gdb: handle invalid DWARF when compilation unit is missing

Replace an abort call in process_psymtab_comp_unit with a real error,
and add a test to cover this case.  The case is question is when badly
formed DWARF is missing a DW_TAG_compile_unit, DW_TAG_partial_unit, or
DW_TAG_type_unit as its top level tag.

I then tested with --target_board=readnow and added additional code to
also validate the top-level tag in this case.

I added an assert that would trigger for the readnow case before I
added the fix.  I suspect there's lots of places where badly formed
DWARF could result in the builder being nullptr when it shouldn't be,
but I only added this one assert, as this is the one that would have
helped me in this case.

gdb/ChangeLog:

	* dwarf2/read.c (process_psymtab_comp_unit): Replace abort with an
	error.
	(process_full_comp_unit): Validate the top-level tag before
	processing the first DIE.
	(read_func_scope): Ensure we have a valid builder.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-missing-cu-tag.c: New file.
	* gdb.dwarf2/dw2-missing-cu-tag.exp: New file.

1 files changed, 20 insertions, 1 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index acbc5fa..2bfb13d 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -7737,7 +7737,10 @@ process_psymtab_comp_unit (dwarf2_per_cu_data *this_cu,
       this_cu->unit_type = DW_UT_type;
       break;
     default:
-      abort ();
+      error (_("Dwarf Error: unexpected tag '%s' at offset %s [in module %s]"),
+	     dwarf_tag_name (reader.comp_unit_die->tag),
+	     sect_offset_str (reader.cu->per_cu->sect_off),
+	     objfile_name (per_objfile->objfile));
     }
 
   if (reader.dummy_p)
@@ -9994,6 +9997,21 @@ process_full_comp_unit (dwarf2_cu *cu, enum language pretend_language)
 
   dwarf2_find_base_address (cu->dies, cu);
 
+  /* Before we start reading the top-level DIE, ensure it has a valid tag
+     type.  */
+  switch (cu->dies->tag)
+    {
+    case DW_TAG_compile_unit:
+    case DW_TAG_partial_unit:
+    case DW_TAG_type_unit:
+      break;
+    default:
+      error (_("Dwarf Error: unexpected tag '%s' at offset %s [in module %s]"),
+	     dwarf_tag_name (cu->dies->tag),
+	     sect_offset_str (cu->per_cu->sect_off),
+	     objfile_name (per_objfile->objfile));
+    }
+
   /* Do line number decoding in read_file_scope () */
   process_die (cu->dies, cu);
 
@@ -13628,6 +13646,7 @@ read_func_scope (struct die_info *die, struct dwarf2_cu *cu)
 	}
     }
 
+  gdb_assert (cu->get_builder () != nullptr);
   newobj = cu->get_builder ()->push_context (0, lowpc);
   newobj->name = new_symbol (die, read_type_die (die, cu), cu,
 			     (struct symbol *) templ_func);