[PATCH bpf-next v3 2/4] nsfs: add an ioctl to discover the network namespace cookie

[Lorenz Bauer <lmb@cloudflare.com>]

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.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
---
 fs/nsfs.c                 | 7 +++++++
 include/uapi/linux/nsfs.h | 2 ++
 2 files changed, 9 insertions(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 800c1d0eb0d0..48198a1b1685 100644
--- a/fs/nsfs.c
+++ b/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 +192,7 @@ 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;
 	uid_t uid;
 
 	switch (ioctl) {
@@ -209,6 +211,11 @@ 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);
+		return put_user(net_ns->net_cookie, (u64 __user *)arg);
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index a0c8552b64ee..86611c2cf908 100644
--- a/include/uapi/linux/nsfs.h
+++ b/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 */
-- 
2.27.0