linux-kernel - less size_t please (was Re: [PATCH net] xfrm: fix integer overflow in xfrm_replay_state_esn

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Message-ID: <03997448-cd88-4b80-ab85-fe1100203339@p183>
Date: Thu, 30 Jan 2025 16:44:42 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: Dan Carpenter <dan.carpenter@...aro.org>
Cc: Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: less size_t please (was Re: [PATCH net] xfrm: fix integer overflow
 in xfrm_replay_state_esn_len())

> -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;