netdev - Re: [PATCH v3 3/3] packet: Deliver VLAN TPID to userspace

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Date:	Tue, 17 Dec 2013 14:51:38 +0900
From:	Atzm Watanabe <atzm@...atosphere.co.jp>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	netdev@...r.kernel.org,
	"Ben Hutchings" <bhutchings@...arflare.com>,
	"David Miller" <davem@...emloft.net>,
	"Daniel Borkmann" <dborkman@...hat.com>,
	"David Laight" <David.Laight@...LAB.COM>
Subject: Re: [PATCH v3 3/3] packet: Deliver VLAN TPID to userspace

At Mon, 16 Dec 2013 09:25:09 -0800,
Stephen Hemminger wrote:
> 
> On Mon, 16 Dec 2013 17:12:09 +0900
> Atzm Watanabe <atzm@...atosphere.co.jp> wrote:
> 
> >  struct tpacket_hdr_variant1 {
> >  	__u32	tp_rxhash;
> >  	__u32	tp_vlan_tci;
> > +	__u32	tp_vlan_tpid;
> >  };
> 
> Is this safe? Won't you break ABI for existing userspace applications.

I think we can do it safely without breaking userspace.
struct tpacket_hdr_variant1 is an union member of struct tpacket3_hdr,
and it is not used for any others in the kernel-userspace ABI.
struct tpacket3_hdr is aligned to a multiple of 16 bytes, its size is
currently 48 bytes.  Against that, the size before aligning is
currently 36 bytes, so there is a gap of 12 bytes that is unused.

Previous patch 2/3 adds padding members for filling the gap, after the
patch 1/3 which adds BUILD_BUG_ON() as sentinel for aligned size.
The patch 3/3 adds tp_vlan_tpid by picking area from the padding added
in the patch 2/3.
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