diff --git a/fs/nsfs.c b/fs/nsfs.c
index 8f20d60..bfd5bde 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -8,8 +8,20 @@
 
 static struct vfsmount *nsfs_mnt;
 
+static void show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct dentry *dentry = f->f_path.dentry;
+	struct inode *inode = d_inode(dentry);
+	const struct proc_ns_operations *ns_ops = dentry->d_fsdata;
+	struct ns_common *ns = inode->i_private;
+
+	if (ns_ops->show_fdinfo)
+		ns_ops->show_fdinfo(m, ns);
+}
+
 static const struct file_operations ns_file_operations = {
 	.llseek		= no_llseek,
+	.show_fdinfo	= show_fdinfo,
 };
 
 static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)
diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index de0e771..fed276b 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h
@@ -18,6 +18,7 @@ struct proc_ns_operations {
 	struct ns_common *(*get)(struct task_struct *task);
 	void (*put)(struct ns_common *ns);
 	int (*install)(struct nsproxy *nsproxy, struct ns_common *ns);
+	void (*show_fdinfo)(struct seq_file *m, struct ns_common *ns);
 };
 
 extern const struct proc_ns_operations netns_operations;
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index a65ba13..910b388 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -388,12 +388,20 @@ static int pidns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return 0;
 }
 
+static void pidns_fdinfo(struct seq_file *m, struct ns_common *ns)
+{
+	struct pid_namespace *pidns = to_pid_ns(ns);
+
+	seq_printf(m, "userns: %u\n", pidns->user_ns->ns.inum);
+}
+
 const struct proc_ns_operations pidns_operations = {
 	.name		= "pid",
 	.type		= CLONE_NEWPID,
 	.get		= pidns_get,
 	.put		= pidns_put,
 	.install	= pidns_install,
+	.show_fdinfo	= pidns_fdinfo,
 };
 
 static __init int pid_namespaces_init(void)