dwarf_loader: Track unspecified types in a separate list

In recent discussion in BPF mailing list ([1]) participants agreed to
add a new DWARF representation for "btf_type_tag" annotations.
The agreed representation of void pointers uses unspecified types.
For example, consider the following C code:

    struct alpha {
      void __attribute__((btf_type_tag("__alpha_a"))) *a;
    } g;

And corresponding DWARF:

0x29:   DW_TAG_structure_type
          DW_AT_name      ("alpha")

0x2e:     DW_TAG_member
            DW_AT_name    ("a")
            DW_AT_type    (0x38 "void *")

0x38:   DW_TAG_pointer_type
          DW_AT_type      (0x41 "void")

0x41:   DW_TAG_unspecified_type
          DW_AT_name      ("void")

0x43:     DW_TAG_LLVM_annotation
            DW_AT_name    ("btf:type_tag")
            DW_AT_const_value     ("__alpha_a")

This is a preparatory patch for new type tags representation support,
specifically it adds `struct unspecified_type` and a new `cu` field
`struct cu::unspecified_types`. These would be used in a subsequent
patch to recode type tags attached to DW_TAG_unspecified_type
as in the example above.

[1] Mailing list discussion regarding `btf:type_tag`
    Various approaches are discussed, Solution acmel#2 is accepted
    https://lore.kernel.org/bpf/87r0w9jjoq.fsf@oracle.com/

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

dwarf_loader.c

@@ -165,14 +165,19 @@ static struct dwarf_cu *dwarf_cu__new(struct cu *cu)
 	return dwarf_cu;
 }
 
+static void unspecified_type__delete(struct cu *cu, struct unspecified_type *utype);
+
 static void dwarf_cu__delete(struct cu *cu)
 {
 	if (cu == NULL || cu->priv == NULL)
 		return;
 
 	struct dwarf_cu *dcu = cu->priv;
+	struct list_head *pos, *n;
 
 	// dcu->hash_tags & dcu->hash_types are on cu->obstack
+	list_for_each_safe(pos, n, &cu->unspecified_types)
+		unspecified_type__delete(cu, container_of(pos, struct unspecified_type, node));
 	cu__free(cu, dcu);
 	cu->priv = NULL;
 }
@@ -1386,6 +1391,33 @@ static struct tag *die__create_new_tag(Dwarf_Die *die, struct cu *cu)
 	return tag;
 }
 
+static struct tag *die__create_new_unspecified_type(Dwarf_Die *die, struct cu *cu,
+						    struct conf_load *conf)
+{
+	struct unspecified_type *tag;
+
+	tag = tag__alloc_with_spec(cu, sizeof(struct unspecified_type));
+	if (tag == NULL)
+		return NULL;
+
+	tag__init(&tag->tag, cu, die);
+	INIT_LIST_HEAD(&tag->node);
+
+	tag->name = attr_string(die, DW_AT_name, conf);
+
+	list_add(&tag->node, &cu->unspecified_types);
+
+	return &tag->tag;
+}
+
+static void unspecified_type__delete(struct cu *cu, struct unspecified_type *utype)
+{
+	struct dwarf_tag *dtag = utype->tag.priv;
+
+	cu__free(cu, dtag);
+	cu__free(cu, utype);
+}
+
 static struct btf_type_tag_ptr_type *die__create_new_btf_type_tag_ptr_type(Dwarf_Die *die, struct cu *cu)
 {
 	struct btf_type_tag_ptr_type *tag;
@@ -2186,10 +2218,10 @@ static struct tag *__die__process_tag(Dwarf_Die *die, struct cu *cu,
 	case DW_TAG_volatile_type:
 	case DW_TAG_atomic_type:
 		tag = die__create_new_tag(die, cu);		break;
-	case DW_TAG_unspecified_type:
-		tag = die__create_new_tag(die, cu);		break;
 	case DW_TAG_pointer_type:
 		tag = die__create_new_pointer_tag(die, cu, conf);	break;
+	case DW_TAG_unspecified_type:
+		tag = die__create_new_unspecified_type(die, cu, conf); break;
 	case DW_TAG_ptr_to_member_type:
 		tag = die__create_new_ptr_to_member_type(die, cu); break;
 	case DW_TAG_enumeration_type:

dwarves.c

@@ -681,6 +681,7 @@ struct cu *cu__new(const char *name, uint8_t addr_size,
 		cu->dfops	= NULL;
 		INIT_LIST_HEAD(&cu->tags);
 		INIT_LIST_HEAD(&cu->tool_list);
+		INIT_LIST_HEAD(&cu->unspecified_types);
 
 		cu->addr_size = addr_size;
 		cu->extra_dbg_info = 0;

dwarves.h

@@ -265,6 +265,7 @@ struct cu {
 	struct list_head node;
 	struct list_head tags;
 	struct list_head tool_list;	/* To be used by tools such as ctracer */
+	struct list_head unspecified_types;
 	struct ptr_table types_table;
 	struct ptr_table functions_table;
 	struct ptr_table tags_table;
@@ -705,6 +706,22 @@ static inline struct btf_type_tag_type *tag__btf_type_tag(struct tag *tag)
 	return (struct btf_type_tag_type *)tag;
 }
 
+/** struct unspecified_type - representation of DW_TAG_unspecified_type.
+ *
+ *  @name   - DW_AT_name associated with this tag
+ *  @node   - a node for cu::unspecified_types list
+ */
+struct unspecified_type {
+	struct tag		tag;
+	const char		*name;
+	struct list_head	node;
+};
+
+static inline struct unspecified_type *tag__unspecified_type(struct tag *tag)
+{
+	return (struct unspecified_type *)tag;
+}
+
 /** struct namespace - base class for enums, structs, unions, typedefs, etc
  *
  * @tags - class_member, enumerators, etc