netdev - Re: [PATCH net-next v4 2/5] net: dsa: add out-of-band tagging protocol

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Date:   Sat, 17 Sep 2022 00:15:22 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Maxime Chevallier <maxime.chevallier@...tlin.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.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>,
        Florian Fainelli <f.fainelli@...il.com>,
        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 v4 2/5] net: dsa: add out-of-band tagging
 protocol

Hi Maxime,

On Fri, Sep 09, 2022 at 05:24:51PM +0200, Maxime Chevallier wrote:
> +int dsa_oob_tag_push(struct sk_buff *skb, struct dsa_oob_tag_info *ti)
> +{
> +	struct dsa_oob_tag_info *tag_info;
> +
> +	tag_info = (struct dsa_oob_tag_info *)skb->head;
> +
> +	tag_info->proto = ti->proto;
> +	tag_info->dp = ti->dp;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(dsa_oob_tag_push);
> +
> +int dsa_oob_tag_pop(struct sk_buff *skb, struct dsa_oob_tag_info *ti)
> +{
> +	struct dsa_oob_tag_info *tag_info;
> +
> +	tag_info = (struct dsa_oob_tag_info *)skb->head;
> +
> +	if (tag_info->proto != DSA_TAG_PROTO_OOB)
> +		return -EINVAL;
> +
> +	ti->proto = tag_info->proto;
> +	ti->dp = tag_info->dp;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(dsa_oob_tag_pop);
> +
> +static struct sk_buff *oob_tag_xmit(struct sk_buff *skb,
> +				    struct net_device *dev)
> +{
> +	struct dsa_port *dp = dsa_slave_to_port(dev);
> +	struct dsa_oob_tag_info tag_info;
> +
> +	tag_info.dp = dp->index;
> +	tag_info.proto = DSA_TAG_PROTO_OOB;
> +
> +	if (dsa_oob_tag_push(skb, &tag_info))
> +		return NULL;
> +
> +	return skb;
> +}

I don't have too many comments on this patch set, except for a very
fundamental one. It is impossible to pass a DSA out of band header
between the switch tagging protocol driver and the host Ethernet
controller via the beginning of skb->head, and just putting some magic
bytes there and hoping that no random junk in the buffer will have the
same value (and that skb_push() calls will not eat into your tag_info
structure which isn't accounted for in any way by skb->data).

Please create an skb extension for this, it is the only unambiguous way
to deal with the given hardware, which will not give lots of headaches
in the future.