mntns: 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 mount
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-v1-3-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

@@ -48,6 +48,7 @@ enum {
 	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

@@ -50,6 +50,7 @@ enum init_ns_ino {
 	CGROUP_NS_INIT_INO	= 0xEFFFFFFBU,
 	TIME_NS_INIT_INO	= 0xEFFFFFFAU,
 	NET_NS_INIT_INO		= 0xEFFFFFF9U,
+	MNT_NS_INIT_INO		= 0xEFFFFFF8U,
 };
 
 #endif /* __LINUX_NSFS_H */