net: lwtunnel: change encap fill order

I plan to add lwtunnel encap attributes to the YNL rt-route.yaml spec.
When decoding submessages, YNL expects to read the "selector" (encap-type)
first. Currently lwtunnel writes the encap payload first, which makes YNL
fail to parse the lwtunnel encap message.

Fixing this within YNL itself would be complicated. Instead, reorder the
netlink attributes in the kernel to output encap_type first.

This is a preparatory change for the upcoming rt-route spec updates.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Hangbin Liu <liuhangbin@kylinos.cn>
Link: https://patch.msgid.link/20261008-ynl_rt_encap-v4-1-4ed2ccf7f949@kylinos.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 18 insertions, 17 deletions

diff --git a/net/core/lwtunnel.c b/net/core/lwtunnel.c
index b01a395d9a96..8223c44f10c8 100644
--- a/net/core/lwtunnel.c
+++ b/net/core/lwtunnel.c
@@ -231,7 +231,7 @@ int lwtunnel_fill_encap(struct sk_buff *skb, struct lwtunnel_state *lwtstate,
 {
 	const struct lwtunnel_encap_ops *ops;
 	struct nlattr *nest;
-	int ret;
+	int ret = 0;
 
 	if (!lwtstate)
 		return 0;
@@ -240,30 +240,31 @@ int lwtunnel_fill_encap(struct sk_buff *skb, struct lwtunnel_state *lwtstate,
 	    lwtstate->type > LWTUNNEL_ENCAP_MAX)
 		return 0;
 
-	nest = nla_nest_start_noflag(skb, encap_attr);
-	if (!nest)
-		return -EMSGSIZE;
-
-	ret = -EOPNOTSUPP;
 	rcu_read_lock();
+
 	ops = rcu_dereference(lwtun_encaps[lwtstate->type]);
-	if (likely(ops && ops->fill_encap))
-		ret = ops->fill_encap(skb, lwtstate);
-	rcu_read_unlock();
+	if (unlikely(!ops || !ops->fill_encap))
+		goto unlock_out;
 
-	if (ret)
-		goto nla_put_failure;
-	nla_nest_end(skb, nest);
 	ret = nla_put_u16(skb, encap_type_attr, lwtstate->type);
 	if (ret)
-		goto nla_put_failure;
+		goto unlock_out;
 
-	return 0;
+	nest = nla_nest_start_noflag(skb, encap_attr);
+	if (!nest) {
+		ret = -EMSGSIZE;
+		goto unlock_out;
+	}
 
-nla_put_failure:
-	nla_nest_cancel(skb, nest);
+	ret = ops->fill_encap(skb, lwtstate);
+	if (ret)
+		nla_nest_cancel(skb, nest);
+	else
+		nla_nest_end(skb, nest);
 
-	return (ret == -EOPNOTSUPP ? 0 : ret);
+unlock_out:
+	rcu_read_unlock();
+	return ret;
 }
 EXPORT_SYMBOL_GPL(lwtunnel_fill_encap);