src: add hash expression

This is special expression that transforms an input expression into a
32-bit unsigned integer. This expression takes a modulus parameter to
scale the result and the random seed so the hash result becomes harder
to predict.

You can use it to set the packet mark, eg.

 # nft add rule x y meta mark set jhash ip saddr . ip daddr mod 2 seed 0xdeadbeef

You can combine this with maps too, eg.

 # nft add rule x y dnat to jhash ip saddr mod 2 seed 0xdeadbeef map { \
	0 : 192.168.20.100, \
	1 : 192.168.30.100 \
   }

Currently, this expression implements the jenkins hash implementation
available in the Linux kernel:

 http://lxr.free-electrons.com/source/include/linux/jhash.h

But it should be possible to extend it to support any other hash
function type.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 60 insertions, 0 deletions

diff --git a/src/hash.c b/src/hash.c
new file mode 100644
index 00000000..125b3202
--- /dev/null
+++ b/src/hash.c
@@ -0,0 +1,60 @@
+/*
+ * Hash expression definitions.
+ *
+ * Copyright (c) 2016 Pablo Neira Ayuso <pablo@netfilter.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <nftables.h>
+#include <expression.h>
+#include <datatype.h>
+#include <gmputil.h>
+#include <hash.h>
+#include <utils.h>
+
+static void hash_expr_print(const struct expr *expr)
+{
+	printf("jhash ");
+	expr_print(expr->hash.expr);
+	printf(" mod %u", expr->hash.mod);
+	if (expr->hash.seed)
+		printf(" seed 0x%x", expr->hash.seed);
+}
+
+static bool hash_expr_cmp(const struct expr *e1, const struct expr *e2)
+{
+	return expr_cmp(e1->hash.expr, e2->hash.expr) &&
+	       e1->hash.mod == e2->hash.mod &&
+	       e1->hash.seed == e2->hash.seed;
+}
+
+static void hash_expr_clone(struct expr *new, const struct expr *expr)
+{
+	new->hash.expr = expr_clone(expr->hash.expr);
+	new->hash.mod = expr->hash.mod;
+	new->hash.seed = expr->hash.seed;
+}
+
+static const struct expr_ops hash_expr_ops = {
+	.type		= EXPR_HASH,
+	.name		= "hash",
+	.print		= hash_expr_print,
+	.cmp		= hash_expr_cmp,
+	.clone		= hash_expr_clone,
+};
+
+struct expr *hash_expr_alloc(const struct location *loc, uint32_t mod,
+			     uint32_t seed)
+{
+	struct expr *expr;
+
+	expr = expr_alloc(loc, &hash_expr_ops, &integer_type,
+			  BYTEORDER_HOST_ENDIAN, 4 * BITS_PER_BYTE);
+	expr->hash.mod  = mod;
+	expr->hash.seed = seed;
+
+	return expr;
+}