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..668131aa5de1 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 that is
+ * anchored in nullfs 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

Christian Brauner <brauner@kernel.org>:
> + * Allow to give a specific kthread a private mount namespace that is
> + * anchored in nullfs so it can mount.

Which nullfs? By the end of this patchset we have two ones: the one in the root
of namespace of every userspace task, and the one used by kernel threads.

You meant userspace one here. (Similar problem may apply to other patches
in this patchset.)

-- 
Askar Safin