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

[Gary Guo <gary@kernel.org>]

On Wed, 2 Jul 2025 12:12:22 +0200, Guillaume Nault wrote:
> Aiden, can you confirm that the following patch fixes the issue on your
> side?

Not Aiden, but I get hit with the same regression after updating kernel on my
router from v6.12.28 to v6.12.35 today. Symptom for me is bird complaining
about "Socket error: Network is unreachable", and strace shows that it's sending
packets to ff02::1:6 and get hit with ENETUNREACH.

I can confirm that applying this patch on top of v6.12.35 fixes the issue for me.
I also took a look of the code, not a net expert, but this approach does look
like a proper fix to me.

Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>

> 
> ---- >8 ----
> 
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index ba2ec7c870cc..870a0bd6c2ba 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -3525,11 +3525,9 @@ static void addrconf_gre_config(struct net_device *dev)
>  
>  	ASSERT_RTNL();
>  
> -	idev = ipv6_find_idev(dev);
> -	if (IS_ERR(idev)) {
> -		pr_debug("%s: add_dev failed\n", __func__);
> +	idev = addrconf_add_dev(dev);
> +	if (IS_ERR(idev))
>  		return;
> -	}
>  
>  	/* Generate the IPv6 link-local address using addrconf_addr_gen(),
>  	 * unless we have an IPv4 GRE device not bound to an IP address and
> @@ -3543,9 +3541,6 @@ static void addrconf_gre_config(struct net_device *dev)
>  	}
>  
>  	add_v4_addrs(idev);
> -
> -	if (dev->flags & IFF_POINTOPOINT)
> -		addrconf_add_mroute(dev);
>  }
>  #endif