Re: [PATCH nf-next v4 00/10] netfilter: nft_bitwise: shift support

[Jeremy Sowden <jeremy@azazel.net>]

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On 2020-01-26, at 12:12:51 +0100, Pablo Neira Ayuso wrote:
> I've been looking into (ab)using bitwise to implement add/sub. I would
> like to not add nft_arith for only this, and it seems to me much of
> your code can be reused.
>
> Do you think something like this would work?

Absolutely.

A couple of questions.  What's the use-case?  I find the combination of
applying the delta to every u32 and having a carry curious.  Do you want
to support bigendian arithmetic (i.e., carrying to the left) as well?

I've suggested a couple of changes below.

J.

> Thanks.
>
> diff --git a/include/uapi/linux/netfilter/nf_tables.h
> b/include/uapi/linux/netfilter/nf_tables.h
> index 065218a20bb7..c4078359b6e4 100644
> --- a/include/uapi/linux/netfilter/nf_tables.h
> +++ b/include/uapi/linux/netfilter/nf_tables.h
> @@ -508,11 +508,15 @@ enum nft_immediate_attributes {
>   *                    XOR boolean operations
>   * @NFT_BITWISE_LSHIFT: left-shift operation
>   * @NFT_BITWISE_RSHIFT: right-shift operation
> + * @NFT_BITWISE_ADD: add operation
> + * @NFT_BITWISE_SUB: subtract operation
>   */
>  enum nft_bitwise_ops {
>  	NFT_BITWISE_BOOL,
>  	NFT_BITWISE_LSHIFT,
>  	NFT_BITWISE_RSHIFT,
> +	NFT_BITWISE_ADD,
> +	NFT_BITWISE_SUB,
>  };
>
>  /**
> diff --git a/net/netfilter/nft_bitwise.c b/net/netfilter/nft_bitwise.c
> index 0ed2281f03be..fd0cd2b4722a 100644
> --- a/net/netfilter/nft_bitwise.c
> +++ b/net/netfilter/nft_bitwise.c
> @@ -60,6 +60,38 @@ static void nft_bitwise_eval_rshift(u32 *dst, const
> u32 *src,
>  	}
>  }
>
> +static void nft_bitwise_eval_add(u32 *dst, const u32 *src,
> +				 const struct nft_bitwise *priv)
> +{
> +	u32 delta = priv->data.data[0];
> +	unsigned int i, words;
> +	u32 tmp = 0;
> +
> +	words = DIV_ROUND_UP(priv->len, sizeof(u32));
> +	for (i = 0; i < words; i++) {
> +		tmp = src[i];
> +		dst[i] = src[i] + delta;
> +		if (dst[i] < tmp && i + 1 < words)
> +			dst[i + 1]++;
> +	}
> +}

for (i = 0; i < words; i++) {
	dst[i] = src[i] + delta + tmp;
	tmp = dst[i] < src[i] ? 1 : 0;
}

> +static void nft_bitwise_eval_sub(u32 *dst, const u32 *src,
> +				 const struct nft_bitwise *priv)
> +{
> +	u32 delta = priv->data.data[0];
> +	unsigned int i, words;
> +	u32 tmp = 0;
> +
> +	words = DIV_ROUND_UP(priv->len, sizeof(u32));
> +	for (i = 0; i < words; i++) {
> +		tmp = src[i];
> +		dst[i] = src[i] - delta;
> +		if (dst[i] > tmp && i + 1 < words)
> +			dst[i + 1]--;
> +	}
> +}

for (i = 0; i < words; i++) {
	dst[i] = src[i] - delta - tmp;
	tmp = dst[i] > src[i] ? 1 : 0;
}

>  void nft_bitwise_eval(const struct nft_expr *expr,
>  		      struct nft_regs *regs, const struct nft_pktinfo *pkt)
>  {
> @@ -77,6 +109,12 @@ void nft_bitwise_eval(const struct nft_expr *expr,
>  	case NFT_BITWISE_RSHIFT:
>  		nft_bitwise_eval_rshift(dst, src, priv);
>  		break;
> +	case NFT_BITWISE_ADD:
> +		nft_bitwise_eval_add(dst, src, priv);
> +		break;
> +	case NFT_BITWISE_SUB:
> +		nft_bitwise_eval_sub(dst, src, priv);
> +		break;
>  	}
>  }
>
> @@ -129,8 +167,8 @@ static int nft_bitwise_init_bool(struct
> nft_bitwise *priv,
>  	return err;
>  }
>
> -static int nft_bitwise_init_shift(struct nft_bitwise *priv,
> -				  const struct nlattr *const tb[])
> +static int nft_bitwise_init_data(struct nft_bitwise *priv,
> +				 const struct nlattr *const tb[])
>  {
>  	struct nft_data_desc d;
>  	int err;
> @@ -191,6 +229,8 @@ static int nft_bitwise_init(const struct nft_ctx
> *ctx,
>  		case NFT_BITWISE_BOOL:
>  		case NFT_BITWISE_LSHIFT:
>  		case NFT_BITWISE_RSHIFT:
> +		case NFT_BITWISE_ADD:
> +		case NFT_BITWISE_SUB:
>  			break;
>  		default:
>  			return -EOPNOTSUPP;
> @@ -205,7 +245,9 @@ static int nft_bitwise_init(const struct nft_ctx
> *ctx,
>  		break;
>  	case NFT_BITWISE_LSHIFT:
>  	case NFT_BITWISE_RSHIFT:
> -		err = nft_bitwise_init_shift(priv, tb);
> +	case NFT_BITWISE_ADD:
> +	case NFT_BITWISE_SUB:
> +		err = nft_bitwise_init_data(priv, tb);
>  		break;
>  	}
>
> @@ -226,8 +268,8 @@ static int nft_bitwise_dump_bool(struct sk_buff
> *skb,
>  	return 0;
>  }
>
> -static int nft_bitwise_dump_shift(struct sk_buff *skb,
> -				  const struct nft_bitwise *priv)
> +static int nft_bitwise_dump_data(struct sk_buff *skb,
> +				 const struct nft_bitwise *priv)
>  {
>  	if (nft_data_dump(skb, NFTA_BITWISE_DATA, &priv->data,
>  			  NFT_DATA_VALUE, sizeof(u32)) < 0)
> @@ -255,7 +297,9 @@ static int nft_bitwise_dump(struct sk_buff *skb,
> const struct nft_expr *expr)
>  		break;
>  	case NFT_BITWISE_LSHIFT:
>  	case NFT_BITWISE_RSHIFT:
> -		err = nft_bitwise_dump_shift(skb, priv);
> +	case NFT_BITWISE_ADD:
> +	case NFT_BITWISE_SUB:
> +		err = nft_bitwise_dump_data(skb, priv);
>  		break;
>  	}
>

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