netdev - Re: [RFC PATCH 00/04]: VLAN vs. packet socket inconsistencies

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Message-Id: <20080708.151233.259338088.davem@davemloft.net>
Date:	Tue, 08 Jul 2008 15:12:33 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	kaber@...sh.net
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC PATCH 00/04]: VLAN vs. packet socket inconsistencies

From: Patrick McHardy <kaber@...sh.net>
Date: Tue, 08 Jul 2008 12:14:51 +0200

> There are mainly two remaining problems with this approach:
> 
> - socket filters for VLAN headers don't work properly since they
>    expect the VLAN header to be present. Since with the approach
>    taken by these patches, userspace has to have knowledge about
>    VLAN acceleration anyway, it seems OK to simply add a filter
>    instruction to filter on skb->vlan_tci and have userspace
>    construct its filters accordingly.
> 
> - packet socket auxdata is only available for non-mmaped sockets.
>    For mmaped sockets the only place to store the data is in
>    struct tpacket_hdr, but that would break compatibility. Not sure
>    what to do about this case.
> 
> Comments welcome.

The mmap socket limitation is pretty serious.  It's the only
thing holding me back from just applying these patches right now.

I think we should make a new version of the tpacket_hdr structure
anyways, in order to make it more compat-friendly.  It currently
has "unsigned long" members and other crap.

We can design this thing to be extensible quite easily, by simply
giving it an "offset" field.  My suggestion is:

1) sed 's/tpacket_hdr/tpacket_hdr_old/'

2) Create packet socket option PACKET_NEW_TPHDR which enables
   use of a new tpacket_hdr layout.

3) Make new tpacket_hdr which has the VLAN tag stuff as well
   as an 'offset' field so that we can add more stuff later
   in a backwards compat way.

4) New new tpacket_hdr layout when PACKET_NEW_TPHDR has been
   set.

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