netdev - Re: [PATCH net-next] net: ipv4: Add ability to have GRE ignore DF bit in IPv4 payloads

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Date:	Tue, 7 Jun 2016 20:22:19 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Philip Prindeville <philipp@...fish-solutions.com>
Cc:	Netdev <netdev@...r.kernel.org>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net-next] net: ipv4: Add ability to have GRE ignore DF bit
 in IPv4 payloads

On Tue, Jun 7, 2016 at 12:48 PM, Philip Prindeville
<philipp@...fish-solutions.com> wrote:
> From: Philip Prindeville <philipp@...fish-solutions.com>
>
> In the presence of firewalls which improperly block ICMP Unreachable
> (including Fragmentation Required) messages, Path MTU Discovery is
> prevented from working.
>
> A workaround is to handle IPv4 payloads opaquely, ignoring the DF bit--as
> is done for other payloads like AppleTalk--and doing transparent
> fragmentation and reassembly.
>
> Reviewed-by: Stephen Hemminger <stephen@...workplumber.org>
> Signed-off-by: Philip Prindeville <philipp@...fish-solutions.com>
> ---
>  include/net/ip_tunnels.h       |    1 +
>  include/uapi/linux/if_tunnel.h |    1 +
>  net/ipv4/ip_gre.c              |   13 +++++++++++--
>  net/ipv4/ip_tunnel.c           |    2 +-
>  4 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
> index dbf4444..9222678 100644
> --- a/include/net/ip_tunnels.h
> +++ b/include/net/ip_tunnels.h
> @@ -132,6 +132,7 @@ struct ip_tunnel {
>         int                     ip_tnl_net_id;
>         struct gro_cells        gro_cells;
>         bool                    collect_md;
> +       bool                    ignore_df;
>  };
>
>  #define TUNNEL_CSUM            __cpu_to_be16(0x01)
> diff --git a/include/uapi/linux/if_tunnel.h b/include/uapi/linux/if_tunnel.h
> index af4de90..1046f55 100644
> --- a/include/uapi/linux/if_tunnel.h
> +++ b/include/uapi/linux/if_tunnel.h
> @@ -113,6 +113,7 @@ enum {
>         IFLA_GRE_ENCAP_SPORT,
>         IFLA_GRE_ENCAP_DPORT,
>         IFLA_GRE_COLLECT_METADATA,
> +       IFLA_GRE_IGNORE_DF,
>         __IFLA_GRE_MAX,
>  };
>
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 4d2025f..cbeecfb 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -846,6 +846,8 @@ static void ipgre_netlink_parms(struct net_device *dev,
>                                 struct nlattr *tb[],
>                                 struct ip_tunnel_parm *parms)
>  {
> +       struct ip_tunnel *t = netdev_priv(dev);
> +
>         memset(parms, 0, sizeof(*parms));
>
>         parms->iph.protocol = IPPROTO_GRE;
> @@ -884,12 +886,13 @@ static void ipgre_netlink_parms(struct net_device *dev,
>                 parms->iph.frag_off = htons(IP_DF);
>
>         if (data[IFLA_GRE_COLLECT_METADATA]) {
> -               struct ip_tunnel *t = netdev_priv(dev);
> -
>                 t->collect_md = true;
>                 if (dev->type == ARPHRD_IPGRE)
>                         dev->type = ARPHRD_NONE;
>         }
> +
> +       if (data[IFLA_GRE_IGNORE_DF])
> +               t->ignore_df = !!nla_get_u8(data[IFLA_GRE_IGNORE_DF]);

Why are you passing this as a u8?  Why not just pass it as a flag
since it defaults to off?

>  }
>
>  /* This function returns true when ENCAP attributes are present in the nl msg */
> @@ -1024,6 +1027,8 @@ static size_t ipgre_get_size(const struct net_device *dev)
>                 nla_total_size(2) +
>                 /* IFLA_GRE_COLLECT_METADATA */
>                 nla_total_size(0) +
> +               /* IFLA_GRE_IGNORE_DF */
> +               nla_total_size(1) +
>                 0;
>  }

Switching it to a flag would help reduce the total size.

> @@ -1057,6 +1062,9 @@ static int ipgre_fill_info(struct sk_buff *skb, const struct net_device *dev)
>                         t->encap.flags))
>                 goto nla_put_failure;
>
> +       if (nla_put_u8(skb, IFLA_GRE_IGNORE_DF, t->ignore_df))
> +               goto nla_put_failure;
> +
>         if (t->collect_md) {
>                 if (nla_put_flag(skb, IFLA_GRE_COLLECT_METADATA))
>                         goto nla_put_failure;
> @@ -1084,6 +1092,7 @@ static const struct nla_policy ipgre_policy[IFLA_GRE_MAX + 1] = {
>         [IFLA_GRE_ENCAP_SPORT]  = { .type = NLA_U16 },
>         [IFLA_GRE_ENCAP_DPORT]  = { .type = NLA_U16 },
>         [IFLA_GRE_COLLECT_METADATA]     = { .type = NLA_FLAG },
> +       [IFLA_GRE_IGNORE_DF]    = { .type = NLA_U8 },
>  };
>

You could just copy the approach used for COLLECT_METADATA and skip
the need to pass any data and instead just use a flag.

>  static struct rtnl_link_ops ipgre_link_ops __read_mostly = {
> diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> index d8f5e0a..95649eb 100644
> --- a/net/ipv4/ip_tunnel.c
> +++ b/net/ipv4/ip_tunnel.c
> @@ -682,7 +682,7 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
>         }
>
>         df = tnl_params->frag_off;
> -       if (skb->protocol == htons(ETH_P_IP))
> +       if (skb->protocol == htons(ETH_P_IP) && !tunnel->ignore_df)
>                 df |= (inner_iph->frag_off&htons(IP_DF));
>
>         max_headroom = LL_RESERVED_SPACE(rt->dst.dev) + sizeof(struct iphdr)

Looking at this it appears that you should probably make the feature
mutually exclusive from the pmtudisc feature.  It wouldn't make much
sense to set the tunnel to ignore the inner DF bit and then turn
around and set it because you are enforcing path MTU discovery on the
tunnel itself.