Lack of SHF_GROUP sections result in ld segfault

[Alan Modra <amodra at gmail dot com>]

ld internal info for SHT_GROUP sections is set up when ld sees a
SHF_GROUP section.  The invalid object file in the pr somehow had lost
all SHF_GROUP flags, resulting in a segfault.  Note that the error
message I'm adding here will be repeated for each matching ELF target,
which is excessive but no different than we do for other errors
reported by elf_object_p.

	PR 20520
	* elf.c (_bfd_elf_setup_sections): Check that SHT_GROUP sections
	have corresponding SHF_GROUP sections.
	(bfd_elf_set_group_contents): Comment.

diff --git a/bfd/elf.c b/bfd/elf.c
index c3630d2..7d0b8a9 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -834,6 +834,14 @@ _bfd_elf_setup_sections (bfd *abfd)
 	      elf_linked_to_section (s) = linksec;
 	    }
 	}
+      else if (this_hdr->sh_type == SHT_GROUP
+	       && elf_next_in_group (s) == NULL)
+	{
+	  (*_bfd_error_handler)
+	    (_("%B: SHT_GROUP section [index %d] has no SHF_GROUP sections"),
+	     abfd, elf_section_data (s)->this_idx);
+	  result = FALSE;
+	}
     }
 
   /* Process section groups.  */
@@ -3403,12 +3411,19 @@ bfd_elf_set_group_contents (bfd *abfd, asection *sec, void *failedptrarg)
       /* The ELF backend linker sets sh_info to -2 when the group
 	 signature symbol is global, and thus the index can't be
 	 set until all local symbols are output.  */
-      asection *igroup = elf_sec_group (elf_next_in_group (sec));
-      struct bfd_elf_section_data *sec_data = elf_section_data (igroup);
-      unsigned long symndx = sec_data->this_hdr.sh_info;
-      unsigned long extsymoff = 0;
+      asection *igroup;
+      struct bfd_elf_section_data *sec_data;
+      unsigned long symndx;
+      unsigned long extsymoff;
       struct elf_link_hash_entry *h;
 
+      /* The point of this little dance to the first SHF_GROUP section
+	 then back to the SHT_GROUP section is that this gets us to
+	 the SHT_GROUP in the input object.  */
+      igroup = elf_sec_group (elf_next_in_group (sec));
+      sec_data = elf_section_data (igroup);
+      symndx = sec_data->this_hdr.sh_info;
+      extsymoff = 0;
       if (!elf_bad_symtab (igroup->owner))
 	{
 	  Elf_Internal_Shdr *symtab_hdr;

-- 
Alan Modra
Australia Development Lab, IBM