Re: [PATCH net 1/1] ovpn: avoid caching stale IPv6 dst after FIB changes

[Ralf Lici <ralf@mandelbit.com>]

On Thu, 17 Sep 2026 13:09:10 -0600, David Ahern <dsahern@kernel.org> wrote:
> On 9/17/26 4:09 AM, Ralf Lici wrote:
> > diff --git a/drivers/net/ovpn/udp.c b/drivers/net/ovpn/udp.c
> > index 7f69e8890b5b..a28974cc36e8 100644
> > --- a/drivers/net/ovpn/udp.c
> > +++ b/drivers/net/ovpn/udp.c
> > @@ -250,7 +252,11 @@ static int ovpn_udp6_output(struct ovpn_peer *peer, struct ovpn_bind *bind,
> >  		dst_cache_reset(cache);
> >  	}
> >  
> > -	dst = ip6_dst_lookup_flow(sock_net(sk), sk, &fl, NULL);
> > +	gen0 = rt_genid_ipv6(net);
>
> this detail about routes should not be buried here. rt_genid_ipv6 really
> should be local to net/ipv6/route.c
>
>

I agree. Keeping this detail inside the IPv6 routing code is exactly why
I sent the RFC linked from the cover letter, and I would appreciate your
thoughts on the proposed approach:

https://lore.kernel.org/netdev/20260901122501.482920-1-ralf@mandelbit.com/

The pattern of obtaining a dst and only later sampling its validation
cookie when publishing it into a persistent cache is widespread in both
drivers and the networking core. It is racy for IPv6 routes with
rt6_info::sernum == 0, because rt6_get_cookie then reads the current
fib6_node::fn_sernum, not necessarily the value corresponding to the
lookup which returned that dst.

-- 
Ralf Lici
Mandelbit Srl