netdev - Re: VLAN packets silently dropped in promiscuous mode

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Message-ID: <AANLkTinH1Q4iq1_wg7HU_==khAtKOTZ2gz79s_EfzLYP@mail.gmail.com>
Date:	Mon, 25 Oct 2010 17:40:57 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Guillaume Gaudonville <guillaume.gaudonville@...nd.com>
Cc:	Roger Luethi <rl@...lgate.ch>, netdev@...r.kernel.org,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: VLAN packets silently dropped in promiscuous mode

On Mon, Oct 25, 2010 at 6:48 AM, Guillaume Gaudonville
<guillaume.gaudonville@...nd.com> wrote:
> Jesse Gross wrote:
>>
>> On Fri, Oct 15, 2010 at 2:16 AM, Guillaume Gaudonville
>> <guillaume.gaudonville@...nd.com> wrote:
>>
>>>
>>> Jesse Gross wrote:
>>>
>>>>
>>>> On Thu, Sep 30, 2010 at 1:07 AM, Roger Luethi <rl@...lgate.ch> wrote:
>>>>
>>>>
>>>>>
>>>>> On Wed, 29 Sep 2010 10:44:26 -0700, Jesse Gross wrote:
>>>>>
>>>>>
>>>>>>
>>>>>> On Wed, Sep 29, 2010 at 4:37 AM, Roger Luethi <rl@...lgate.ch> wrote:
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> I noticed packets for unknown VLANs getting silently dropped even in
>>>>>>> promiscuous mode (this is true only for the hardware accelerated
>>>>>>> path).
>>>>>>> netif_nit_deliver was introduced specifically to prevent that, but
>>>>>>> the
>>>>>>> function gets called only _after_ packets from unknown VLANs have
>>>>>>> been
>>>>>>> dropped.
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> Some drivers are fixing this on a case by case basis by disabling
>>>>>> hardware accelerated VLAN stripping when in promiscuous mode, i.e.:
>>>>>>
>>>>>>
>>>>>> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=5f6c01819979afbfec7e0b15fe52371b8eed87e8
>>>>>>
>>>>>> However, at this point it is more or less random which drivers do
>>>>>> this.  It would obviously be much better if it were consistent.
>>>>>>
>>>>>>
>>>>>
>>>>> My understanding is this. Hardware VLAN tagging and stripping can
>>>>> always
>>>>> be
>>>>> enabled. The kernel passes 802.1Q information along with the stripped
>>>>> header to libpcap which reassembles the original header where
>>>>> necessary.
>>>>> Works for me.
>>>>>
>>>>>
>>>>
>>>> Sorry, I misread your original post as saying that the VLAN header
>>>> gets dropped, rather than the entire packet.  I agree that this is how
>>>> it should work but not necessarily how it does work (again, depending
>>>> on the driver).  Here's the problem that I was talking about:
>>>>
>>>> Most drivers have a snippet of code that looks something like this
>>>> (taken from ixgbe):
>>>>
>>>> if (adapter->vlgrp && is_vlan && (tag & VLAN_VID_MASK))
>>>>       vlan_gro_receive(napi, adapter->vlgrp, tag, skb);
>>>> else
>>>>       napi_gro_receive(napi, skb);
>>>>
>>>> At this point the VLAN has already been stripped in hardware.  If
>>>> there is no VLAN group configured on the device then we hit the second
>>>> case.  The VLAN header was removed from the SKB and the tag variable
>>>> is unused.  It is no longer possible for libpcap to reconstruct the
>>>> header because the information was thrown away (even the fact that
>>>> there was a VLAN tag at all).
>>>>
>>>> There are a couple ways to fix this:
>>>>
>>>> * Turn off VLAN stripping when in promiscuous mode (as done by the ixgbe
>>>> driver)
>>>>
>>>>
>>>
>>> This is not totally true: if changing the MTU ixgbe_change_mtu will call:
>>> ixgbe_reinit_locked--> ixgbe_up --> ixgbe_configure:
>>>               --> ixgbe_set_rx_mode: flag IFF_PROMISC is tested
>>> ixgbe_vlan_filter_enable is not called
>>>               --> ixgbe_restore_vlan --> ixgbe_vlan_rx_register: flag
>>> IFF_PROMISC is not tested ixgbe_vlan_filter_enable
>>>                    will be called.
>>>
>>> In fact it should happen each time we configure something which needs a
>>> reset of the device. Why don't add a test
>>> on flag promiscuous directly in ixgbe_vlan_filter_enable? Or do it on
>>> each
>>> call, if we want to allow a device in promiscuous
>>> mode to enable this feature.
>>>
>>> What do you think?
>>>
>>
>> I can believe that there are paths that lead to this not working
>> correctly.  That was actually my larger point: this is something that
>> is commonly not implemented correctly in drivers.  Rather than try to
>> study every driver my goal is to just avoid the problem completely by
>> handling vlan acceleration centrally in the networking core.  I sent
>> out an RFC patch series a few days ago that should solve this problem:
>>
>> http://marc.info/?l=linux-netdev&m=128700022614170&w=3
>> --
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>>
>
> Thank you, I'm going to check these patches and try to apply them in our
> kernel.

An updated set of patches has been merged into net-2.6, so you might
want to try that instead.
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