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

[Guillaume Nault <gnault@redhat.com>]

On Sun, Jul 06, 2025 at 04:40:30PM +0100, Gary Guo wrote:
> 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.

Thanks Gary, it's good to have such feedback.
I'm going to formally send the patch soon.

> 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
>