libbpf: Fix a couple of missed btf_type_tag handling in btf.c

Commit 2dc1e48 ("libbpf: Support BTF_KIND_TYPE_TAG") added the
BTF_KIND_TYPE_TAG support. But to test vmlinux build with ...

  #define __user __attribute__((btf_type_tag("user")))

... I needed to sync libbpf repo and manually copy libbpf sources to
pahole. To simplify process, I used BTF_KIND_RESTRICT to simulate the
BTF_KIND_TYPE_TAG with vmlinux build as "restrict" modifier is barely
used in kernel.

But this approach missed one case in dedup with structures where
BTF_KIND_RESTRICT is handled and BTF_KIND_TYPE_TAG is not handled in
btf_dedup_is_equiv(), and this will result in a pahole dedup failure.
This patch fixed this issue and a selftest is added in the subsequent
patch to test this scenario.

The other missed handling is in btf__resolve_size(). Currently the compiler
always emit like PTR->TYPE_TAG->... so in practice we don't hit the missing
BTF_KIND_TYPE_TAG handling issue with compiler generated code. But lets
add case BTF_KIND_TYPE_TAG in the switch statement to be future proof.

Fixes: 2dc1e48 ("libbpf: Support BTF_KIND_TYPE_TAG")
Signed-off-by: Yonghong Song <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

tools/lib/bpf/btf.c

@@ -610,6 +610,7 @@ __s64 btf__resolve_size(const struct btf *btf, __u32 type_id)
  case BTF_KIND_RESTRICT:
  case BTF_KIND_VAR:
  case BTF_KIND_DECL_TAG:
+ case BTF_KIND_TYPE_TAG:
  type_id = t->type;
  break;
  case BTF_KIND_ARRAY:
@@ -4023,6 +4024,7 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32 cand_id,
  case BTF_KIND_PTR:
  case BTF_KIND_TYPEDEF:
  case BTF_KIND_FUNC:
+ case BTF_KIND_TYPE_TAG:
  if (cand_type->info != canon_type->info)
  return 0;
  return btf_dedup_is_equiv(d, cand_type->type, canon_type->type);