bpf: Let callers of btf_parse_kptr() track life cycle of prog btf

btf_parse_kptr() and btf_record_free() do btf_get() and btf_put()
respectively when working on btf_record in program and map if there are
kptr fields. If the kptr is from program BTF, since both callers has
already tracked the life cycle of program BTF, it is safe to remove the
btf_get() and btf_put().

This change prevents memory leak of program BTF later when we start
searching for kptr fields when building btf_record for program. It can
happen when the btf fd is closed. The btf_put() corresponding to the
btf_get() in btf_parse_kptr() was supposed to be called by
btf_record_free() in btf_free_struct_meta_tab() in btf_free(). However,
it will never happen since the invocation of btf_free() depends on the
refcount of the btf to become 0 in the first place.
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Amery Hung <amery.hung@bytedance.com>
Link: https://lore.kernel.org/r/20240813212424.2871455-2-amery.hung@bytedance.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -3754,6 +3754,7 @@ static int btf_find_field(const struct btf *btf, const struct btf_type *t,
 	return -EINVAL;
 }
 
+/* Callers have to ensure the life cycle of btf if it is program BTF */
 static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
 			  struct btf_field_info *info)
 {
@@ -3782,7 +3783,6 @@ static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
 		field->kptr.dtor = NULL;
 		id = info->kptr.type_id;
 		kptr_btf = (struct btf *)btf;
-		btf_get(kptr_btf);
 		goto found_dtor;
 	}
 	if (id < 0)

kernel/bpf/syscall.c

@@ -550,6 +550,7 @@ void btf_record_free(struct btf_record *rec)
 		case BPF_KPTR_PERCPU:
 			if (rec->fields[i].kptr.module)
 				module_put(rec->fields[i].kptr.module);
+			if (btf_is_kernel(rec->fields[i].kptr.btf))
 				btf_put(rec->fields[i].kptr.btf);
 			break;
 		case BPF_LIST_HEAD:
@@ -596,6 +597,7 @@ struct btf_record *btf_record_dup(const struct btf_record *rec)
 		case BPF_KPTR_UNREF:
 		case BPF_KPTR_REF:
 		case BPF_KPTR_PERCPU:
+			if (btf_is_kernel(fields[i].kptr.btf))
 				btf_get(fields[i].kptr.btf);
 			if (fields[i].kptr.module && !try_module_get(fields[i].kptr.module)) {
 				ret = -ENXIO;