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

From: Jarod Wilson <[email-masked]>
Date: Mon, 3 Oct 2016 13:46:43 -0400

On Sun, Oct 02, 2016 at 10:43:22PM -0400, David Miller wrote:
> 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.
> 
> Agreed, the < 0 test must be reintroduced.

Gah, yeah, okay, will add it back in. Thinking like this:

if (new_mtu < 0 || new_mtu < dev->min_mtu) {

Alternatively, could have the negative value check on it's own, with a
harsher warning about negative values.

-- 
Jarod Wilson
[email-masked]