Re: [PATCH net-next 1/3] net: gre_demux: add skb drop reasons to gre_rcv()

[Menglong Dong]

On Wed, Mar 16, 2022 at 11:08 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Mon, 14 Mar 2022 21:33:10 +0800 menglong8.dong@xxxxxxxxx wrote:
> > +     reason = SKB_DROP_REASON_NOT_SPECIFIED;
> >       if (!pskb_may_pull(skb, 12))
> >               goto drop;
>
> REASON_HDR_TRUNC ?

I'm still not sure about such a 'pskb_pull' failure, whose reasons may be
complex, such as no memory or packet length too small. I see somewhere
return a '-NOMEM' when skb pull fails.

So maybe such cases can be ignored? In my opinion, not all skb drops
need a reason.


>
> >       ver = skb->data[1]&0x7f;
> > -     if (ver >= GREPROTO_MAX)
> > +     if (ver >= GREPROTO_MAX) {
> > +             reason = SKB_DROP_REASON_GRE_VERSION;
>
> TBH I'm still not sure what level of granularity we should be shooting
> for with the reasons. I'd throw all unexpected header values into one
> bucket, not go for a reason per field, per protocol. But as I'm said
> I'm not sure myself, so we can keep what you have..
>
> >               goto drop;
> > +     }
> >
> >       rcu_read_lock();
> >       proto = rcu_dereference(gre_proto[ver]);
> > -     if (!proto || !proto->handler)
> > +     if (!proto || !proto->handler) {
> > +             reason = SKB_DROP_REASON_GRE_NOHANDLER;
>
> I think the ->handler check is defensive programming, there's no
> protocol upstream which would leave handler NULL.
>
> This is akin to SKB_DROP_REASON_PTYPE_ABSENT, we can reuse that or add
> a new reason, but I'd think the phrasing should be kept similar.

With the handler not NULL, does it mean the gre version is not supported here,
and this 'SKB_DROP_REASON_GRE_NOHANDLER' can be replaced with
SKB_DROP_REASON_GRE_VERSION above?

>
> >               goto drop_unlock;
> > +     }
> >       ret = proto->handler(skb);