netdev - Re: [PATCH net-next v2 01/12] flow_dissector: Parse ETH_P

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Message-ID: <66c4c493543_aed9229422@willemb.c.googlers.com.notmuch>
Date: Tue, 20 Aug 2024 12:30:11 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Tom Herbert <tom@...bertland.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, 
 kuba@...nel.org, 
 edumazet@...gle.com, 
 netdev@...r.kernel.org, 
 felipe@...anda.io
Subject: Re: [PATCH net-next v2 01/12] flow_dissector: Parse ETH_P_TEB and
 move out of GRE

Tom Herbert wrote:
> On Fri, Aug 16, 2024 at 11:54 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Tom Herbert wrote:
> > > ETH_P_TEB (Trans Ether Bridging) is the EtherType to carry
> > > a plain Etherent frame. Add case in skb_flow_dissect to parse
> > > packets of this type
> > >
> > > If the GRE protocol is ETH_P_TEB then just process that as any
> > > another EtherType since it's now supported in the main loop
> > >
> > > Signed-off-by: Tom Herbert <tom@...bertland.com>
> >
> > Reviewed-by: Willem de Bruijn <willemb@...gle.com>
> >
> > > -     if (gre_ver == 0) {
> > > -             if (*p_proto == htons(ETH_P_TEB)) {
> > > -                     const struct ethhdr *eth;
> > > -                     struct ethhdr _eth;
> > > -
> > > -                     eth = __skb_header_pointer(skb, *p_nhoff + offset,
> > > -                                                sizeof(_eth),
> > > -                                                data, *p_hlen, &_eth);
> > > -                     if (!eth)
> > > -                             return FLOW_DISSECT_RET_OUT_BAD;
> > > -                     *p_proto = eth->h_proto;
> > > -                     offset += sizeof(*eth);
> > > -
> > > -                     /* Cap headers that we access via pointers at the
> > > -                      * end of the Ethernet header as our maximum alignment
> > > -                      * at that point is only 2 bytes.
> > > -                      */
> > > -                     if (NET_IP_ALIGN)
> > > -                             *p_hlen = *p_nhoff + offset;
> > > -             }
> > > -     } else { /* version 1, must be PPTP */
> >
> > > @@ -1284,6 +1268,27 @@ bool __skb_flow_dissect(const struct net *net,
> > >
> > >               break;
> > >       }
> > > +     case htons(ETH_P_TEB): {
> > > +             const struct ethhdr *eth;
> > > +             struct ethhdr _eth;
> > > +
> > > +             eth = __skb_header_pointer(skb, nhoff, sizeof(_eth),
> > > +                                        data, hlen, &_eth);
> > > +             if (!eth)
> > > +                     goto out_bad;
> > > +
> > > +             proto = eth->h_proto;
> > > +             nhoff += sizeof(*eth);
> > > +
> > > +             /* Cap headers that we access via pointers at the
> > > +              * end of the Ethernet header as our maximum alignment
> > > +              * at that point is only 2 bytes.
> > > +              */
> > > +             if (NET_IP_ALIGN)
> > > +                     hlen = nhoff;
> >
> > I wonder why this exists. But besides the point of this move.
> 
> Willem,
> 
> Ethernet header breaks 4-byte alignment of encapsulated protocols
> since it's 14 bytes, so the NET_IP_ALIGN can be used on architectures
> that don't like unaligned loads.

I understand how NET_IP_ALIGN is used by drivers.

I don't understand its use here in the flow dissector. Why is hlen
capped if it is set?