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

[David Ahern <dsahern@kernel.org>]

On 10/8/26 6:33 PM, Antonio Quartulli wrote:
> You're right that a cached dst can go stale later and that reuse-time
> validation handles that. This case is different, because it breaks
> reuse-time validation.
> 
> dst_cache_set_ip6() samples the cookie itself:
> 
>         dst_cache_per_cpu_dst_set(idst, dst,
>                                   rt6_get_cookie(dst_rt6_info(dst)));
> 
> rt6_get_cookie() only returns a route-tied value when rt->sernum is
> non-zero, which happens in ip6_rt_pcpu_alloc() and only for nexthop
> routes. Everything else reads the current node generation:
> 
>         *cookie = READ_ONCE(fn->fn_sernum);

Have you tried setting sernum for all dst_entry when they are created?

This deferred reading of fn_sernum seems to be the problem. fib entries
have sernum set. dst_entries are created from the fib entries and should
carry the sernum from the fib entry at that moment.