netdev - Re: [PATCH net-next v4 4/5] net: hdlc_fr: Do skb_reset_mac_header for skbs received on normal PVC devices

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Date:   Fri, 30 Oct 2020 15:52:09 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net-next v4 4/5] net: hdlc_fr: Do skb_reset_mac_header for
 skbs received on normal PVC devices

On Fri, Oct 30, 2020 at 3:22 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> It's indirect:
>
>         skb_reset_network_header(skb);
>         if (!skb_transport_header_was_set(skb))
>                 skb_reset_transport_header(skb);
>         skb_reset_mac_len(skb);

Oh. I see. skb_reset_mac_len would set skb->mac_len. Not sure where
skb->mac_len would be used though.

> > I thought only AF_PACKET/RAW sockets would need this information
> > because other upper layers would not care about what happened in L2.
>
> I think that's a reasonable assumption. I don't have a good
> counterexample ready. Specific to this case, it seems to have been
> working with no one complaining so far ;)

Yeah. It seems to me that a lot of drivers (without header_ops) have
this problem. The comment in af_packet.c before my commit b79a80bd6dd8
also indicated this problem was widespread. It seemed to not cause any
issues.