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, 21 insertions, 0 deletions

diff --git a/src/evaluate.c b/src/evaluate.c
index ed722df9..8f7824b5 100644
--- a/src/evaluate.c
+++ b/src/evaluate.c
@@ -1183,6 +1183,25 @@ static int expr_evaluate_numgen(struct eval_ctx *ctx, struct expr **exprp)
 	return expr_evaluate_primary(ctx, exprp);
 }
 
+static int expr_evaluate_hash(struct eval_ctx *ctx, struct expr **exprp)
+{
+	struct expr *expr = *exprp;
+
+	expr_dtype_integer_compatible(ctx, expr);
+
+	expr_set_context(&ctx->ectx, NULL, 0);
+	if (expr_evaluate(ctx, &expr->hash.expr) < 0)
+		return -1;
+
+	/* expr_evaluate_primary() sets the context to what to the input
+         * expression to be hashed. Since this input is transformed to a 4 bytes
+	 * integer, restore context to the datatype that results from hashing.
+	 */
+	expr_set_context(&ctx->ectx, expr->dtype, expr->len);
+
+	return 0;
+}
+
 /*
  * Transfer the invertible binops to the constant side of an equality
  * expression. A left shift is only invertible if the low n bits are
@@ -1587,6 +1606,8 @@ static int expr_evaluate(struct eval_ctx *ctx, struct expr **expr)
 		return expr_evaluate_relational(ctx, expr);
 	case EXPR_NUMGEN:
 		return expr_evaluate_numgen(ctx, expr);
+	case EXPR_HASH:
+		return expr_evaluate_hash(ctx, expr);
 	default:
 		BUG("unknown expression type %s\n", (*expr)->ops->name);
 	}