bpf: use type_may_be_null() helper for nullable-param check

Commit 980ca8c ("bpf: check bpf_dummy_struct_ops program params for
test runs") does bitwise AND between reg_type and PTR_MAYBE_NULL, which
is correct, but due to type difference the compiler complains:

  net/bpf/bpf_dummy_struct_ops.c:118:31: warning: bitwise operation between different enumeration types ('const enum bpf_reg_type' and 'enum bpf_type_flag') [-Wenum-enum-conversion]
    118 |                 if (info && (info->reg_type & PTR_MAYBE_NULL))
        |                              ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~

Workaround the warning by moving the type_may_be_null() helper from
verifier.c into bpf_verifier.h, and reuse it here to check whether param
is nullable.

Fixes: 980ca8c ("bpf: check bpf_dummy_struct_ops program params for test runs")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404241956.HEiRYwWq-lkp@intel.com/
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240905055233.70203-1-shung-hsi.yu@suse.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: shunghsiyu

include/linux/bpf_verifier.h

@@ -927,6 +927,11 @@ static inline bool type_is_sk_pointer(enum bpf_reg_type type)
 		type == PTR_TO_XDP_SOCK;
 }
 
+static inline bool type_may_be_null(u32 type)
+{
+	return type & PTR_MAYBE_NULL;
+}
+
 static inline void mark_reg_scratched(struct bpf_verifier_env *env, u32 regno)
 {
 	env->scratched_regs |= 1U << regno;

kernel/bpf/verifier.c

@@ -383,11 +383,6 @@ static void verbose_invalid_scalar(struct bpf_verifier_env *env,
 	verbose(env, " should have been in [%d, %d]\n", range.minval, range.maxval);
 }
 
-static bool type_may_be_null(u32 type)
-{
-	return type & PTR_MAYBE_NULL;
-}
-
 static bool reg_not_null(const struct bpf_reg_state *reg)
 {
 	enum bpf_reg_type type;

net/bpf/bpf_dummy_struct_ops.c

@@ -115,7 +115,7 @@ static int check_test_run_args(struct bpf_prog *prog, struct bpf_dummy_ops_test_
 
 		offset = btf_ctx_arg_offset(bpf_dummy_ops_btf, func_proto, arg_no);
 		info = find_ctx_arg_info(prog->aux, offset);
-		if (info && (info->reg_type & PTR_MAYBE_NULL))
+		if (info && type_may_be_null(info->reg_type))
 			continue;
 
 		return -EINVAL;