From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [net-next 2/2] ifb: add device MTU validation check
Date: Thu, 21 Sep 2017 08:10:10 -0700
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Cc: davem@davemloft.net, willemb@google.com, netdev@vger.kernel.org
To: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
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On Thu, 21 Sep 2017 21:32:02 +0800
Zhang Shengju <zhangshengju@cmss.chinamobile.com> wrote:

> Currently, any mtu value can be assigned when adding a new ifb device:
> [~]# ip link add name ifb2 mtu 100000 type ifb
> [~]# ip link show ifb2
> 18: ifb2: <BROADCAST,NOARP> mtu 100000 qdisc noop state DOWN mode DEFAULT group default qlen 32
>     link/ether 7a:bf:f4:63:da:d1 brd ff:ff:ff:ff:ff:ff
> 
> This patch adds device MTU validation check.
> 
> Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
> ---
>  drivers/net/ifb.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/net/ifb.c b/drivers/net/ifb.c
> index 8870bd2..ce84ad2 100644
> --- a/drivers/net/ifb.c
> +++ b/drivers/net/ifb.c
> @@ -282,6 +282,14 @@ static int ifb_validate(struct nlattr *tb[], struct nlattr *data[],
>  		if (!is_valid_ether_addr(nla_data(tb[IFLA_ADDRESS])))
>  			return -EADDRNOTAVAIL;
>  	}
> +
> +	if (tb[IFLA_MTU]) {
> +		u32 mtu = nla_get_u32(tb[IFLA_MTU]);
> +
> +		if (mtu < ETH_MIN_MTU || mtu > ETH_DATA_LEN)
> +			return -EINVAL;
> +	}
> +
>  	return 0;
>  }
>  

What about running ifb with packets coming from devices with jumbo frames?
Also since ifb is an input only device, MTU doesn't matter.