[PATCH v2 dwarves] btf_loader: handle union forward declaration properly

[Andrii Nakryiko <andrii@xxxxxxxxxx>]

Differentiate between struct and union forwards in BTF. BTF type representing
forward declaration (BTF_KIND_FWD) encodes whether the forward declaration is
for struct or union type by using kflag. So use that kflag to create a proper
internal representation of forward declaration.

Tested with btfdiff on vmlinux:

$ PAHOLE=build/pahole ./btfdiff ~/linux-build/default/vmlinux | wc -l
0

Also tested manually:

$ cat test.c
struct struct_fwd;

union union_fwd;

struct s {
        struct struct_fwd *f1;
        union union_fwd *f2;
};

int func(struct s *s) {
        return !!s->f1 + !!s->f2;
}
$ clang -g -target bpf -c test.c -o test.o
$ bpftool btf dump file test.o | grep fwd_kind
[6] FWD 'struct_fwd' fwd_kind=struct
[8] FWD 'union_fwd' fwd_kind=union
$ build/pahole -F btf test.o
struct s {
        struct struct_fwd *        f1;                   /*     0     8 */
        union union_fwd *          f2;                   /*     8     8 */

        /* size: 16, cachelines: 1, members: 2 */
        /* last cacheline: 16 bytes */
};

Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 btf_loader.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/btf_loader.c b/btf_loader.c
index 9b5da3a4997a..6ea207ea65b4 100644
--- a/btf_loader.c
+++ b/btf_loader.c
@@ -134,12 +134,13 @@ static struct type *type__new(uint16_t tag, strings_t name, size_t size)
 	return type;
 }
 
-static struct class *class__new(strings_t name, size_t size)
+static struct class *class__new(strings_t name, size_t size, bool is_union)
 {
 	struct class *class = tag__alloc(sizeof(*class));
+	uint32_t tag = is_union ? DW_TAG_union_type : DW_TAG_structure_type;
 
 	if (class != NULL) {
-		type__init(&class->type, DW_TAG_structure_type, name, size);
+		type__init(&class->type, tag, name, size);
 		INIT_LIST_HEAD(&class->vtable);
 	}
 
@@ -228,7 +229,7 @@ static int create_members(struct btf_elf *btfe, const struct btf_type *tp,
 
 static int create_new_class(struct btf_elf *btfe, const struct btf_type *tp, uint32_t id)
 {
-	struct class *class = class__new(tp->name_off, tp->size);
+	struct class *class = class__new(tp->name_off, tp->size, false);
 	int member_size = create_members(btfe, tp, &class->type);
 
 	if (member_size < 0)
@@ -313,7 +314,7 @@ static int create_new_subroutine_type(struct btf_elf *btfe, const struct btf_typ
 
 static int create_new_forward_decl(struct btf_elf *btfe, const struct btf_type *tp, uint32_t id)
 {
-	struct class *fwd = class__new(tp->name_off, 0);
+	struct class *fwd = class__new(tp->name_off, 0, btf_kflag(tp));
 
 	if (fwd == NULL)
 		return -ENOMEM;
-- 
2.24.1