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@xxxxxxxxxx>
---
fs/overlayfs/ovl_entry.h | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index c6c7d09b494e..e5207c4bf5b8 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/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;
--
2.34.1
