netdev - Re: [PATCH] IPv4: Tunnel: Fix effective path mtu calculation

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Message-ID: <a3d6baa0-7f72-adf3-0082-df42a05858b1@gmail.com>
Date:   Wed, 24 Jun 2020 09:43:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Oliver Herms <oliver.peter.herms@...il.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        kuba@...nel.org
Subject: Re: [PATCH] IPv4: Tunnel: Fix effective path mtu calculation



On 6/24/20 4:48 AM, Oliver Herms wrote:
> The calculation of the effective tunnel mtu, that is used to create
> mtu exceptions if necessary, is currently not done correctly. This
> leads to unnecessary entries in the IPv6 route cache for any packet
> to be sent through the tunnel.
> 
> The root cause is, that "dev->hard_header_len" is subtracted from the
> tunnel destinations path mtu. Thus subtracting too much, if
> dev->hard_header_len is filled in. This is that case for SIT tunnels
> where hard_header_len is the underlyings dev's hard_header_len (e.g. 14
> for ethernet) + 20 bytes IP header (see net/ipv6/sit.c:1091).
> 
> However, the MTU of the path is exclusive of the ethernet header
> and the 20 bytes for the IP header are being subtracted separately
> already. Thus hard_header_len is removed from this calculation.
> 
> For IPIP and GRE tunnels this doesn't change anything as hard_header_len
> is zero in those cases anyways.
> 
> Signed-off-by: Oliver Herms <oliver.peter.herms@...il.com>
> ---
>  net/ipv4/ip_tunnel.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> index f4f1d11eab50..871d28bd29fa 100644
> --- a/net/ipv4/ip_tunnel.c
> +++ b/net/ipv4/ip_tunnel.c
> @@ -495,8 +495,7 @@ static int tnl_update_pmtu(struct net_device *dev, struct sk_buff *skb,
>  	pkt_size = skb->len - tunnel_hlen - dev->hard_header_len;
>  
>  	if (df)
> -		mtu = dst_mtu(&rt->dst) - dev->hard_header_len
> -					- sizeof(struct iphdr) - tunnel_hlen;
> +		mtu = dst_mtu(&rt->dst) - sizeof(struct iphdr) - tunnel_hlen;
>  	else
>  		mtu = skb_valid_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
>  
> 


Can you add a Fixes: tag, to help stable teams work ?

Thanks.