netns: use stable inode number for initial mount ns

Apart from the network and mount namespace all other namespaces expose a
stable inode number and userspace has been relying on that for a very
long time now. It's very much heavily used API. Align the network
namespace and use a stable inode number from the reserved procfs inode
number space so this is consistent across all namespaces.

Link: https://lore.kernel.org/20250606-work-nsfs-v1-2-b8749c9a8844@kernel.org
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

include/linux/proc_ns.h

@@ -47,6 +47,7 @@ enum {
 	PROC_PID_INIT_INO	= PID_NS_INIT_INO,
 	PROC_CGROUP_INIT_INO	= CGROUP_NS_INIT_INO,
 	PROC_TIME_INIT_INO	= TIME_NS_INIT_INO,
+	PROC_NET_INIT_INO	= NET_NS_INIT_INO,
 };
 
 #ifdef CONFIG_PROC_FS

include/uapi/linux/nsfs.h

@@ -49,6 +49,7 @@ enum init_ns_ino {
 	PID_NS_INIT_INO		= 0xEFFFFFFCU,
 	CGROUP_NS_INIT_INO	= 0xEFFFFFFBU,
 	TIME_NS_INIT_INO	= 0xEFFFFFFAU,
+	NET_NS_INIT_INO		= 0xEFFFFFF9U,
 };
 
 #endif /* __LINUX_NSFS_H */

net/core/net_namespace.c

@@ -796,11 +796,19 @@ static __net_init int net_ns_net_init(struct net *net)
 #ifdef CONFIG_NET_NS
 	net->ns.ops = &netns_operations;
 #endif
+	if (net == &init_net) {
+		net->ns.inum = PROC_NET_INIT_INO;
+		return 0;
+	}
 	return ns_alloc_inum(&net->ns);
 }
 
 static __net_exit void net_ns_net_exit(struct net *net)
 {
+	/*
+	 * Initial network namespace doesn't exit so we don't need any
+	 * special checks here.
+	 */
 	ns_free_inum(&net->ns);
 }