Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging protocol

From: Vladimir Oltean <vladimir.oltean@nxp.com>
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: Maxime Chevallier <maxime.chevallier@bootlin.com>,
	"davem@davemloft.net" <davem@davemloft.net>,
	Rob Herring <robh+dt@kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"thomas.petazzoni@bootlin.com" <thomas.petazzoni@bootlin.com>,
	Andrew Lunn <andrew@lunn.ch>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Luka Perkov <luka.perkov@sartura.hr>,
	Robert Marko <robert.marko@sartura.hr>
Subject: Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging protocol
Date: Thu, 19 May 2022 17:34:37 +0000	[thread overview]
Message-ID: <20220519173436.z7g2sshz5ivqlpe7@skbuf> (raw)
In-Reply-To: <c16384a3-d868-11e7-ceed-bc8e7029962a@gmail.com>

On Thu, May 19, 2022 at 10:11:13AM -0700, Florian Fainelli wrote:
> unless we somehow manage to put it in the linear portion of
> the SKB to avoid using any control buffer or extension.

But how? Essentially the DSA master has to look at a packet and
determine whether it came from DSA based on something which non-DSA
code could not have done. In fact, I'm looking at the calls to
skb_reset_mac_{header,len} from net/core/skbuff.c, specifically at VLAN
and MPLS, and I believe (but haven't tested) that pushing such headers
would also alter skb->mac_len to some value != ETH_HLEN. So simply
having the DSA master check whether DSA was there by checking whether
skb->mac_len is ETH_HLEN + DSA tag len could easily confuse DSA with
some other protocol of same header size.