Allow kthreads to create a private mount namespace.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c        | 30 ++++++++++++++++++++++++++++++
 include/linux/mount.h |  1 +
 2 files changed, 31 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 854f4fc66469..e23d2fa7e255 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -6200,6 +6200,36 @@ static void __init init_mount_tree(void)
 	ns_tree_add(&init_mnt_ns);
 }
 
+/*
+ * Allow to give a specific kthread a private mount namespace anchored
+ * in the userspace nullfs (mount id 1) so it can mount.
+ */
+int __init kthread_mntns(void)
+{
+	struct mount *m;
+	struct path root;
+	int ret;
+
+	/* Only allowed for kthreads in the initial mount namespace. */
+	VFS_WARN_ON_ONCE(!(current->flags & PF_KTHREAD));
+	VFS_WARN_ON_ONCE(current->nsproxy->mnt_ns != &init_mnt_ns);
+
+	/*
+	 * TODO: switch to creating a completely empty mount namespace
+	 * once that series lands.
+	 */
+	ret = ksys_unshare(CLONE_NEWNS);
+	if (ret)
+		return ret;
+
+	m = current->nsproxy->mnt_ns->root;
+	root.mnt = &m->mnt;
+	root.dentry = root.mnt->mnt_root;
+	set_fs_pwd(current->fs, &root);
+	set_fs_root(current->fs, &root);
+	return 0;
+}
+
 void __init mnt_init(void)
 {
 	int err;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index acfe7ef86a1b..69d61f21b548 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -106,6 +106,7 @@ int do_mount(const char *, const char __user *,
 extern const struct path *collect_paths(const struct path *, struct path *, unsigned);
 extern void drop_collected_paths(const struct path *, const struct path *);
 extern void kern_unmount_array(struct vfsmount *mnt[], unsigned int num);
+int __init kthread_mntns(void);
 
 extern int cifs_root_data(char **dev, char **opts);
 

-- 
2.47.3