netdev - [PATCH net-next v2] ip6_tunnel: enable to change proto of fb tunnels

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Message-ID: <20250626215919.2825347-1-nicolas.dichtel@6wind.com>
Date: Thu, 26 Jun 2025 23:55:09 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>
Subject: [PATCH net-next v2] ip6_tunnel: enable to change proto of fb tunnels

This is possible via the ioctl API:
> ip -6 tunnel change ip6tnl0 mode any

Let's align the netlink API:
> ip link set ip6tnl0 type ip6tnl mode any

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
---

I finally checked  all params, let's do this properly (:

v1 -> v2:
 - returns an error if the user attempts to change anything other than the proto

 net/ipv6/ip6_tunnel.c | 44 ++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index a885bb5c98ea..8dcad289b8c5 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1562,11 +1562,22 @@ static void ip6_tnl_update(struct ip6_tnl *t, struct __ip6_tnl_parm *p)
 	netdev_state_change(t->dev);
 }
 
-static void ip6_tnl0_update(struct ip6_tnl *t, struct __ip6_tnl_parm *p)
+static int ip6_tnl0_update(struct ip6_tnl *t, struct __ip6_tnl_parm *p,
+			   bool strict)
 {
-	/* for default tnl0 device allow to change only the proto */
+	/* For the default ip6tnl0 device, allow changing only the protocol (the
+	 * IP6_TNL_F_CAP_PER_PACKET flag is set on ip6tnl0, and all other
+	 * parameters are 0).
+	 */
+	if (strict &&
+	    (!ipv6_addr_any(&p->laddr) || !ipv6_addr_any(&p->raddr) ||
+	     p->flags != t->parms.flags || p->hop_limit || p->encap_limit ||
+	     p->flowinfo || p->link || p->fwmark || p->collect_md))
+		return -EINVAL;
+
 	t->parms.proto = p->proto;
 	netdev_state_change(t->dev);
+	return 0;
 }
 
 static void
@@ -1680,7 +1691,7 @@ ip6_tnl_siocdevprivate(struct net_device *dev, struct ifreq *ifr,
 			} else
 				t = netdev_priv(dev);
 			if (dev == ip6n->fb_tnl_dev)
-				ip6_tnl0_update(t, &p1);
+				ip6_tnl0_update(t, &p1, false);
 			else
 				ip6_tnl_update(t, &p1);
 		}
@@ -2053,8 +2064,31 @@ static int ip6_tnl_changelink(struct net_device *dev, struct nlattr *tb[],
 	struct ip6_tnl_net *ip6n = net_generic(net, ip6_tnl_net_id);
 	struct ip_tunnel_encap ipencap;
 
-	if (dev == ip6n->fb_tnl_dev)
-		return -EINVAL;
+	if (dev == ip6n->fb_tnl_dev) {
+		struct ip6_tnl *t = netdev_priv(ip6n->fb_tnl_dev);
+
+		if (ip_tunnel_netlink_encap_parms(data, &ipencap)) {
+			/* iproute2 always sets TUNNEL_ENCAP_FLAG_CSUM6, so
+			 * let's ignore this flag.
+			 */
+			ipencap.flags &= ~TUNNEL_ENCAP_FLAG_CSUM6;
+			if (memchr_inv(&ipencap, 0, sizeof(ipencap))) {
+				NL_SET_ERR_MSG(extack,
+					       "Only protocol can be changed for fallback tunnel, not encap params");
+				return -EINVAL;
+			}
+		}
+
+		ip6_tnl_netlink_parms(data, &p);
+		if (ip6_tnl0_update(netdev_priv(ip6n->fb_tnl_dev), &p,
+				    true) < 0) {
+			NL_SET_ERR_MSG(extack,
+				       "Only protocol can be changed for fallback tunnel");
+			return -EINVAL;
+		}
+
+		return 0;
+	}
 
 	if (ip_tunnel_netlink_encap_parms(data, &ipencap)) {
 		int err = ip6_tnl_encap_setup(t, &ipencap);
-- 
2.47.1