Re: [PATCH net-next 2/3] ipv6: take fib6_sernum from a sequence shared by all netns

[kuniyu@google.com]

On Fri, 16 Oct 2026 14:09:54 +0800, Qiliang Yuan wrote:
> A per-cpu IPv6 route of a nexthop object records fib6_sernum of the
> netns it was created in, and rt6_is_valid() compares it with fib6_sernum
> of the netns its device belongs to when the route is checked. Every netns
> starts its fib6_sernum at 1 and increments it on each tree change, so
> different netns often hold the same value.
> 
> When a device moves to another netns, such a route remains usable until
> nexthop_free_rcu() releases it, and fib6_info_destroy_rcu() queues that
> callback from inside an RCU callback, after the device has moved. If the
> fib6_sernum of the new netns matches the cached one in that window,
> ip6_dst_check() accepts the route and a socket in the old netns keeps
> sending through a device that now belongs to another netns.
> 
> Take every fib6_sernum, including the initial one, from one global
> sequence, so that a cached route never matches the fib6_sernum of
> another netns.
> 
> A connected UDP socket in netns A caches a route through an IPv6 nexthop
> object on a veth. The veth is moved to netns B and brought up with one
> RTM_NEWLINK, then the socket sends 200000 datagrams. A route is added
> to and removed from B 0 to 20 times before the move, 63 trials:
> 
>                                 before     after
>   trials sending after move          3         0
>   datagrams sent after move       8187         0
> 
> Fixes: 8f34e53b60b3 ("ipv6: Use global sernum for dst validation with nexthop objects")
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>