ELF: Support the section flag 'o' in .section directive

As shown in

https://sourceware.org/bugzilla/show_bug.cgi?id=25490

--gc-sections will silently remove __patchable_function_entries section
and generate corrupt result.  This patch adds the section flag 'o' to
.section directive:

.section __patchable_function_entries,"awo",@progbits,foo
.section __patchable_function_entries,"awoG",@progbits,foo,foo,comdat
.section __patchable_function_entries,"awo",@progbits,bar,unique,4
.section __patchable_function_entries,"awoG",@progbits,foo,foo,comdat,unique,1

which specifies the symbol name which the section references.  Assmebler
will set its elf_linked_to_section to a local section where the symbol
is defined.

Linker is updated to call mark_hook if gc_mark of any of its linked-to
sections is set after all sections, except for backend specific ones,
have been garbage collected.

bfd/

	PR gas/25381
	* bfd-in2.h: Regenerated.
	* elflink.c (_bfd_elf_gc_mark_extra_sections): Call mark_hook
	on section if gc_mark of any of its linked-to sections is set
	and don't set gc_mark again.
	* section.c (asection): Add linked_to_symbol_name to map_head
	union.

gas/

	PR gas/25381
	* config/obj-elf.c (get_section): Also check
	linked_to_symbol_name.
	(obj_elf_change_section): Also set map_head.linked_to_symbol_name.
	(obj_elf_parse_section_letters): Handle the 'o' flag.
	(build_group_lists): Renamed to ...
	(build_additional_section_info): This.  Set elf_linked_to_section
	from map_head.linked_to_symbol_name.
	(elf_adjust_symtab): Updated.
	* config/obj-elf.h (elf_section_match): Add linked_to_symbol_name.
	* doc/as.texi: Document the 'o' flag.
	* testsuite/gas/elf/elf.exp: Run PR gas/25381 tests.
	* testsuite/gas/elf/section18.d: New file.
	* testsuite/gas/elf/section18.s: Likewise.
	* testsuite/gas/elf/section19.d: Likewise.
	* testsuite/gas/elf/section19.s: Likewise.
	* testsuite/gas/elf/section20.d: Likewise.
	* testsuite/gas/elf/section20.s: Likewise.
	* testsuite/gas/elf/section21.d: Likewise.
	* testsuite/gas/elf/section21.l: Likewise.
	* testsuite/gas/elf/section21.s: Likewise.

ld/

	PR ld/24526
	PR ld/25021
	PR ld/25490
	* testsuite/ld-elf/elf.exp: Run PR ld/25490 tests.
	* testsuite/ld-elf/pr24526.d: New file.
	* testsuite/ld-elf/pr24526.s: Likewise.
	* testsuite/ld-elf/pr25021.d: Likewise.
	* testsuite/ld-elf/pr25021.s: Likewise.
	* testsuite/ld-elf/pr25490-2-16.rd: Likewise.
	* testsuite/ld-elf/pr25490-2-32.rd: Likewise.
	* testsuite/ld-elf/pr25490-2-64.rd: Likewise.
	* testsuite/ld-elf/pr25490-2.s: Likewise.
	* testsuite/ld-elf/pr25490-3-16.rd: Likewise.
	* testsuite/ld-elf/pr25490-3-32.rd: Likewise.
	* testsuite/ld-elf/pr25490-3-64.rd: Likewise.
	* testsuite/ld-elf/pr25490-3.s: Likewise.
	* testsuite/ld-elf/pr25490-4-16.rd: Likewise.
	* testsuite/ld-elf/pr25490-4-32.rd: Likewise.
	* testsuite/ld-elf/pr25490-4-64.rd: Likewise.
	* testsuite/ld-elf/pr25490-4.s: Likewise.
	* testsuite/ld-elf/pr25490-5-16.rd: Likewise.
	* testsuite/ld-elf/pr25490-5-32.rd: Likewise.
	* testsuite/ld-elf/pr25490-5-64.rd: Likewise.
	* testsuite/ld-elf/pr25490-5.s: Likewise.
	* testsuite/ld-elf/pr25490-6-16.rd: Likewise.
	* testsuite/ld-elf/pr25490-6-32.rd: Likewise.
	* testsuite/ld-elf/pr25490-6-64.rd: Likewise.
	* testsuite/ld-elf/pr25490-6.s: Likewise.

