netdev - Re: [PATCH net 1/1] net: stmmac: Prevent DSA tags from breaking COE

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Date: Thu, 21 Dec 2023 00:00:52 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>, 
	Romain Gantois <romain.gantois@...tlin.com>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Miquel Raynal <miquel.raynal@...tlin.com>, 
	Sylvain Girard <sylvain.girard@...com>, Pascal EBERHARD <pascal.eberhard@...com>, 
	Richard Tresidder <rtresidd@...ctromag.com.au>, netdev@...r.kernel.org, 
	linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH net 1/1] net: stmmac: Prevent DSA tags from breaking COE

On Wed, Dec 20, 2023 at 1:43 AM Linus Walleij <linus.walleij@...aro.org> wrote:

> Then for *this* driver I need to check for the ethertype
> ETH_P_8021Q what is inside it, one level down, and that is a
> separate helper. And I named it skb_vlan_raw_inner_ethertype()
> It will retrieve the inner type no matter
>
> include/linux/if_vlan.h
>
> +/* This determines the inner ethertype incoded into the skb data without
> + * relying on skb->protocol which is not always identical.
> + */
> +static inline u16 skb_vlan_raw_inner_ethertype(const struct sk_buff *skb)
> +{
> +       struct vlan_ethhdr *vhdr;
> +
> +       if (!skb_pointer_if_linear(skb, 0, VLAN_ETH_HLEN))
> +               return 0x0000U;
> +
> +       vhdr = vlan_eth_hdr(skb);
> +       return ntohs(vhdr->h_vlan_encapsulated_proto);
> +}
>
> (We can bikeshed the name of the function. *_inner_protocol maybe.)
>
> It does not handle nested VLANs and I don't see why it should since
> the immediate siblings in if_vlan.h does not, i.e.
> vlan_eth_hdr(), skb_vlan_eth_hdr(). It's pretty clear these helpers
> all go just one level down. (We can add a *_descend_*()
> helper the day someone needs that.)

Forget this whole discussion because in <linux/if_vlan.h>
there is already vlan_get_protocol() and vlan_get_protocol_and_depth()
so this problem is already solved, just better.

Yours,
Linus Walleij