netdev - Re: [PATCH net-next v2] net/packet: Fix a comment about hard_header

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Message-ID: <CAF=yD-Kq4fgNtYae0s9Z4KhYgyCYZo6Ws8Syf+zp0bh-sNArjQ@mail.gmail.com>
Date:   Mon, 14 Sep 2020 11:48:17 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Brian Norris <briannorris@...omium.org>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [PATCH net-next v2] net/packet: Fix a comment about
 hard_header_len and headroom allocation

On Mon, Sep 14, 2020 at 9:42 AM Xie He <xie.he.0141@...il.com> wrote:
>
> This comment is outdated and no longer reflects the actual implementation
> of af_packet.c.
>
> Reasons for the new comment:
>
> 1.
>
> In af_packet.c, the function packet_snd first reserves a headroom of
> length (dev->hard_header_len + dev->needed_headroom).
> Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
> which calls dev->header_ops->create, to create the link layer header.
> If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
> length (dev->hard_header_len), and checks if the user has provided a
> header sized between (dev->min_header_len) and (dev->hard_header_len)
> (in dev_validate_header).
> This shows the developers of af_packet.c expect hard_header_len to
> be consistent with header_ops.
>
> 2.
>
> In af_packet.c, the function packet_sendmsg_spkt has a FIXME comment.
> That comment states that prepending an LL header internally in a driver
> is considered a bug. I believe this bug can be fixed by setting
> hard_header_len to 0, making the internal header completely invisible
> to af_packet.c (and requesting the headroom in needed_headroom instead).
>
> 3.
>
> There is a commit for a WiFi driver:
> commit 9454f7a895b8 ("mwifiex: set needed_headroom, not hard_header_len")
> According to the discussion about it at:
>   https://patchwork.kernel.org/patch/11407493/
> The author tried to set the WiFi driver's hard_header_len to the Ethernet
> header length, and request additional header space internally needed by
> setting needed_headroom.
> This means this usage is already adopted by driver developers.
>
> Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Brian Norris <briannorris@...omium.org>
> Cc: Cong Wang <xiyou.wangcong@...il.com>
> Signed-off-by: Xie He <xie.he.0141@...il.com>

Acked-by: Willem de Bruijn <willemb@...gle.com>