ovl: check type and offset of struct vfsmount in ovl_entry

Porting overlayfs to the new amount api I started experiencing random
crashes that couldn't be explained easily. So after much debugging and
reasoning it became clear that struct ovl_entry requires the point to
struct vfsmount to be the first member and of type struct vfsmount.

During the port I added a new member at the beginning of struct
ovl_entry which broke all over the place in the form of random crashes
and cache corruptions. While there's a comment in ovl_free_fs() to the
effect of "Hack! Reuse ofs->layers as a vfsmount array before freeing
it" there's no such comment on struct ovl_entry which makes this easy to
trip over.

Add a comment and two static asserts for both the offset and the type of
pointer in struct ovl_entry.
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

fs/overlayfs/ovl_entry.h

@@ -32,6 +32,7 @@ struct ovl_sb {
 };
 
 struct ovl_layer {
+	/* ovl_free_fs() relies on @mnt being the first member! */
 	struct vfsmount *mnt;
 	/* Trap in ovl inode cache */
 	struct inode *trap;
@@ -42,6 +43,14 @@ struct ovl_layer {
 	int fsid;
 };
 
+/*
+ * ovl_free_fs() relies on @mnt being the first member when unmounting
+ * the private mounts created for each layer. Let's check both the
+ * offset and type.
+ */
+static_assert(offsetof(struct ovl_layer, mnt) == 0);
+static_assert(__same_type(typeof_member(struct ovl_layer, mnt), struct vfsmount *));
+
 struct ovl_path {
 	const struct ovl_layer *layer;
 	struct dentry *dentry;