Subject: Re: [PATCH] veth: Optionally pad packets to minimum Ethernet length

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Tue, 12 Dec 2017 16:18:17 -0200

On Tue, Dec 12, 2017 at 11:32:46AM -0600, Dan Williams wrote:
> On Tue, 2017-12-12 at 08:13 -0800, Ed Swierk wrote:
> > Most physical Ethernet devices pad short packets to the minimum
> > length
> > of 64 bytes (including FCS) on transmit. It can be useful to simulate
> > this behavior when debugging a problem that results from it (such as
> > incorrect L4 checksum calculation).
> > 
> > Padding is unnecessary for most applications so leave it off by
> > default. Enable padding only when the otherwise unused IFF_AUTOMEDIA
> > flag is set (e.g. by writing 0x5003 to flags in sysfs).
> 
> This seems like a weird overload of AUTOMEDIA, which no other driver
> uses for this purpose.  Seems like the only other user of AUTOMEDIA is
> 8390/etherh.c for some 10BaseT/10Base2 stuff.
> 
> I'm not sure what the interface should be, but perhaps a sysfs
> attribute would be better than overloading IFF_AUTOMEDIA?

What about using some tc action (i.e. skbmod) for this?

  Marcelo

> 
> Dan
> 
> > Signed-off-by: Ed Swierk <[email-masked]>
> > ---
> >  drivers/net/veth.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index f5438d0978ca..292029bf4bb2 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -111,6 +111,12 @@ static netdev_tx_t veth_xmit(struct sk_buff
> > *skb, struct net_device *dev)
> >  		goto drop;
> >  	}
> >  
> > +	if (unlikely(dev->flags & IFF_AUTOMEDIA)) {
> > +		/* if eth_skb_pad returns an error the skb was freed
> > */
> > +		if (eth_skb_pad(skb))
> > +			goto drop;
> > +	}
> > +
> >  	if (likely(dev_forward_skb(rcv, skb) == NET_RX_SUCCESS)) {
> >  		struct pcpu_vstats *stats = this_cpu_ptr(dev-
> > >vstats);
> >  
>