netfilter: nft_osf: track register operations

Allow to recycle the previous output of the OS fingerprint expression
if flags and ttl are the same.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_osf.c

@@ -120,6 +120,30 @@ static int nft_osf_validate(const struct nft_ctx *ctx,
 						    (1 << NF_INET_FORWARD));
 }
 
+static bool nft_osf_reduce(struct nft_regs_track *track,
+			   const struct nft_expr *expr)
+{
+	struct nft_osf *priv = nft_expr_priv(expr);
+	struct nft_osf *osf;
+
+	if (!nft_reg_track_cmp(track, expr, priv->dreg)) {
+		nft_reg_track_update(track, expr, priv->dreg, NFT_OSF_MAXGENRELEN);
+		return false;
+	}
+
+	osf = nft_expr_priv(track->regs[priv->dreg].selector);
+	if (priv->flags != osf->flags ||
+	    priv->ttl != osf->ttl) {
+		nft_reg_track_update(track, expr, priv->dreg, NFT_OSF_MAXGENRELEN);
+		return false;
+	}
+
+	if (!track->regs[priv->dreg].bitwise)
+		return true;
+
+	return false;
+}
+
 static struct nft_expr_type nft_osf_type;
 static const struct nft_expr_ops nft_osf_op = {
 	.eval		= nft_osf_eval,
@@ -128,6 +152,7 @@ static const struct nft_expr_ops nft_osf_op = {
 	.dump		= nft_osf_dump,
 	.type		= &nft_osf_type,
 	.validate	= nft_osf_validate,
+	.reduce		= nft_osf_reduce,
 };
 
 static struct nft_expr_type nft_osf_type __read_mostly = {