netdev - Re: [PATCH 1/2] net/ipv6: always honour route mtu during forwarding

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Message-ID: <CAKD1Yr3idc3zz1AT5kmqBE4A9QaOYVF-XvU9zh29gW66tjHQ3g@mail.gmail.com>
Date:   Thu, 8 Oct 2020 15:04:39 +0900
From:   Lorenzo Colitti <lorenzo@...gle.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
Cc:     Maciej Żenczykowski <maze@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Network Development Mailing List 
        <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Sunmeet Gill <sgill@...cinc.com>,
        Vinay Paradkar <vparadka@....qualcomm.com>,
        Tyler Wear <twear@...cinc.com>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH 1/2] net/ipv6: always honour route mtu during forwarding

On Thu, Oct 8, 2020 at 12:31 PM Maciej Żenczykowski
<zenczykowski@...il.com> wrote:
> diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
> index 2a5277758379..598415743f46 100644
> --- a/include/net/ip6_route.h
> +++ b/include/net/ip6_route.h
> @@ -311,19 +311,13 @@ static inline bool rt6_duplicate_nexthop(struct fib6_info *a, struct fib6_info *
>  static inline unsigned int ip6_dst_mtu_forward(const struct dst_entry *dst)
>  {
>         struct inet6_dev *idev;
> -       unsigned int mtu;
> +       unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
> +       if (mtu)
> +               return mtu;

What should happen here if mtu is less than idev->cnf.mtu6? Should the
code pick the minimum? If not: will picking the higher value work, or
will the packet be dropped? I suppose we already have this problem
today if the administrator configures a route with a locked MTU.