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Date:   Mon, 14 Sep 2020 11:48:17 +0200
From:   Willem de Bruijn <>
To:     Xie He <>
Cc:     "David S. Miller" <>,
        Jakub Kicinski <>,
        Network Development <>,
        LKML <>,
        Eric Dumazet <>,
        Brian Norris <>,
        Cong Wang <>
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 <> 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 This comment is outdated and no longer reflects the actual implementation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:
> 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 <>
> Cc: Eric Dumazet <>
> Cc: Brian Norris <>
> Cc: Cong Wang <>
> Signed-off-by: Xie He <>

Acked-by: Willem de Bruijn <>

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