Subject: Re: [PATCH net v5] net: stmmac: Prevent DSA tags from breaking COE

From: Jakub Kicinski <[email-masked]>
Date: Fri, 12 Jan 2024 18:13:27 -0800

On Thu, 11 Jan 2024 15:58:51 +0100 Romain Gantois wrote:
> Some DSA tagging protocols change the EtherType field in the MAC header
> e.g.  DSA_TAG_PROTO_(DSA/EDSA/BRCM/MTK/RTL4C_A/SJA1105). On TX these tagged
> frames are ignored by the checksum offload engine and IP header checker of
> some stmmac cores.
> 
> On RX, the stmmac driver wrongly assumes that checksums have been computed
> for these tagged packets, and sets CHECKSUM_UNNECESSARY.
> 
> Add an additional check in the stmmac TX and RX hotpaths so that COE is
> deactivated for packets with ethertypes that will not trigger the COE and
> IP header checks.
> 
> Fixes: 6b2c6e4a938f ("net: stmmac: propagate feature flags to vlan")
> Cc:  <[email-masked]>

nit: double space

> +/**
> + * stmmac_has_ip_ethertype() - Check if packet has IP ethertype
> + * @skb: socket buffer to check
> + *
> + * Check if a packet has an ethertype that will trigger the IP header checks
> + * and IP/TCP checksum engine of the stmmac core.
> + *
> + * Return: true if the ethertype can trigger the checksum engine, false otherwise

nit: please don't go over 80 chars unless there's a good reason.
we are old school and stick to checkpatch --max-line-length=80 in netdev

>  	if (csum_insertion &&
> -	    priv->plat->tx_queues_cfg[queue].coe_unsupported) {
> +	    (priv->plat->tx_queues_cfg[queue].coe_unsupported ||
> +	    !stmmac_has_ip_ethertype(skb))) {

nit: minor misalignment here, the '!' should be under 'p'

>  		if (unlikely(skb_checksum_help(skb)))
>  			goto dma_map_err;
>  		csum_insertion = !csum_insertion;
> @@ -4997,7 +5020,7 @@ static void stmmac_dispatch_skb_zc(struct stmmac_priv *priv, u32 queue,
>  	stmmac_rx_vlan(priv->dev, skb);
>  	skb->protocol = eth_type_trans(skb, priv->dev);
>  
> -	if (unlikely(!coe))
> +	if (unlikely(!coe) || !stmmac_has_ip_ethertype(skb))

The lack of Rx side COE checking in this driver is kinda crazy.
Looking at enh_desc_coe_rdes0() it seems like RDES0_FRAME_TYPE
may be the indication we need here? 

We can dig into it as a follow up but I'm guessing that sending
an IPv6 packet with extension headers will also make the device
skip checksumming, or a UDP packet with csum of 0?