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

Introduce a new helper function to retrieve the cgroup ID from a net_cls
cgroup directory.

Signed-off-by: Yafang Shao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/cgroup_helpers.c

···
 }
 
 /**
- * get_cgroup_id() - Get cgroup id for a particular cgroup path
- * @relative_path: The cgroup path, relative to the workdir, to join
+ * get_cgroup_id_from_path - Get cgroup id for a particular cgroup path
+ * @cgroup_workdir: The absolute cgroup path
  *
  * On success, it returns the cgroup id. On failure it returns 0,
  * which is an invalid cgroup id.
  * If there is a failure, it prints the error to stderr.
  */
-unsigned long long get_cgroup_id(const char *relative_path)
+unsigned long long get_cgroup_id_from_path(const char *cgroup_workdir)
 {
 	int dirfd, err, flags, mount_id, fhsize;
 	union {
 		unsigned long long cgid;
 		unsigned char raw_bytes[8];
 	} id;
-	char cgroup_workdir[PATH_MAX + 1];
 	struct file_handle *fhp, *fhp2;
 	unsigned long long ret = 0;
-
-	format_cgroup_path(cgroup_workdir, relative_path);
 
 	dirfd = AT_FDCWD;
 	flags = 0;
···
 free_mem:
 	free(fhp);
 	return ret;
+}
+
+unsigned long long get_cgroup_id(const char *relative_path)
+{
+	char cgroup_workdir[PATH_MAX + 1];
+
+	format_cgroup_path(cgroup_workdir, relative_path);
+	return get_cgroup_id_from_path(cgroup_workdir);
 }
 
 int cgroup_setup_and_join(const char *path) {
···
 	format_classid_path(cgroup_workdir);
 	join_cgroup_from_top(NETCLS_MOUNT_PATH);
 	nftw(cgroup_workdir, nftwfunc, WALK_FD_LIMIT, FTW_DEPTH | FTW_MOUNT);
+}
+
+/**
+ * get_classid_cgroup_id - Get the cgroup id of a net_cls cgroup
+ */
+unsigned long long get_classid_cgroup_id(void)
+{
+	char cgroup_workdir[PATH_MAX + 1];
+
+	format_classid_path(cgroup_workdir);
+	return get_cgroup_id_from_path(cgroup_workdir);
 }

tools/testing/selftests/bpf/cgroup_helpers.h

···
 /* cgroupv1 related */
 int set_classid(void);
 int join_classid(void);
+unsigned long long get_classid_cgroup_id(void);
 
 int setup_classid_environment(void);
 void cleanup_classid_environment(void);