From: ebiederm@xmission•com (Eric W. Biederman)
To: Jesse Gross <jesse@nicira•com>
Cc: "David Miller" <davem@davemloft•net>,
netdev@vger•kernel.org, "Michał Mirosław" <mirqus@gmail•com>,
"Ben Hutchings" <bhutchings@solarflare•com>,
"Eric Dumazet" <eric.dumazet@gmail•com>,
"John Fastabend" <john.r.fastabend@intel•com>
Subject: Re: [PATCH] vlan: Fix duplicate delivery of vlan 0 packets to ETH_P_ALL packet sockets
Date: Sat, 26 Mar 2011 23:27:38 -0700 [thread overview]
Message-ID: <m1zkoh9ixx.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <AANLkTinYDOAbF8TO_LA9UccEnBNCqLEyx7mpDBtfmH8z@mail.gmail.com> (Jesse Gross's message of "Wed, 23 Mar 2011 13:59:25 -0700")
Jesse Gross <jesse@nicira•com> writes:
> On Mon, Mar 21, 2011 at 2:35 PM, Eric W. Biederman
> <ebiederm@xmission•com> wrote:
>>
>> For vlan data coming in from nics without vlan hardware accelleration we
>> get two copies of vlan packets with vlan id 0 on pf_packet sockets, causing
>> userspace to break. This is caused by delivering the same packet to the same
>> networking device more than once.
>
> I agree that this is a problem and the code consolidation is very nice
> but I'm concerned that there is extra complexity for the rest of the
> system to counterbalance what is saved here.
>
>> diff --git a/net/8021q/vlan_core.c b/net/8021q/vlan_core.c
>> index ce8e3ab..a0849b9 100644
>> --- a/net/8021q/vlan_core.c
>> +++ b/net/8021q/vlan_core.c
>> +void emulate_vlan_hwaccel(struct sk_buff *skb)
>> +{
>> + struct vlan_hdr *vhdr = (struct vlan_hdr *)skb->data;
>> + __be16 proto;
>> +
>> + if (!pskb_may_pull(skb, VLAN_HLEN))
>> + return;
>> +
>> + __vlan_hwaccel_put_tag(skb, vhdr->h_vlan_TCI);
>> + skb_pull_rcsum(skb, VLAN_HLEN);
>
> Doesn't this break things which push the header back on? Bridging
> pushes ETH_HLEN before forwarding but here it will be a garbage value
> due to the extra vlan header. AF_PACKET pushes the mac header back
> on, which in this case includes the original vlan header. However,
> since we've also put the tag in skb->vlan_tci, won't it appear to be
> double tagged?
Probably that part does indeed look like a bug, and my testing certainly
shows that there are problems with my patch.
> More generally, even though we pull the tag off the skb it's pretty
> common on the receive path to look backwards into previous headers.
> Given that this can happen, I think it's somewhat confusing/fragile to
> have packet data which effectively should not be there. It also adds
> a third case to any generic vlan handling code: tag in packet (can
> still happen, such as on transmit), received on vlan accelerated NIC -
> tag in skb but not in packet, receive on non-vlan accelerated NIC -
> tag in both skb and packet.
>
> If we actually removed the tag in the emulated case that would avoid
> these concerns but would, of course, add extra overhead in some
> situations.
The only extra overhead I can really see is the need to put the vlan
tag back on in a few instances. Moving the ethernet addresses around
in the packet (the cost of adding/removing the vlan header) since they
are in a hot cacheline doesn't concern me very much.
But we definitely need to do something to fix the regression for
pf_packet sockets.
Eric
next prev parent reply other threads:[~2011-03-27 6:27 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-21 21:35 [PATCH] vlan: Fix duplicate delivery of vlan 0 packets to ETH_P_ALL packet sockets Eric W. Biederman
2011-03-23 20:59 ` Jesse Gross
2011-03-27 6:27 ` Eric W. Biederman [this message]
2011-04-02 1:41 ` Jesse Gross
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