Re: [Lxc-users] Bad checksums and lost packets with macvlan on dummy

[Patrick McHardy <kaber@trash.net>]

Am 02.03.2011 17:03, schrieb Daniel Lezcano:
> On 03/02/2011 12:03 PM, Patrick McHardy wrote:
>> Am 01.03.2011 21:04, schrieb Daniel Lezcano:
>>> On 03/01/2011 05:51 PM, Patrick McHardy wrote:
>>>>> Patrick, do you have any suggestions to fix this ?
>>>> Since the frames are only looped back locally, I suppose the easiest
>>>> fix would be to mark them with CHECKSUM_UNNECESSARY. Alternatively
>>>> we need to complete the checksum manually, similar to what
>>>> dev_hard_start_xmit() does.
>>> That sounds very simple to fix, maybe too much simple :)
>>>
>>> I did the following change:
>>>
>>> --- linux-next.orig/drivers/net/macvlan.c
>>> +++ linux-next/drivers/net/macvlan.c
>>> @@ -222,6 +222,7 @@ static int macvlan_queue_xmit(struct sk_
>>>
>>>          if (vlan->mode == MACVLAN_MODE_BRIDGE) {
>>>                  const struct ethhdr *eth = (void *)skb->data;
>>> +               skb->ip_summed = CHECKSUM_UNNECESSARY;
>>>
>>>                  /* send to other bridge ports directly */
>>>                  if (is_multicast_ether_addr(eth->h_dest)) {
>>>
>>>
>>> and that fixed the problem. Do you think it is acceptable ?
>> The only problem I see is if the packets are bridged to a
>> different networking device (or redirected using the mirred
>> action), in this case the checksum will not be completed.
>> This would be a very strange setup though and probably wouldn't
>> be using dummy as lower device, so I'm not sure we have to
>> worry about this case.
> 
> I am not sure to get it, do you say the patch is correct ?

Its correct with a short-coming that doesn't seem to matter.

> If my understanding is correct, the packet will be flagged
> CHECKSUM_UNNECESSARY only for the macvlan devices, right ?

Only for packets bridged between macvlan devices. A setup like
the following would cause problems:

                        br0
                         |
                   .----------.
                   |          |
macvlan0	macvlan1    eth0
   |               |
    -------.-------
	 dummy0

In this case packets sent from macvlan0 will show up on
eth0 with incorrect setups. However this setup doesn't
seem realistic to me, you would simply use eth0 instead
of dummy0.

> By the way, this problem occurs for any lower device with offloading
> capabilities with a macvlan port in bridge mode.

True. This doesn't affect outgoing packets since their checksum
will be completed in dev_hard_start_xmit(), but it affects
packets bridged between macvlans.

As I said, the full solution would be to complete the checksum
for CHECKSUM_PARTIAL packets when bridging between macvlans.