selftests/bpf: Add a new cgroup helper open_classid()

This new helper allows us to obtain the fd of a net_cls cgroup, which will
be utilized in the subsequent patch.
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20231206115326.4295-3-laoar.shao@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -689,3 +689,19 @@ int get_cgroup1_hierarchy_id(const char *subsys_name)
 	fclose(file);
 	return found ? id : -1;
 }
+
+/**
+ * open_classid() - Open a cgroupv1 net_cls classid
+ *
+ * This function expects the cgroup work dir to be already created, as we
+ * open it here.
+ *
+ * On success, it returns the file descriptor. On failure it returns -1.
+ */
+int open_classid(void)
+{
+	char cgroup_workdir[PATH_MAX + 1];
+
+	format_classid_path(cgroup_workdir);
+	return open(cgroup_workdir, O_RDONLY);
+}

tools/testing/selftests/bpf/cgroup_helpers.h

@@ -33,6 +33,7 @@ void cleanup_cgroup_environment(void);
 int set_classid(void);
 int join_classid(void);
 unsigned long long get_classid_cgroup_id(void);
+int open_classid(void);
 
 int setup_classid_environment(void);
 void cleanup_classid_environment(void);