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Re: [Xen-devel] [PATCH] xen-netfront: drop skb when skb->len > 65535

On Wed, 2013-03-06 at 17:20 +0000, Nick Pegg wrote:
> On 3/2/13 8:32 AM, Wei Liu wrote:
> > 
> > As Nick (npegg@xxxxxxxxxx) pointed out in his email, he saw size 65538.
> > I can reproduce this as well by setting vif's mtu to 100 then run iperf.
> > 100 was just a random number I came up with when I played with
> > fragmentation.
> > 
> Wei: How exactly did you set the MTU on the vif and what were your
> arguments to iperf? I tried this on a test host and was unable to
> trigger the XSA-39 protection.

Sorry I didn't state this clearly. I tend to call the interface inside
VM as vif as well. I did this inside a VM, ifconfig eth0 mtu 100.
Nothing fancy added to iperf command line, just `iperf -c XXX` in VM and
`iperf -s` in host (I limited the test time with -t though).

> As for a more graceful punishment, I noticed that xenvif_carrier_off()
> uses netif_carrier_off() to discard queued packets. Would doing a
> netif_carrier_off() and then a netif_carrier_on() be sufficient to drop
> the bad packets and keep netback from spinning? Would blinking the vif's
> carrier in this way allow the DomU to gracefully resume network traffic?

I think the spinning has something to do with the ring, not packets --
I'm not very sure though, because I didn't take part in the discussion /
development of XSA-39. Draining host queues is not likely to fix this.


> -Nick

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