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

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Sat, May 14, 2022 at 05:06:53PM +0200, Maxime Chevallier wrote:
> This tagging protocol is designed for the situation where the link
> between the MAC and the Switch is designed such that the Destination
> Port, which is usually embedded in some part of the Ethernet Header, is
> sent out-of-band, and isn't present at all in the Ethernet frame.
> 
> This can happen when the MAC and Switch are tightly integrated on an
> SoC, as is the case with the Qualcomm IPQ4019 for example, where the DSA
> tag is inserted directly into the DMA descriptors. In that case,
> the MAC driver is responsible for sending the tag to the switch using
> the out-of-band medium. To do so, the MAC driver needs to have the
> information of the destination port for that skb.
> 
> This out-of-band tagging protocol is using the very beggining of the skb
> headroom to store the tag. The drawback of this approch is that the
> headroom isn't initialized upon allocating it, therefore we have a
> chance that the garbage data that lies there at allocation time actually
> ressembles a valid oob tag. This is only problematic if we are
> sending/receiving traffic on the master port, which isn't a valid DSA
> use-case from the beggining. When dealing from traffic to/from a slave
> port, then the oob tag will be initialized properly by the tagger or the
> mac driver through the use of the dsa_oob_tag_push() call.
> 
> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> ---

Why put the DSA pseudo-header at skb->head rather than push it using
skb_push()? I thought you were going to check for the presence of a DSA
header using something like skb->mac_len == ETH_HLEN + tag len, but
right now it sounds like treating garbage in the headroom as a valid DSA
tag is indeed a potential problem. If you can't sort that out using
information from the header offsets alone, maybe an skb extension is
required?