netfilter: nft_meta: track register operations

Check if the destination register already contains the data that this
meta store expression performs. This allows to skip this redundant
operation. If the destination contains a different selector, update
the register tracking information.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_meta.c

@@ -750,12 +750,40 @@ static int nft_meta_get_offload(struct nft_offload_ctx *ctx,
 	return 0;
 }
 
+static bool nft_meta_get_reduce(struct nft_regs_track *track,
+				const struct nft_expr *expr)
+{
+	const struct nft_meta *priv = nft_expr_priv(expr);
+	const struct nft_meta *meta;
+
+	if (!track->regs[priv->dreg].selector ||
+	    track->regs[priv->dreg].selector->ops != expr->ops) {
+		track->regs[priv->dreg].selector = expr;
+		track->regs[priv->dreg].bitwise = NULL;
+		return false;
+	}
+
+	meta = nft_expr_priv(track->regs[priv->dreg].selector);
+	if (priv->key != meta->key ||
+	    priv->dreg != meta->dreg) {
+		track->regs[priv->dreg].selector = expr;
+		track->regs[priv->dreg].bitwise = NULL;
+		return false;
+	}
+
+	if (!track->regs[priv->dreg].bitwise)
+		return true;
+
+	return false;
+}
+
 static const struct nft_expr_ops nft_meta_get_ops = {
 	.type		= &nft_meta_type,
 	.size		= NFT_EXPR_SIZE(sizeof(struct nft_meta)),
 	.eval		= nft_meta_get_eval,
 	.init		= nft_meta_get_init,
 	.dump		= nft_meta_get_dump,
+	.reduce		= nft_meta_get_reduce,
 	.validate	= nft_meta_get_validate,
 	.offload	= nft_meta_get_offload,
 };