Re: [PATCH v2 4/4] serial: Add kserial_rs485 to avoid wasted space due to .padding

[Greg Kroah-Hartman]

On Tue, Aug 30, 2022 at 10:29:56AM +0300, Ilpo Järvinen wrote:
> -static int serial_rs485_from_user(struct serial_rs485 *rs485,
> +static int serial_rs485_from_user(struct kserial_rs485 *rs485,
>  				  const struct serial_rs485 __user *rs485_user)
>  {
> -	if (copy_from_user(rs485, rs485_user, sizeof(*rs485)))
> +	struct serial_rs485 rs485_uapi;
> +
> +	if (copy_from_user(&rs485_uapi, rs485_user, sizeof(*rs485)))
>  		return -EFAULT;
>  
> +	*rs485 = *((struct kserial_rs485 *)&rs485_uapi);

Ah, you are mapping this on top of the existing structure, so there was
no padding in the original one, why say that?

> +/*
> + * Compile-time asserts for struct kserial_rs485 and struct serial_rs485 equality
> + * (except padding).

This does not take into account any padding, in fact it's the opposite
as all of this:

> + */
> +static_assert(offsetof(struct kserial_rs485, flags) ==
> +	      offsetof(struct serial_rs485, flags));
> +static_assert(offsetof(struct kserial_rs485, delay_rts_before_send) ==
> +	      offsetof(struct serial_rs485, delay_rts_before_send));
> +static_assert(offsetof(struct kserial_rs485, delay_rts_after_send) ==
> +	      offsetof(struct serial_rs485, delay_rts_after_send));
> +static_assert(offsetof(struct kserial_rs485, addr_recv) ==
> +	      offsetof(struct serial_rs485, addr_recv));
> +static_assert(offsetof(struct kserial_rs485, addr_dest) ==
> +	      offsetof(struct serial_rs485, addr_dest));
> +static_assert(sizeof(struct kserial_rs485) <= sizeof(struct serial_rs485));

Is there to ensure that the offsets are exactly the same, no padding
involved anywhere.

So I don't understand the problem you are trying to solve here,

greg k-h