netdev - Re: [PATCH net-next] ip6_vti: adjust vti mtu according to mtu of output device

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Date:   Fri, 8 Dec 2017 08:02:15 +0100
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Alexey Kodanev <alexey.kodanev@...cle.com>
CC:     <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>,
        Petr Vorel <pvorel@...e.cz>
Subject: Re: [PATCH net-next] ip6_vti: adjust vti mtu according to mtu of
 output device

On Wed, Dec 06, 2017 at 07:38:19PM +0300, Alexey Kodanev wrote:
> LTP/udp6_ipsec_vti tests fail when sending large UDP datagrams
> that require fragmentation and underlying device MTU <= 1500.
> This happens because ip6_vti sets mtu to ETH_DATA_LEN and not
> updating it depending on a destiantion address.
> 
> Futhure attempts to send UDP packets may succeed because pmtu
> get updated on ICMPV6_PKT_TOOBIG in vti6_err().
> 
> Here is the example when output device MTU set to 9000:
> 
>   # ip a sh ltp_ns_veth2
>     ltp_ns_veth2@if7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 ...
>       inet 10.0.0.2/24 scope global ltp_ns_veth2
>       inet6 fd00::2/64 scope global
>       ...
>   # ip li add vti6 type vti6 local fd00::2 remote fd00::1
>   # ip li show vti6
>     vti6@...E: <POINTOPOINT,NOARP> mtu 1500 ...
>       link/tunnel6 fd00::2 peer fd00::1
> 
> After the patch:
> 
>   # ip li add vti6 type vti6 local fd00::2 remote fd00::1
>   # ip li show vti6
>     vti6@...E: <POINTOPOINT,NOARP> mtu 8832 ...
>       link/tunnel6 fd00::2 peer fd00::1
> 
> Regarding ip_vti, it already tunes mtu with ip_tunnel_bind_dev():
> 
>   # ip li add vti4 type vti local 10.0.0.2 remote 10.0.0.1
>   # ip li sh vti4
>     vti4@...E: <POINTOPOINT,NOARP> mtu 8832 ...
>       link/ipip 10.0.0.2 peer 10.0.0.1
> 
> Reported-by: Petr Vorel <pvorel@...e.cz>
> Signed-off-by: Alexey Kodanev <alexey.kodanev@...cle.com>
> ---
> 
> ip6_vti mtu offset is the same (168) as in ip_vti because ip_vti
> offset includes two sizes of struct iphdr: in dev->hard_header_len
> and in t_hlen in ip_tunnel_bind_dev(). I'm not sure if it's correct.
> 
>  net/ipv6/ip6_vti.c |   18 ++++++++++++++++++
>  1 files changed, 18 insertions(+), 0 deletions(-)
> 
> diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
> index dbb74f3..47e6464 100644
> --- a/net/ipv6/ip6_vti.c
> +++ b/net/ipv6/ip6_vti.c
> @@ -638,6 +638,24 @@ static void vti6_link_config(struct ip6_tnl *t)
>  		dev->flags |= IFF_POINTOPOINT;
>  	else
>  		dev->flags &= ~IFF_POINTOPOINT;
> +
> +	if (p->flags & IP6_TNL_F_CAP_XMIT) {
> +		int strict = (ipv6_addr_type(&p->raddr) &
> +			      (IPV6_ADDR_MULTICAST | IPV6_ADDR_LINKLOCAL));
> +
> +		struct rt6_info *rt = rt6_lookup(t->net,
> +						 &p->raddr, &p->laddr,
> +						 p->link, strict);
> +
> +		if (!rt)
> +			return;
> +
> +		if (rt->dst.dev) {
> +			dev->mtu = max(rt->dst.dev->mtu - dev->hard_header_len,
> +				       IPV6_MIN_MTU);

Hm, I'm gettting this when compiling with your patch:

In file included from /home/klassert/git/ipsec-next/include/linux/list.h:9:0,
from /home/klassert/git/ipsec-next/include/linux/module.h:9,
from /home/klassert/git/ipsec-next/net/ipv6/ip6_vti.c:18:
/home/klassert/git/ipsec-next/net/ipv6/ip6_vti.c: In function ‘vti6_link_config’:
/home/klassert/git/ipsec-next/include/linux/kernel.h:808:16: warning: comparison of distinct pointer types lacks a cast
 (void) (&max1 == &max2);   \
               ^
/home/klassert/git/ipsec-next/include/linux/kernel.h:817:2: note: in expansion of macro ‘__max’
 __max(typeof(x), typeof(y),   \
 ^~~~~
/home/klassert/git/ipsec-next/net/ipv6/ip6_vti.c:654:15: note: in expansion of macro ‘max’
  dev->mtu = max(rt->dst.dev->mtu - dev->hard_header_len,