cgroups: handle fallback gracefully

src/lxc/cgroups/cgfsng.c

@@ -3458,7 +3458,7 @@ int cgroup_get(const char *name, const char *lxcpath,
 
 	unified_fd = lxc_cmd_get_limiting_cgroup2_fd(name, lxcpath);
 	if (unified_fd < 0)
-		return ret_errno(ENOCGROUP2);
+		return ret_errno(ENOSYS);
 
 	ret = lxc_read_try_buf_at(unified_fd, filename, buf, len);
 	if (ret < 0)
@@ -3480,7 +3480,7 @@ int cgroup_set(const char *name, const char *lxcpath,
 
 	unified_fd = lxc_cmd_get_limiting_cgroup2_fd(name, lxcpath);
 	if (unified_fd < 0)
-		return ret_errno(ENOCGROUP2);
+		return ret_errno(ENOSYS);
 
 	if (strnequal(filename, "devices.", STRLITERALLEN("devices."))) {
 		struct device_item device = {};

src/lxc/lxccontainer.c

@@ -3290,7 +3290,7 @@ static bool do_lxcapi_set_cgroup_item(struct lxc_container *c, const char *subsy
 		return false;
 
 	ret = cgroup_set(c->name, c->config_path, subsys, value);
-	if (ret == -ENOCGROUP2) {
+	if (ret < 0 && ERRNO_IS_NOT_SUPPORTED(ret)) {
 		cgroup_ops = cgroup_init(c->lxc_conf);
 		if (!cgroup_ops)
 			return false;
@@ -3315,7 +3315,7 @@ static int do_lxcapi_get_cgroup_item(struct lxc_container *c, const char *subsys
 		return -1;
 
 	ret = cgroup_get(c->name, c->config_path, subsys, retv, inlen);
-	if (ret == -ENOCGROUP2) {
+	if (ret < 0 && ERRNO_IS_NOT_SUPPORTED(ret)) {
 		cgroup_ops = cgroup_init(c->lxc_conf);
 		if (!cgroup_ops)
 			return -1;