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@xxxxxxxxxxx> > --- 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?
