less size_t please (was Re: [PATCH net] xfrm: fix integer overflow in xfrm_replay_state_esn_len())

[Alexey Dobriyan <adobriyan@gmail.com>]

> -static inline unsigned int xfrm_replay_state_esn_len(struct xfrm_replay_state_esn *replay_esn)
> +static inline size_t xfrm_replay_state_esn_len(struct xfrm_replay_state_esn *replay_esn)
>  {
> -	return sizeof(*replay_esn) + replay_esn->bmp_len * sizeof(__u32);
> +	return size_add(sizeof(*replay_esn), size_mul(replay_esn->bmp_len, sizeof(__u32)));

Please don't do this.

You can (and should!) make calculations and check for overflow at the
same time. It's very efficient.

> 1) Use size_add() and size_mul().  This change is necessary for 32bit systems.

This bloats code on 32-bit.

	int len;
	if (__builtin_mul_overflow(replay_esn->bmp_len, 4, &len)) {
		return true;
	}
	if (__builtin_add_overflow(len, sizeof(*replay_esn), &len)) {
		return true;
	}
	*plen = len;
	return false;