Subject: Re: [PATCH v2 net-next 1/2] net: centralize net_device min/max MTU checking

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 30 Sep 2016 11:37:24 +0200

On Wed, Sep 28, 2016 at 10:20 PM GMT, Jarod Wilson wrote:
> While looking into an MTU issue with sfc, I started noticing that almost
> every NIC driver with an ndo_change_mtu function implemented almost
> exactly the same range checks, and in many cases, that was the only
> practical thing their ndo_change_mtu function was doing. Quite a few
> drivers have either 68, 64, 60 or 46 as their minimum MTU value checked,
> and then various sizes from 1500 to 65535 for their maximum MTU value. We
> can remove a whole lot of redundant code here if we simple store min_mtu
> and max_mtu in net_device, and check against those in net/core/dev.c's
> dev_set_mtu().
>
> In theory, there should be zero functional change with this patch, it just
> puts the infrastructure in place. Subsequent patches will attempt to start
> using said infrastructure, with theoretically zero change in
> functionality.
>
> CC: "David S. Miller" <[email-masked]>
> CC: [email-masked]
> Signed-off-by: Jarod Wilson <[email-masked]>
> ---

[...]

> diff --git a/net/core/dev.c b/net/core/dev.c
> index c0c291f..5343799 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6493,9 +6493,17 @@ int dev_set_mtu(struct net_device *dev, int new_mtu)
>  	if (new_mtu == dev->mtu)
>  		return 0;
>  
> -	/*	MTU must be positive.	 */
> -	if (new_mtu < 0)
> +	if (new_mtu < dev->min_mtu) {

Ouch, integral promotions. Looks like you need to keep the < 0 check.
Otherwise new_mtu gets promoted to unsigned int and negative values will
pass the check.

Thanks,
Jakub