netdev - Re: [PATCH] [PATCH net v2] ipv6: remove min MTU check for ipsec tunnels

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Message-ID: <a63d9866-4a61-4aab-4ed5-5d544c999fc7@gmail.com>
Date:   Thu, 10 May 2018 10:05:05 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ashwanth Goli <ashwanth@...eaurora.org>, netdev@...r.kernel.org,
        davem@...emloft.net
Cc:     pabeni@...hat.com, dsahern@...il.com
Subject: Re: [PATCH] [PATCH net v2] ipv6: remove min MTU check for ipsec
 tunnels



On 05/10/2018 09:48 AM, Ashwanth Goli wrote:
> With 749439bfac "fix udpv6 sendmsg crash caused by too small MTU"
> ipsec tunnels that report a MTU less than IPV6_MIN_MTU are broken
> even for packets that are smaller than IPV6_MIN_MTU.
> 
> According to rfc2473#section-7.1
> 
>     if the original IPv6 packet is equal or  smaller  than  the
>     IPv6 minimum link MTU, the tunnel entry-point node
>     encapsulates the original packet, and subsequently
>     fragments the resulting IPv6 tunnel packet into IPv6
>     fragments that do not exceed the Path MTU to the tunnel
>     exit-point.
> 
> Dropping the MTU check for ipsec tunnel destinations.
> 
> Signed-off-by: Ashwanth Goli <ashwanth@...eaurora.org>
> ---
>  net/ipv6/ip6_output.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 2e891d2..c4c3313 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1235,7 +1235,7 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
>  		if (np->frag_size)
>  			mtu = np->frag_size;
>  	}
> -	if (mtu < IPV6_MIN_MTU)
> +	if (!(rt->dst.flags & DST_XFRM_TUNNEL) && mtu < IPV6_MIN_MTU)
>  		return -EINVAL;
>  	cork->base.fragsize = mtu;
>  	if (dst_allfrag(xfrm_dst_path(&rt->dst)))
> 

Given that the (mtu < IPV6_MIN_MTU) condition is unlikely, I would reorder to :

if (mtu < IPV6_MIN_MTU && !(rt->dst.flags & DST_XFRM_TUNNEL))
	return -EINVAL;

The fast path should not look at dst.flags