[ovs-dev] [PATCH] dpif-netlink-rtnl: Work around MTU bug in kernel GRE driver.

Eric Garver e at erig.me
Fri Jan 12 21:35:06 UTC 2018


On Fri, Jan 12, 2018 at 12:44:59PM -0800, Ben Pfaff wrote:
> The kernel GRE driver ignores IFLA_MTU in RTM_NEWLINK requests and
> overrides the MTU to 1472 bytes.  This commit works around the problem by
> following up a request to create a GRE device with a second request to set
> the MTU.
> 
> Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=1488484
> Reported-by: Eric Garver <e at erig.me>
> Reported-by: James Page <james.page at ubuntu.com>
> Signed-off-by: Ben Pfaff <blp at ovn.org>
> ---
> This is not properly tested.  It needs to be tested before it makes sense
> to commit it.
> 
>  lib/dpif-netlink-rtnl.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/lib/dpif-netlink-rtnl.c b/lib/dpif-netlink-rtnl.c
> index fe9c8ed7104f..0bf965d29f41 100644
> --- a/lib/dpif-netlink-rtnl.c
> +++ b/lib/dpif-netlink-rtnl.c
> @@ -329,6 +329,19 @@ dpif_netlink_rtnl_create(const struct netdev_tunnel_config *tnl_cfg,
>      nl_msg_end_nested(&request, linkinfo_off);
>  
>      err = nl_transact(NETLINK_ROUTE, &request, NULL);
> +    if (!err && type == OVS_VPORT_TYPE_GRE) {
> +        /* Work around a bug in kernel GRE driver, which ignores IFLA_MTU in
> +         * RTM_NEWLINK, by setting the MTU again.  See
> +         * https://bugzilla.redhat.com/show_bug.cgi?id=1488484. */
> +        ofpbuf_clear(&request);
> +        nl_msg_put_nlmsghdr(&request, 0, RTM_SETLINK,
> +                            NLM_F_REQUEST | NLM_F_ACK);
> +        ofpbuf_put_zeros(&request, sizeof(struct ifinfomsg));
> +        nl_msg_put_string(&request, IFLA_IFNAME, name);
> +        nl_msg_put_u32(&request, IFLA_MTU, UINT16_MAX);
> +
> +        err = nl_transact(NETLINK_ROUTE, &request, NULL);
> +    }
>  
>  exit:
>      ofpbuf_uninit(&request);
> -- 
> 2.10.2
> 

This looks sane to me. It would be nice if James could test it though.

Acked-by: Eric Garver <e at erig.me>


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