Re: Discard zero address range eh_frame FDEs

[Alan Modra <amodra at gmail dot com>]

On Sun, Oct 12, 2014 at 10:09:25PM +1030, Alan Modra wrote:
> misguided actions..  The underlying problem, with the potential for
> nasty exception handling bugs at runtime, has been there since
> https://sourceware.org/ml/binutils/2003-02/msg00471.html 

Trying to clean up after ld -r at final link time proved to be messy.

The idea here is to drop .eh_frame FDEs corresponding to dropped
comdat group sections or linkonce sections for ld -r, but not perform
changes in encoding.  The least amount of editing for ld -r, the
better.

Changing bfd_elf_reloc_symbol_deleted_p does have the potential to
affect mips, score and xtensa, but as far as I know, .pdr and the
xtensa .xt.insn, .xt.lit and .xt.prop sections all hold info for
their own object file rather than about other object files.  If I'm
wrong about that, please let me know.

bfd/
	PR 17467
	* elf-eh-frame.c (ENSURE_NO_RELOCS): Don't stop at first NONE reloc.
	(_bfd_elf_parse_eh_frame): When relocatable output, don't set
	flags enabling conversion of CIEs and FDEs to use relative encoding.
	(find_merged_cie): Similarly.
	(_bfd_elf_write_section_eh_frame): Don't edit FDEs when
	relocatable, except for CIE pointer.
	* elflink.c (bfd_elf_reloc_symbol_deleted_p): Return true for
	relocs against symbols in dropped comdat group sections.
	(bfd_elf_discard_info): Do some eh_frame optimisation when
	relocatable.
ld/
	* ldlang.c (lang_add_section): Set up map_head.s and map_tail.s when
	relocatable.

diff --git a/bfd/elf-eh-frame.c b/bfd/elf-eh-frame.c
index 331570a..e481f34 100644
--- a/bfd/elf-eh-frame.c
+++ b/bfd/elf-eh-frame.c
@@ -556,10 +556,13 @@ _bfd_elf_parse_eh_frame (bfd *abfd, struct bfd_link_info *info,
 
   /* FIXME: octets_per_byte.  */
 #define ENSURE_NO_RELOCS(buf)				\
-  REQUIRE (!(cookie->rel < cookie->relend		\
-	     && (cookie->rel->r_offset			\
-		 < (bfd_size_type) ((buf) - ehbuf))	\
-	     && cookie->rel->r_info != 0))
+  while (cookie->rel < cookie->relend			\
+	 && (cookie->rel->r_offset			\
+	     < (bfd_size_type) ((buf) - ehbuf)))	\
+    {							\
+      REQUIRE (cookie->rel->r_info == 0);		\
+      cookie->rel++;					\
+    }
 
   /* FIXME: octets_per_byte.  */
 #define SKIP_RELOCS(buf)				\
@@ -726,6 +729,7 @@ _bfd_elf_parse_eh_frame (bfd *abfd, struct bfd_link_info *info,
 	  /* For shared libraries, try to get rid of as many RELATIVE relocs
 	     as possible.  */
 	  if (info->shared
+	      && !info->relocatable
 	      && (get_elf_backend_data (abfd)
 		  ->elf_backend_can_make_relative_eh_frame
 		  (abfd, info, sec)))
@@ -763,10 +767,12 @@ _bfd_elf_parse_eh_frame (bfd *abfd, struct bfd_link_info *info,
 	  ENSURE_NO_RELOCS (buf);
 
 	  if (!info->relocatable)
-	    /* Keep info for merging cies.  */
-	    this_inf->u.cie.u.full_cie = cie;
-	  this_inf->u.cie.per_encoding_relative
-	    = (cie->per_encoding & 0x70) == DW_EH_PE_pcrel;
+	    {
+	      /* Keep info for merging cies.  */
+	      this_inf->u.cie.u.full_cie = cie;
+	      this_inf->u.cie.per_encoding_relative
+		= (cie->per_encoding & 0x70) == DW_EH_PE_pcrel;
+	    }
 	}
       else
 	{
@@ -1071,6 +1077,7 @@ find_merged_cie (bfd *abfd, struct bfd_link_info *info, asection *sec,
 
       if (per_binds_local
 	  && info->shared
+	  && !info->relocatable
 	  && (cie->per_encoding & 0x70) == DW_EH_PE_absptr
 	  && (get_elf_backend_data (abfd)
 	      ->elf_backend_can_make_relative_eh_frame (abfd, info, sec)))
@@ -1577,6 +1584,8 @@ _bfd_elf_write_section_eh_frame (bfd *abfd,
 	  value = ((ent->new_offset + sec->output_offset + 4)
 		   - (cie->new_offset + cie->u.cie.u.sec->output_offset));
 	  bfd_put_32 (abfd, value, buf);
+	  if (info->relocatable)
+	    continue;
 	  buf += 4;
 	  width = get_DW_EH_PE_width (ent->fde_encoding, ptr_size);
 	  value = read_value (abfd, buf, width,
diff --git a/bfd/elflink.c b/bfd/elflink.c
index d33efe0..c8068c0 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -12602,10 +12602,10 @@ bfd_elf_reloc_symbol_deleted_p (bfd_vma offset, void *cookie)
 
 	  if ((h->root.type == bfd_link_hash_defined
 	       || h->root.type == bfd_link_hash_defweak)
-	      && discarded_section (h->root.u.def.section))
+	      && (h->root.u.def.section->owner != rcookie->abfd
+		  || h->root.u.def.section->kept_section != NULL
+		  || discarded_section (h->root.u.def.section)))
 	    return TRUE;
-	  else
-	    return FALSE;
 	}
       else
 	{
@@ -12618,7 +12618,9 @@ bfd_elf_reloc_symbol_deleted_p (bfd_vma offset, void *cookie)
 	  /* Need to: get the symbol; get the section.  */
 	  isym = &rcookie->locsyms[r_symndx];
 	  isec = bfd_section_from_elf_index (rcookie->abfd, isym->st_shndx);
-	  if (isec != NULL && discarded_section (isec))
+	  if (isec != NULL
+	      && (isec->kept_section != NULL
+		  || discarded_section (isec)))
 	    return TRUE;
 	}
       return FALSE;
@@ -12672,9 +12674,7 @@ bfd_elf_discard_info (bfd *output_bfd, struct bfd_link_info *info)
 	}
     }
 
-  o = NULL;
-  if (!info->relocatable)
-    o = bfd_get_section_by_name (output_bfd, ".eh_frame");
+  o = bfd_get_section_by_name (output_bfd, ".eh_frame");
   if (o != NULL)
     {
       asection *i;
diff --git a/ld/ldlang.c b/ld/ldlang.c
index 899f710..5960e5c 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -2411,8 +2411,7 @@ lang_add_section (lang_statement_list_type *ptr,
 
   section->output_section = output->bfd_section;
 
-  if (!link_info.relocatable
-      && !map_head_is_link_order)
+  if (!map_head_is_link_order)
     {
       asection *s = output->bfd_section->map_tail.s;
       output->bfd_section->map_tail.s = section;

-- 
Alan Modra
Australia Development Lab, IBM