netdev - Re: [PATCH net v2] net: Reset MAC header for direct packet transmission

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Date:   Mon, 29 Mar 2021 14:13:25 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Antoine Tenart <atenart@...nel.org>,
        Wei Wang <weiwan@...gle.com>,
        Cong Wang <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2] net: Reset MAC header for direct packet transmission

On Mon, Mar 29, 2021 at 12:30 PM Kurt Kanzenbach <kurt@...utronix.de> wrote:
>
> On Mon Mar 29 2021, Eric Dumazet wrote:
> > Note that last year, I addressed the issue differently in commit
> > 96cc4b69581db68efc9749ef32e9cf8e0160c509
> > ("macvlan: do not assume mac_header is set in macvlan_broadcast()")
> > (amended with commit 1712b2fff8c682d145c7889d2290696647d82dab
> > "macvlan: use skb_reset_mac_header() in macvlan_queue_xmit()")
> >
> > My reasoning was that in TX path, when ndo_start_xmit() is called, MAC
> > header is essentially skb->data,
> > so I was hoping to _remove_ skb_reset_mac_header(skb) eventually from
> > the fast path (aka __dev_queue_xmit),
> > because most drivers do not care about MAC header, they just use skb->data.
> >
> > I understand it is more difficult to review drivers instead of just
> > adding more code in  __dev_direct_xmit()
> >
> > In hsr case, I do not really see why the existing check can not be
> > simply reworked ?
>
> It can be reworked, no problem. I just thought it might be better to add
> it to the generic code just in case there are more drivers suffering
> from the issue.

Note that I have a similar issue pending in ipvlan.

Still, I think I prefer the non easy way to not add more stuff in fast path.

>
> >
> > mac_header really makes sense in input path, when some layer wants to
> > get it after it has been pulled.
> >
> > diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
> > index ed82a470b6e154be28d7e53be57019bccd4a964d..cda495cb1471e23e6666c1f2e9d27a01694f997f
> > 100644
> > --- a/net/hsr/hsr_forward.c
> > +++ b/net/hsr/hsr_forward.c
> > @@ -555,11 +555,7 @@ void hsr_forward_skb(struct sk_buff *skb, struct
> > hsr_port *port)
> >  {
> >         struct hsr_frame_info frame;
> >
> > -       if (skb_mac_header(skb) != skb->data) {
> > -               WARN_ONCE(1, "%s:%d: Malformed frame (port_src %s)\n",
> > -                         __FILE__, __LINE__, port->dev->name);
> > -               goto out_drop;
> > -       }
> > +       skb_reset_mac_header(skb);
>
> hsr_forward_skb() has four call sites. Three of them make sure that the
> header is properly set. The Tx path does not. So, maybe something like
> below?

Yes, this should be fine.

>
> Thanks,
> Kurt
>
> diff --git a/net/hsr/hsr_device.c b/net/hsr/hsr_device.c
> index 7444ec6e298e..bfcdc75fc01e 100644
> --- a/net/hsr/hsr_device.c
> +++ b/net/hsr/hsr_device.c
> @@ -217,6 +217,7 @@ static netdev_tx_t hsr_dev_xmit(struct sk_buff *skb, struct net_device *dev)
>         master = hsr_port_get_hsr(hsr, HSR_PT_MASTER);
>         if (master) {
>                 skb->dev = master->dev;
> +               skb_reset_mac_header(skb);
>                 hsr_forward_skb(skb, master);
>         } else {
>                 atomic_long_inc(&dev->tx_dropped);
> diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
> index ed82a470b6e1..b218e4594009 100644
> --- a/net/hsr/hsr_forward.c
> +++ b/net/hsr/hsr_forward.c
> @@ -555,12 +555,6 @@ void hsr_forward_skb(struct sk_buff *skb, struct hsr_port *port)
>  {
>         struct hsr_frame_info frame;
>
> -       if (skb_mac_header(skb) != skb->data) {
> -               WARN_ONCE(1, "%s:%d: Malformed frame (port_src %s)\n",
> -                         __FILE__, __LINE__, port->dev->name);
> -               goto out_drop;
> -       }
> -