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

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Message-ID: <20220519173436.z7g2sshz5ivqlpe7@skbuf>
Date:   Thu, 19 May 2022 17:34:37 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Florian Fainelli <f.fainelli@...il.com>
CC:     Maxime Chevallier <maxime.chevallier@...tlin.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "thomas.petazzoni@...tlin.com" <thomas.petazzoni@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Luka Perkov <luka.perkov@...tura.hr>,
        Robert Marko <robert.marko@...tura.hr>
Subject: Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging
 protocol

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.