One quick review note:

On Fri, Mar 07, 2025 at 12:57:35AM +0000, Pratyush Yadav wrote:

+/**
+ * struct fdbox - A box of FDs.
+ * @name: Name of the box. Must be unique.
+ * @rwsem: Used to ensure exclusive access to the box during SEAL/UNSEAL
+ *         operations.
+ * @dev: Backing device for the character device.
+ * @cdev: Character device which accepts ioctls from userspace.

You now have a structure that contains 2 different reference counts,
which is going to be impossible to handle properly.  Which one defines
the lifetime of the object?  That's not going to work, please fix.

thanks,

greg k-h