[PATCH 04/13] vfs: introduce clone_private_mount()

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@xxxxxxx>

Overlayfs needs a private clone of the mount, so create a function for
this and export to modules.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---
 fs/namespace.c        | 27 +++++++++++++++++++++++++++
 include/linux/mount.h |  3 +++
 2 files changed, 30 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 182bc41cd887..9ad57948f527 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1550,6 +1550,33 @@ void drop_collected_mounts(struct vfsmount *mnt)
 	namespace_unlock();
 }
 
+/**
+ * clone_private_mount - create a private clone of a path
+ *
+ * This creates a new vfsmount, which will be the clone of @path.  The new will
+ * not be attached anywhere in the namespace and will be private (i.e. changes
+ * to the originating mount won't be propagated into this).
+ *
+ * Release with mntput().
+ */
+struct vfsmount *clone_private_mount(struct path *path)
+{
+	struct mount *old_mnt = real_mount(path->mnt);
+	struct mount *new_mnt;
+
+	if (IS_MNT_UNBINDABLE(old_mnt))
+		return ERR_PTR(-EINVAL);
+
+	down_read(&namespace_sem);
+	new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
+	up_read(&namespace_sem);
+	if (IS_ERR(new_mnt))
+		return ERR_CAST(new_mnt);
+
+	return &new_mnt->mnt;
+}
+EXPORT_SYMBOL_GPL(clone_private_mount);
+
 int iterate_mounts(int (*f)(struct vfsmount *, void *), void *arg,
 		   struct vfsmount *root)
 {
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 839bac270904..31617fcfac8c 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -74,6 +74,9 @@ extern void mnt_pin(struct vfsmount *mnt);
 extern void mnt_unpin(struct vfsmount *mnt);
 extern int __mnt_is_readonly(struct vfsmount *mnt);
 
+struct path;
+extern struct vfsmount *clone_private_mount(struct path *path);
+
 struct file_system_type;
 extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
 				      int flags, const char *name,
-- 
1.8.1.4

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