netdev - Re: [PATCH net v3 1/3] net/sched: act_csum: pull all VLAN headers before checksumming

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Date:   Fri, 31 May 2019 15:50:16 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Davide Caratti <dcaratti@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Shuang Li <shuali@...hat.com>,
        Eli Britstein <elibr@...lanox.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net v3 1/3] net/sched: act_csum: pull all VLAN headers
 before checksumming

On Fri, May 31, 2019 at 3:01 PM Davide Caratti <dcaratti@...hat.com> wrote:
>
> Please note: this loop was here also before this patch (the 'goto again;'
> line is only patch context). It has been introduced with commit
> 2ecba2d1e45b ("net: sched: act_csum: Fix csum calc for tagged packets").
>

This is exactly why I ask...


> > Why do you still need to loop here? tc_skb_pull_vlans() already
> > contains a loop to pop all vlan tags?
>
> The reason why the loop is here is:
> 1) in case there is a stripped vlan tag, it replaces tc_skb_protocol(skb)
> with the inner ethertype (i.e. skb->protocol)
>
> 2) in case there is one or more unstripped VLAN tags, it pulls them. At
> the last iteration, when it does:

Let me ask it in another way:

The original code, without your patch, has a loop (the "goto again") to
pop all vlan tags.

The code with your patch adds yet another loop (the while loop inside your
tc_skb_pull_vlans()) to pop all vlan tags.

So, after your patch, we have both loops. So, I am confused why we need
these two nested loops to just pop all vlan tags? I think one is sufficient.


>
> >
> > >         }
> > >
> > > diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> > > index d4699156974a..382ee69fb1a5 100644
> > > --- a/net/sched/cls_api.c
> > > +++ b/net/sched/cls_api.c
> > > @@ -3300,6 +3300,28 @@ unsigned int tcf_exts_num_actions(struct tcf_exts *exts)
> > >  }
> > >  EXPORT_SYMBOL(tcf_exts_num_actions);
> > >
> > > +int tc_skb_pull_vlans(struct sk_buff *skb, unsigned int *hdr_count,
> > > +                     __be16 *proto)
> >
> > It looks like this function fits better in net/core/skbuff.c, because
> > I don't see anything TC specific.
>
> Ok, I don't know if other parts of the kernel really need it. Its use
> should be combined with tc_skb_protocol(), which is in pkt_sched.h.
>
> But i can move it to skbuff, or elsewhwere, unless somebody disagrees.
>
> >
> > > +{
> > > +       if (skb_vlan_tag_present(skb))
> > > +               *proto = skb->protocol;
> > > +
> > > +       while (eth_type_vlan(*proto)) {
> > > +               struct vlan_hdr *vlan;
> > > +
> > > +               if (unlikely(!pskb_may_pull(skb, VLAN_HLEN)))
> > > +                       return -ENOMEM;
> > > +
> > > +               vlan = (struct vlan_hdr *)skb->data;
> > > +               *proto = vlan->h_vlan_encapsulated_proto;
> > > +               skb_pull(skb, VLAN_HLEN);
> > > +               skb_reset_network_header(skb);
>
> Again, this code was in act_csum.c also before. The only intention of this
> patch is to ensure that pskb_may_pull() is called before skb_pull(), as
> per Eric suggestion, and move this code out of act_csum to use it with
> other TC actions.

Sure, no one says the code before yours is more correct, right? :)

>
> > Any reason not to call __skb_vlan_pop() directly?
>
> I think we can't use __skb_vlan_pop(), because 'act_csum' needs to read
> the innermost ethertype in the packet to understand if it's IPv4, IPv6 or
> else (ARP, EAPOL, ...).
>
> If I well read __skb_vlan_pop(), it returns the VLAN ID, which is useless
> here.
>

I am confused, this could be checked by eth_type_vlan(skb->protocol),
right? So why it stops you from considering __skb_vlan_pop() or
skb_vlan_pop()? They both should return error or zero, not vlan ID.

Thanks.