selftests/bpf: Add cgroup helper remove_cgroup()

Add remove_cgroup() to remove a cgroup which doesn't have any children
or live processes. It will be used by the following patch to test cgroup
iterator on a dead cgroup.

Signed-off-by: Hou Tao <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/cgroup_helpers.c

···
 	return fd;
 }
 
+/*
+ * remove_cgroup() - Remove a cgroup
+ * @relative_path: The cgroup path, relative to the workdir, to remove
+ *
+ * This function expects a cgroup to already be created, relative to the cgroup
+ * work dir. It also expects the cgroup doesn't have any children or live
+ * processes and it removes the cgroup.
+ *
+ * On failure, it will print an error to stderr.
+ */
+void remove_cgroup(const char *relative_path)
+{
+	char cgroup_path[PATH_MAX + 1];
+
+	format_cgroup_path(cgroup_path, relative_path);
+	if (rmdir(cgroup_path))
+		log_err("rmdiring cgroup %s .. %s", relative_path, cgroup_path);
+}
+
 /**
  * create_and_get_cgroup() - Create a cgroup, relative to workdir, and get the FD
  * @relative_path: The cgroup path, relative to the workdir, to join

tools/testing/selftests/bpf/cgroup_helpers.h

···
 int cgroup_setup_and_join(const char *relative_path);
 int get_root_cgroup(void);
 int create_and_get_cgroup(const char *relative_path);
+void remove_cgroup(const char *relative_path);
 unsigned long long get_cgroup_id(const char *relative_path);
 
 int join_cgroup(const char *relative_path);