Merge patch series "nsfs: expose the stable inode numbers in a public header"

Christian Brauner <brauner@kernel.org> says:

Userspace heavily relies on the root inode numbers for namespaces to
identify the initial namespaces. That's already a hard dependency. So we
cannot change that anymore. Move the initial inode numbers to a public
header and align the only two namespaces that currently don't do that
with all the other namespaces.

* patches from https://lore.kernel.org/20250606-work-nsfs-v1-0-b8749c9a8844@kernel.org:
  mntns: use stable inode number for initial mount ns
  netns: use stable inode number for initial mount ns
  nsfs: move root inode number to uapi

Link: https://lore.kernel.org/20250606-work-nsfs-v1-0-b8749c9a8844@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/namespace.c

@@ -6203,9 +6203,11 @@ static void __init init_mount_tree(void)
 	if (IS_ERR(mnt))
 		panic("Can't create rootfs");
 
-	ns = alloc_mnt_ns(&init_user_ns, false);
+	ns = alloc_mnt_ns(&init_user_ns, true);
 	if (IS_ERR(ns))
 		panic("Can't allocate initial namespace");
+	ns->seq = atomic64_inc_return(&mnt_ns_seq);
+	ns->ns.inum = PROC_MNT_INIT_INO;
 	m = real_mount(mnt);
 	ns->root = m;
 	ns->nr_mounts = 1;

include/linux/proc_ns.h

@@ -6,6 +6,7 @@
 #define _LINUX_PROC_NS_H
 
 #include <linux/ns_common.h>
+#include <uapi/linux/nsfs.h>
 
 struct pid_namespace;
 struct nsset;
@@ -40,12 +41,14 @@ extern const struct proc_ns_operations timens_for_children_operations;
  */
 enum {
 	PROC_ROOT_INO		= 1,
-	PROC_IPC_INIT_INO	= 0xEFFFFFFFU,
-	PROC_UTS_INIT_INO	= 0xEFFFFFFEU,
-	PROC_USER_INIT_INO	= 0xEFFFFFFDU,
-	PROC_PID_INIT_INO	= 0xEFFFFFFCU,
-	PROC_CGROUP_INIT_INO	= 0xEFFFFFFBU,
-	PROC_TIME_INIT_INO	= 0xEFFFFFFAU,
+	PROC_IPC_INIT_INO	= IPC_NS_INIT_INO,
+	PROC_UTS_INIT_INO	= UTS_NS_INIT_INO,
+	PROC_USER_INIT_INO	= USER_NS_INIT_INO,
+	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,
+	PROC_MNT_INIT_INO	= MNT_NS_INIT_INO,
 };
 
 #ifdef CONFIG_PROC_FS

include/uapi/linux/nsfs.h

@@ -42,4 +42,15 @@ struct mnt_ns_info {
 /* Get previous namespace. */
 #define NS_MNT_GET_PREV		_IOR(NSIO, 12, struct mnt_ns_info)
 
+enum init_ns_ino {
+	IPC_NS_INIT_INO		= 0xEFFFFFFFU,
+	UTS_NS_INIT_INO		= 0xEFFFFFFEU,
+	USER_NS_INIT_INO	= 0xEFFFFFFDU,
+	PID_NS_INIT_INO		= 0xEFFFFFFCU,
+	CGROUP_NS_INIT_INO	= 0xEFFFFFFBU,
+	TIME_NS_INIT_INO	= 0xEFFFFFFAU,
+	NET_NS_INIT_INO		= 0xEFFFFFF9U,
+	MNT_NS_INIT_INO		= 0xEFFFFFF8U,
+};
+
 #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);
 }