netdev - RE: [PATCH net-next] ip_gre: check packet length and mtu correctly in erspan_fb

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Date:   Fri, 6 Oct 2017 09:38:50 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'William Tu' <u9012063@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Xin Long <lucien.xin@...il.com>
Subject: RE: [PATCH net-next] ip_gre: check packet length and mtu correctly
 in erspan_fb_xmit

From: William Tu
> Sent: 05 October 2017 22:21
...
> >> -     if (skb->len > dev->mtu) {
> >> +     if (skb->len - dev->hard_header_len > dev->mtu) {
> >
> > Can you guarantee that skb->len > dev_hard_header_len?
> > It is probably safer to check skb->len > dev->hard_header_len + dev->mtu
> > since that addition isn't going to overflow.
> Sure, I will fix it.
> 
> >
> >>               pskb_trim(skb, dev->mtu);
> >>               truncate = true;
> >
> > Is that pskb_trim() now truncating to the correct size?
> 
> You're right, now I should truncate to (dev->mtu + dev_hard_header_len)

It might be worth caching that length in the dev structure
to avoid the arithmetic on every packet.

	David