gc sections and .eh_frame

[Jonathan Larmour <jifl at eCosCentric dot com>]

After an update to binutils 2.16 from 2.15 I have found that sections 
referenced by .eh_frame with relocs do not get pulled in if linking 
(statically) with --gc-sections. Initially I just put a 
KEEP(*(.gcc_except_table)) in my ldscript to work round this.

But now I'm using the powerpc architecture where the stuff usually in 
.gcc_except_table is placed in .rodata instead. If the object file has 
nothing else in .rodata, then that section is also being GC'd in the final 
link. And Bad Things Happen.

I believe I have tracked this down to a change enclosed in full at the end 
of this mail, made by Eric Botcazou on 2004-04-21. I've looked at the 
mailing list archives, and not found any posting or discussion of the 
patch, or its rationale. Is that right?

Anyway, the most relevant bit in elflink.c:bfd_elf_gc_sections() is:

       for (o = sub->sections; o != NULL; o = o->next)
        {
          if (o->flags & SEC_KEEP)
-           if (!elf_gc_mark (info, o, gc_mark_hook))
-             return FALSE;
+           {
+             /* _bfd_elf_discard_section_eh_frame knows how to discard
+                orphaned FDEs so don't mark sections referenced by the
+                EH frame section.  */
+             if (strcmp (o->name, ".eh_frame") == 0)
+               o->gc_mark = 1;
+             else if (!elf_gc_mark (info, o, gc_mark_hook))
+               return FALSE;
+           }
        }
     }

This change doesn't make sense to me. _bfd_elf_discard_section_eh_frame 
does not set gc_mark on any section. So as a result, I would assume none 
of the reloc dependencies of .eh_frame would ever get marked and that's 
how those sections get GC'd. Am I missing something?

There seems to have been some effort at the time to allow GC of sections 
in dynamically linked programs (I think). Perhaps this was a change which 
is only relevant for dynamic, not static linking? Certainly from my view, 
the correct thing to do is simply revert this part of the change.

Jifl

Index: ChangeLog
===================================================================
RCS file: /cvs/src/src/bfd/ChangeLog,v
retrieving revision 1.2501
retrieving revision 1.2502
diff -u -5 -p -r1.2501 -r1.2502
--- ChangeLog   20 Apr 2004 12:17:12 -0000      1.2501
+++ ChangeLog   21 Apr 2004 07:14:15 -0000      1.2502
@@ -1,5 +1,14 @@
+2004-04-21  Eric Botcazou  <ebotcazou@act-europe.fr>
+
+       * elflink.c (elf_gc_mark_dynamic_ref_symbol): New function.
+       (bfd_elf_gc_sections): Fail if a shared object is being created.
+       Do not fail if dynamic sections have been created.  Instead call
+       elf_gc_mark_dynamic_ref_symbol to mark sections that contain
+       dynamically referenced symbols.  Do not mark the whole graph
+       rooted at .eh_frame, only the section proper.
+
 2004-04-20  DJ Delorie  <dj@redhat.com>

        * reloc.c: Add BFD_RELOC_32_SECREL.
        * bfd-in2.h: Regenerate.
        * libbfd.h: Likewise.
Index: elflink.c
===================================================================
RCS file: /cvs/src/src/bfd/elflink.c,v
retrieving revision 1.65
retrieving revision 1.66
diff -u -5 -p -r1.65 -r1.66
--- elflink.c   15 Apr 2004 02:55:20 -0000      1.65
+++ elflink.c   21 Apr 2004 07:14:15 -0000      1.66
@@ -8410,10 +8410,28 @@ elf_gc_smash_unused_vtentry_relocs (stru
       }

   return TRUE;
 }

+/* Mark sections containing dynamically referenced symbols.  This is called
+   through elf_link_hash_traverse.  */
+
+static bfd_boolean
+elf_gc_mark_dynamic_ref_symbol (struct elf_link_hash_entry *h,
+                               void *okp ATTRIBUTE_UNUSED)
+{
+  if (h->root.type == bfd_link_hash_warning)
+    h = (struct elf_link_hash_entry *) h->root.u.i.link;
+
+  if ((h->root.type == bfd_link_hash_defined
+       || h->root.type == bfd_link_hash_defweak)
+      && (h->elf_link_hash_flags & ELF_LINK_HASH_REF_DYNAMIC))
+    h->root.u.def.section->flags |= SEC_KEEP;
+
+  return TRUE;
+}
+
 /* Do mark and sweep of unused sections.  */

 bfd_boolean
 bfd_elf_gc_sections (bfd *abfd, struct bfd_link_info *info)
 {
@@ -8424,12 +8442,12 @@ bfd_elf_gc_sections (bfd *abfd, struct b
      struct elf_link_hash_entry *h, Elf_Internal_Sym *);

   if (!get_elf_backend_data (abfd)->can_gc_sections
       || info->relocatable
       || info->emitrelocations
-      || !is_elf_hash_table (info->hash)
-      || elf_hash_table (info)->dynamic_sections_created)
+      || info->shared
+      || !is_elf_hash_table (info->hash))
     {
       (*_bfd_error_handler)(_("Warning: gc-sections option ignored"));
       return TRUE;
     }

@@ -8445,12 +8463,19 @@ bfd_elf_gc_sections (bfd *abfd, struct b
                          elf_gc_smash_unused_vtentry_relocs,
                          &ok);
   if (!ok)
     return FALSE;

-  /* Grovel through relocs to find out who stays ...  */
+  /* Mark dynamically referenced symbols.  */
+  if (elf_hash_table (info)->dynamic_sections_created)
+    elf_link_hash_traverse (elf_hash_table (info),
+                           elf_gc_mark_dynamic_ref_symbol,
+                           &ok);
+  if (!ok)
+    return FALSE;

+  /* Grovel through relocs to find out who stays ...  */
   gc_mark_hook = get_elf_backend_data (abfd)->gc_mark_hook;
   for (sub = info->input_bfds; sub != NULL; sub = sub->link_next)
     {
       asection *o;

@@ -8458,12 +8483,19 @@ bfd_elf_gc_sections (bfd *abfd, struct b
        continue;

       for (o = sub->sections; o != NULL; o = o->next)
        {
          if (o->flags & SEC_KEEP)
-           if (!elf_gc_mark (info, o, gc_mark_hook))
-             return FALSE;
+           {
+             /* _bfd_elf_discard_section_eh_frame knows how to discard
+                orphaned FDEs so don't mark sections referenced by the
+                EH frame section.  */
+             if (strcmp (o->name, ".eh_frame") == 0)
+               o->gc_mark = 1;
+             else if (!elf_gc_mark (info, o, gc_mark_hook))
+               return FALSE;
+           }
        }
     }

   /* ... and mark SEC_EXCLUDE for those that go.  */
   if (!elf_gc_sweep (info, get_elf_backend_data (abfd)->gc_sweep_hook))

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