nsfs: add an ioctl to discover the network namespace cookie

Network namespaces have a globally unique non-zero identifier aka a
cookie, in line with socket cookies. Add an ioctl to retrieve the
cookie from user space without going via BPF.

Cc: linux-api@vger.kernel.org
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>

fs/nsfs.c

@@ -11,6 +11,7 @@
 #include <linux/user_namespace.h>
 #include <linux/nsfs.h>
 #include <linux/uaccess.h>
+#include <net/net_namespace.h>
 
 #include "internal.h"
 
@@ -191,6 +192,8 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 	struct user_namespace *user_ns;
 	struct ns_common *ns = get_proc_ns(file_inode(filp));
 	uid_t __user *argp;
+	struct net *net_ns;
+	u64 cookie;
 	uid_t uid;
 
 	switch (ioctl) {
@@ -209,6 +212,12 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 		argp = (uid_t __user *) arg;
 		uid = from_kuid_munged(current_user_ns(), user_ns->owner);
 		return put_user(uid, argp);
+	case NS_GET_COOKIE:
+		if (ns->ops->type != CLONE_NEWNET)
+			return -EINVAL;
+		net_ns = container_of(ns, struct net, ns);
+		cookie = net_gen_cookie(net_ns);
+		return put_user(cookie, (u64 __user *)arg);
 	default:
 		return -ENOTTY;
 	}

include/net/net_namespace.h

@@ -226,6 +226,17 @@ struct net *get_net_ns_by_fd(int fd);
 
 u64 __net_gen_cookie(struct net *net);
 
+static inline u64 net_gen_cookie(struct net *net)
+{
+	u64 cookie;
+
+	preempt_disable();
+	cookie = __net_gen_cookie(net);
+	preempt_enable();
+
+	return cookie;
+}
+
 #ifdef CONFIG_SYSCTL
 void ipx_register_sysctl(void);
 void ipx_unregister_sysctl(void);

include/uapi/linux/nsfs.h

@@ -15,5 +15,7 @@
 #define NS_GET_NSTYPE		_IO(NSIO, 0x3)
 /* Get owner UID (in the caller's user namespace) for a user namespace */
 #define NS_GET_OWNER_UID	_IO(NSIO, 0x4)
+/* Returns a unique non-zero identifier for a network namespace */
+#define NS_GET_COOKIE		_IO(NSIO, 0x5)
 
 #endif /* __LINUX_NSFS_H */