[PATCH v2] netfilter: nft_hash: Add hash offset value

[Laura Garcia Liebana <nevola@gmail.com>]

Add support to pass through an offset to the hash value. With this
feature, the sysadmin is able to generate a hash with a given
offset value.

Example:

	meta mark set jhash ip saddr mod 2 seed 0xabcd sum 100

This option generates marks according to the source address from 100 to
101.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
---
Changes in v2:
	- Add check for hash + sum overflow.

 include/uapi/linux/netfilter/nf_tables.h |  2 ++
 net/netfilter/nft_hash.c                 | 16 ++++++++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/netfilter/nf_tables.h b/include/uapi/linux/netfilter/nf_tables.h
index 4dbeeed..8026684 100644
--- a/include/uapi/linux/netfilter/nf_tables.h
+++ b/include/uapi/linux/netfilter/nf_tables.h
@@ -764,6 +764,7 @@ enum nft_meta_keys {
  * @NFTA_HASH_LEN: source data length (NLA_U32)
  * @NFTA_HASH_MODULUS: modulus value (NLA_U32)
  * @NFTA_HASH_SEED: seed value (NLA_U32)
+ * @NFTA_HASH_SUM: Hash offset value (NLA_U32)
  */
 enum nft_hash_attributes {
 	NFTA_HASH_UNSPEC,
@@ -772,6 +773,7 @@ enum nft_hash_attributes {
 	NFTA_HASH_LEN,
 	NFTA_HASH_MODULUS,
 	NFTA_HASH_SEED,
+	NFTA_HASH_SUM,
 	__NFTA_HASH_MAX,
 };
 #define NFTA_HASH_MAX	(__NFTA_HASH_MAX - 1)
diff --git a/net/netfilter/nft_hash.c b/net/netfilter/nft_hash.c
index b7e3b40..2102c94 100644
--- a/net/netfilter/nft_hash.c
+++ b/net/netfilter/nft_hash.c
@@ -23,6 +23,7 @@ struct nft_hash {
 	u8			len;
 	u32			modulus;
 	u32			seed;
+	u32			sum;
 };
 
 static void nft_hash_eval(const struct nft_expr *expr,
@@ -35,7 +36,7 @@ static void nft_hash_eval(const struct nft_expr *expr,
 
 	h = reciprocal_scale(jhash(data, priv->len, priv->seed), priv->modulus);
 
-	regs->data[priv->dreg] = h;
+	regs->data[priv->dreg] = priv->sum + h;
 }
 
 const struct nla_policy nft_hash_policy[NFTA_HASH_MAX + 1] = {
@@ -44,6 +45,7 @@ const struct nla_policy nft_hash_policy[NFTA_HASH_MAX + 1] = {
 	[NFTA_HASH_LEN]			= { .type = NLA_U32 },
 	[NFTA_HASH_MODULUS]		= { .type = NLA_U32 },
 	[NFTA_HASH_SEED]		= { .type = NLA_U32 },
+	[NFTA_HASH_SUM]			= { .type = NLA_U32 },
 };
 
 static int nft_hash_init(const struct nft_ctx *ctx,
@@ -60,6 +62,11 @@ static int nft_hash_init(const struct nft_ctx *ctx,
 	    !tb[NFTA_HASH_MODULUS])
 		return -EINVAL;
 
+	if (tb[NFTA_HASH_SUM])
+		priv->sum = ntohl(nla_get_be32(tb[NFTA_HASH_SUM]));
+	else
+		priv->sum = 0;
+
 	priv->sreg = nft_parse_register(tb[NFTA_HASH_SREG]);
 	if (priv->sreg < 0)
 		return -ERANGE;
@@ -77,6 +84,9 @@ static int nft_hash_init(const struct nft_ctx *ctx,
 	if (priv->modulus <= 1)
 		return -ERANGE;
 
+	if (priv->sum + priv->modulus - 1 < U32_MAX)
+		return -EOVERFLOW;
+
 	priv->seed = ntohl(nla_get_be32(tb[NFTA_HASH_SEED]));
 
 	return nft_validate_register_load(priv->sreg, priv->len) &&
@@ -99,7 +109,9 @@ static int nft_hash_dump(struct sk_buff *skb,
 		goto nla_put_failure;
 	if (nft_dump_register(skb, NFTA_HASH_SEED, priv->seed))
 		goto nla_put_failure;
-
+	if (priv->sum != 0)
+		if (nla_put_be32(skb, NFTA_HASH_SUM, htonl(priv->sum)))
+			goto nla_put_failure;
 	return 0;
 
 nla_put_failure:
-- 
2.8.1