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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Message-ID: <CA+FuTSefJk9xkPQU8K5Ew6ZmnSbMo0S4izAoc=h7-cDrN98jUQ@mail.gmail.com>
Date:   Fri, 30 Oct 2020 18:22:10 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        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 5:49 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Fri, Oct 30, 2020 at 2:28 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Yes, it might require holding off the other patches until net is
> > merged into net-next.
> >
> > Packet sockets are likely not the only way these packets are received?
> > It changes mac_len as computed in __netif_receive_skb_core.
>
> I looked at __netif_receive_skb_core. I didn't see it computing mac_len?

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);

> 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 ;)

> I see mac_len is computed in netif_receive_generic_xdp. I'm not clear
> about the reason why it calculates it. But it seems that it considers
> the L2 header as an Ethernet header, which is incorrect for this
> driver.
>
> > If there is no real bug that is fixed, net-next is fine.