Re: [BUG] net: gre: IPv6 link-local multicast is silently dropped (Regression)

[Ido Schimmel <idosch@idosch.org>]

+ Guillaume

Report is here: https://lore.kernel.org/netdev/CANR=AhRM7YHHXVxJ4DmrTNMeuEOY87K2mLmo9KMed1JMr20p6g@mail.gmail.com/

On Sun, Jun 29, 2025 at 02:40:27PM +0800, Aiden Yang wrote:
> This report details a regression in the Linux kernel that prevents
> IPv6 link-local all-nodes multicast packets (ff02::1) from being
> transmitted over a GRE tunnel. The issue is confirmed to have been
> introduced between kernel versions 6.1.0-35-cloud-amd64 (working) and
> 6.1.0-37-cloud-amd64 (failing) on Debian 12 (Bookworm).

Apparently 6.1.0-35-cloud-amd64 is v6.1.137 and 6.1.0-37-cloud-amd64 is
v6.1.140. Probably started with:

a51dc9669ff8 gre: Fix again IPv6 link-local address generation.

In v6.1.139.

It skips creating an IPv6 multicast route for some ipgre devices. Can
you try the following diff?

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index ba2ec7c870cc..d0a202d0d93e 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -3537,12 +3537,10 @@ static void addrconf_gre_config(struct net_device *dev)
 	 * case). Such devices fall back to add_v4_addrs() instead.
 	 */
 	if (!(dev->type == ARPHRD_IPGRE && *(__be32 *)dev->dev_addr == 0 &&
-	      idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_EUI64)) {
+	      idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_EUI64))
 		addrconf_addr_gen(idev, true);
-		return;
-	}
-
-	add_v4_addrs(idev);
+	else
+		add_v4_addrs(idev);
 
 	if (dev->flags & IFF_POINTOPOINT)
 		addrconf_add_mroute(dev);

Guillaume, AFAICT, after commit d3623dd5bd4e ("ipv6: Simplify link-local
address generation for IPv6 GRE.") in net-next an IPv6 multicast route
will be created for every ip6gre device, regardless of IFF_POINTOPOINT.
It should restore the behavior before commit e5dd729460ca ("ip/ip6_gre:
use the same logic as SIT interfaces when computing v6LL address"). We
can extend gre_ipv6_lladdr.sh to test this once the fix is in net-next.