4 files changed, 36 insertions, 5 deletions

diff --git a/bfd/ChangeLog b/bfd/ChangeLog
index 95152ad..9971885 100644
--- a/bfd/ChangeLog
+++ b/bfd/ChangeLog
@@ -1,3 +1,13 @@
+2020-02-06  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR gas/25381
+	* bfd-in2.h: Regenerated.
+	* elflink.c (_bfd_elf_gc_mark_extra_sections): Call mark_hook
+	on section if gc_mark of any of its linked-to sections is set
+	and don't set gc_mark again.
+	* section.c (asection): Add linked_to_symbol_name to map_head
+	union.
+
 2020-02-06  Maciej W. Rozycki  <macro@wdc.com>
 
 	* elf32-v850.c (v850_elf_relax_section): Fix the index used for
diff --git a/bfd/bfd-in2.h b/bfd/bfd-in2.h
index 09a5a39..2d26b81 100644
--- a/bfd/bfd-in2.h
+++ b/bfd/bfd-in2.h
@@ -1185,10 +1185,12 @@ typedef struct bfd_section
   /* Early in the link process, map_head and map_tail are used to build
      a list of input sections attached to an output section.  Later,
      output sections use these fields for a list of bfd_link_order
-     structs.  */
+     structs.  The linked_to_symbol_name field is for ELF assembler
+     internal use.  */
   union {
     struct bfd_link_order *link_order;
     struct bfd_section *s;
+    const char *linked_to_symbol_name;
   } map_head, map_tail;
 } asection;
 
diff --git a/bfd/elflink.c b/bfd/elflink.c
index 5217528..30a572d 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -13316,7 +13316,7 @@ _bfd_elf_gc_mark_debug_special_section_group (asection *grp)
 
 bfd_boolean
 _bfd_elf_gc_mark_extra_sections (struct bfd_link_info *info,
-				 elf_gc_mark_hook_fn mark_hook ATTRIBUTE_UNUSED)
+				 elf_gc_mark_hook_fn mark_hook)
 {
   bfd *ibfd;
 
@@ -13345,6 +13345,21 @@ _bfd_elf_gc_mark_extra_sections (struct bfd_link_info *info,
 		   && (isec->flags & SEC_ALLOC) != 0
 		   && elf_section_type (isec) != SHT_NOTE)
 	    some_kept = TRUE;
+	  else
+	    {
+	      /* Since all sections, except for backend specific ones,
+		 have been garbage collected, call mark_hook on this
+		 section if any of its linked-to sections is marked.  */
+	      asection *linked_to_sec = elf_linked_to_section (isec);
+	      for (; linked_to_sec != NULL;
+		   linked_to_sec = elf_linked_to_section (linked_to_sec))
+		if (linked_to_sec->gc_mark)
+		  {
+		    if (!_bfd_elf_gc_mark (info, isec, mark_hook))
+		      return FALSE;
+		    break;
+		  }
+	    }
 
 	  if (!debug_frag_seen
 	      && (isec->flags & SEC_DEBUGGING)
@@ -13359,14 +13374,16 @@ _bfd_elf_gc_mark_extra_sections (struct bfd_link_info *info,
 
       /* Keep debug and special sections like .comment when they are
 	 not part of a group.  Also keep section groups that contain
-	 just debug sections or special sections.  */
+	 just debug sections or special sections.  NB: Sections with
+	 linked-to section has been handled above.  */
       for (isec = ibfd->sections; isec != NULL; isec = isec->next)
 	{
 	  if ((isec->flags & SEC_GROUP) != 0)
 	    _bfd_elf_gc_mark_debug_special_section_group (isec);
 	  else if (((isec->flags & SEC_DEBUGGING) != 0
 		    || (isec->flags & (SEC_ALLOC | SEC_LOAD | SEC_RELOC)) == 0)
-		   && elf_next_in_group (isec) == NULL)
+		   && elf_next_in_group (isec) == NULL
+		   && elf_linked_to_section (isec) == NULL)
 	    isec->gc_mark = 1;
 	  if (isec->gc_mark && (isec->flags & SEC_DEBUGGING) != 0)
 	    has_kept_debug_info = TRUE;
diff --git a/bfd/section.c b/bfd/section.c
index 0c15a0d..7de0a2d 100644
--- a/bfd/section.c
+++ b/bfd/section.c
@@ -544,10 +544,12 @@ CODE_FRAGMENT
 .  {* Early in the link process, map_head and map_tail are used to build
 .     a list of input sections attached to an output section.  Later,
 .     output sections use these fields for a list of bfd_link_order
-.     structs.  *}
+.     structs.  The linked_to_symbol_name field is for ELF assembler
+.     internal use.  *}
 .  union {
 .    struct bfd_link_order *link_order;
 .    struct bfd_section *s;
+.    const char *linked_to_symbol_name;
 .  } map_head, map_tail;
 .} asection;
 .