Point init_task's fs_struct (root and pwd) at a private nullfs instance
instead of the mutable rootfs. All kthreads now start isolated in nullfs
and must use scoped_with_init_fs() for any path resolution.

PID 1 is moved from nullfs into the initramfs by init_userspace_fs().
Usermodehelper threads use userspace_init_fs via the umh flag in
copy_fs(). All subsystems that need init's filesystem state for path
resolution already use scoped_with_init_fs() from earlier commits in
this series.

This isolates kthreads from userspace filesystem state and makes it
hard to perform filesystem operations from kthread context.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 668131aa5de1..2a530109eb36 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -6188,12 +6188,14 @@ static void __init init_mount_tree(void)
 		init_mnt_ns.nr_mounts++;
 	}
 
+	nullfs_mnt = kern_mount(&nullfs_fs_type);
+	if (IS_ERR(nullfs_mnt))
+		panic("VFS: Failed to create private nullfs instance");
+	root.mnt	= nullfs_mnt;
+	root.dentry	= nullfs_mnt->mnt_root;
+
 	init_task.nsproxy->mnt_ns = &init_mnt_ns;
 	get_mnt_ns(&init_mnt_ns);
-
-	/* The root and pwd always point to the mutable rootfs. */
-	root.mnt	= mnt;
-	root.dentry	= mnt->mnt_root;
 	set_fs_pwd(current->fs, &root);
 	set_fs_root(current->fs, &root);
 
-- 
2.47.3