netdev - Re: [PATCH net-next] net: xdp: Update pkt_type if generic XDP changes unicast MAC

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Message-ID: <f14da35f8cfa4b8f888dadfe4c9ebcd031d8e870.camel@strongswan.org>
Date:   Tue, 20 Apr 2021 07:15:00 +0200
From:   Martin Willi <martin@...ongswan.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next] net: xdp: Update pkt_type if generic XDP
 changes unicast MAC

Hi,

Thanks for your comments.

> >  	eth = (struct ethhdr *)xdp->data;
> > +	orig_host = ether_addr_equal_64bits(eth->h_dest, skb->dev->dev_addr);
> 
> ether_addr_equal_64bits() seems to assume that the addresses passed to 
> it are padded to be 8 bytes long, which is not the case for eth->h_dest.
> AFAICT the only reason the _64bits variant works for multicast is that
> it happens to be only checking the top-most bit, but unless I'm missing
> something you'll have to use the boring old ether_addr_equal() here, no?

This is what eth_type_trans() uses below, so I assumed it is safe to
use. Isn't that working on the same data?

Also, the destination address in Ethernet is followed by the source
address, so two extra bytes in the source are used as padding. These
are then shifted out by ether_addr_equal_64bits(), no?

> > +		skb->pkt_type = PACKET_HOST;
> >  		skb->protocol = eth_type_trans(skb, skb->dev);
> >  	}
> 
> Okay, so this was a bit confusing to me at fist glance:
> eth_type_trans() will reset the type, but not back to PACKET_HOST. So
> this works, just a bit confusing :)

Indeed. I considered changing eth_type_trans() to always reset
pkt_type, but I didn't want to take the risk for any side effects.

Thanks!
Martin