netfilter: nft_lookup: only cancel tracking for clobbered dregs

In most cases, nft_lookup will be read-only, i.e. won't clobber
registers.  In case of map, we need to cancel the registers that will
see stores.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_lookup.c

@@ -253,6 +253,17 @@ static int nft_lookup_validate(const struct nft_ctx *ctx,
 	return 0;
 }
 
+static bool nft_lookup_reduce(struct nft_regs_track *track,
+			      const struct nft_expr *expr)
+{
+	const struct nft_lookup *priv = nft_expr_priv(expr);
+
+	if (priv->set->flags & NFT_SET_MAP)
+		nft_reg_track_cancel(track, priv->dreg, priv->set->dlen);
+
+	return false;
+}
+
 static const struct nft_expr_ops nft_lookup_ops = {
 	.type		= &nft_lookup_type,
 	.size		= NFT_EXPR_SIZE(sizeof(struct nft_lookup)),
@@ -263,6 +274,7 @@ static const struct nft_expr_ops nft_lookup_ops = {
 	.destroy	= nft_lookup_destroy,
 	.dump		= nft_lookup_dump,
 	.validate	= nft_lookup_validate,
+	.reduce		= nft_lookup_reduce,
 };
 
 struct nft_expr_type nft_lookup_type __read_mostly = {