bpf: Refactor NULL-ness check in check_reg_type().

check_reg_type() unconditionally disallows PTR_TO_BTF_ID | PTR_MAYBE_NULL.
It's problematic for helpers that allow ARG_PTR_TO_BTF_ID_OR_NULL like
bpf_sk_storage_get(). Allow passing PTR_TO_BTF_ID | PTR_MAYBE_NULL into such
helpers. That technically includes bpf_kptr_xchg() helper, but in practice:
  bpf_kptr_xchg(..., bpf_cpumask_create());
is still disallowed because bpf_cpumask_create() returns ref counted pointer
with ref_obj_id > 0.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230404045029.82870-6-alexei.starovoitov@gmail.com

kernel/bpf/verifier.c

@@ -7168,6 +7168,8 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BTF_ID | PTR_TRUSTED:
 	case PTR_TO_BTF_ID | MEM_RCU:
+	case PTR_TO_BTF_ID | PTR_MAYBE_NULL:
+	case PTR_TO_BTF_ID | PTR_MAYBE_NULL | MEM_RCU:
 	{
 		/* For bpf_sk_release, it needs to match against first member
 		 * 'struct sock_common', hence make an exception for it. This
@@ -7176,6 +7178,12 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 		bool strict_type_match = arg_type_is_release(arg_type) &&
 					 meta->func_id != BPF_FUNC_sk_release;
 
+		if (type_may_be_null(reg->type) &&
+		    (!type_may_be_null(arg_type) || arg_type_is_release(arg_type))) {
+			verbose(env, "Possibly NULL pointer passed to helper arg%d\n", regno);
+			return -EACCES;
+		}
+
 		if (!arg_btf_id) {
 			if (!compatible->btf_id) {
 				verbose(env, "verifier internal error: missing arg compatible BTF ID\n");
@@ -7206,10 +7214,6 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 		}
 		break;
 	}
-	case PTR_TO_BTF_ID | PTR_MAYBE_NULL:
-	case PTR_TO_BTF_ID | PTR_MAYBE_NULL | MEM_RCU:
-		verbose(env, "Possibly NULL pointer passed to helper arg%d\n", regno);
-		return -EACCES;
 	case PTR_TO_BTF_ID | MEM_ALLOC:
 		if (meta->func_id != BPF_FUNC_spin_lock && meta->func_id != BPF_FUNC_spin_unlock &&
 		    meta->func_id != BPF_FUNC_kptr_xchg) {