   allocated local dynamic syms, followed by the rest of the global
+   symbol for each output section, which come first.  Next come all of
+   the back-end allocated local dynamic syms.  Then symbols which have
+   been forced to local binding, followed by the rest of the global
    symbols.  */
 
 static unsigned long
@@ -726,10 +728,6 @@ _bfd_elf_link_renumber_dynsyms (bfd *out
     }
   *section_sym_count = dynsymcount;
 
-  elf_link_hash_traverse (elf_hash_table (info),
-			  elf_link_renumber_local_hash_table_dynsyms,
-			  &dynsymcount);
-
   if (elf_hash_table (info)->dynlocal)
     {
       struct elf_link_local_dynamic_entry *p;
@@ -738,6 +736,10 @@ _bfd_elf_link_renumber_dynsyms (bfd *out
     }
 
   elf_link_hash_traverse (elf_hash_table (info),
+			  elf_link_renumber_local_hash_table_dynsyms,
+			  &dynsymcount);
+
+  elf_link_hash_traverse (elf_hash_table (info),
 			  elf_link_renumber_hash_table_dynsyms,
 			  &dynsymcount);
 
@@ -1694,6 +1696,7 @@ _bfd_elf_link_assign_sym_version (struct
   struct elf_info_failed eif;
   char *p;
   bfd_size_type amt;
+  bfd_boolean forced_global;
 
   sinfo = data;
   info = sinfo->info;
@@ -1716,6 +1719,10 @@ _bfd_elf_link_assign_sym_version (struct
   if (!h->def_regular)
     return TRUE;
 
+  /* Check if a forced local symbol is marked as global explicitly in
+     version script.  */
+  forced_global = FALSE;
+
   bed = get_elf_backend_data (sinfo->output_bfd);
   p = strchr (h->root.root.string, ELF_VER_CHR);
   if (p != NULL && h->verinfo.vertree == NULL)
@@ -1778,6 +1785,8 @@ _bfd_elf_link_assign_sym_version (struct
 		      && ! info->export_dynamic)
 		    (*bed->elf_backend_hide_symbol) (info, h, TRUE);
 		}
+	      else if (d)
+		forced_global = TRUE;
 
 	      free (alc);
 	      break;
@@ -1868,6 +1877,7 @@ _bfd_elf_link_assign_sym_version (struct
 		    h->verinfo.vertree = t;
 		    local_ver = NULL;
 		    d->script = 1;
+		    forced_global = TRUE;
 		    break;
 		  }
 	      if (d != NULL)
@@ -1904,11 +1914,20 @@ _bfd_elf_link_assign_sym_version (struct
 	      && info->shared
 	      && ! info->export_dynamic)
 	    {
+	      forced_global = FALSE;
 	      (*bed->elf_backend_hide_symbol) (info, h, TRUE);
 	    }
 	}
     }
 
+  if (h->forced_local && forced_global)
+    {
+      h->forced_global = 1;
+
+      if (! bfd_elf_link_record_dynamic_symbol (info, h))
+	return FALSE;
+    }
+
   return TRUE;
 }
 \f
@@ -6316,12 +6335,12 @@ elf_link_output_extsym (struct elf_link_
   /* Decide whether to output this symbol in this pass.  */
   if (eoinfo->localsyms)
     {
-      if (!h->forced_local)
+      if (!h->forced_local || h->forced_global)
 	return TRUE;
     }
   else
     {
-      if (h->forced_local)
+      if (h->forced_local && !h->forced_global)
 	return TRUE;
     }
 
@@ -6351,6 +6370,7 @@ elf_link_output_extsym (struct elf_link_
   if (! finfo->info->relocatable
       && (! finfo->info->shared)
       && h->forced_local
+      && !h->forced_global
       && h->ref_dynamic
       && !h->dynamic_def
       && !h->dynamic_weak
@@ -6404,7 +6424,14 @@ elf_link_output_extsym (struct elf_link_
   sym.st_value = 0;
   sym.st_size = h->size;
   sym.st_other = h->other;
-  if (h->forced_local)
+  if (h->forced_global)
+    {
+      /* A forced global symbol has the default visibility.  */
+      sym.st_other
+	= STV_DEFAULT | (h->other & ~ ELF_ST_VISIBILITY (-1));
+      sym.st_info = ELF_ST_INFO (STB_GLOBAL, h->type);
+    }
+  else if (h->forced_local)
     sym.st_info = ELF_ST_INFO (STB_LOCAL, h->type);
   else if (h->root.type == bfd_link_hash_undefweak
 	   || h->root.type == bfd_link_hash_defweak)