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Date:   Sun, 9 Aug 2020 11:12:33 +0200
From:   Willem de Bruijn <>
To:     Xie He <>
Cc:     "David S. Miller" <>,
        Jakub Kicinski <>,
        Network Development <>,
        linux-kernel <>,
        Linux X25 <>,
        Willem de Bruijn <>,
        Martin Schiller <>
Subject: Re: [PATCH net] drivers/net/wan/x25_asy: Added needed_headroom and a
 skb->len check

On Sun, Aug 9, 2020 at 4:36 AM Xie He <> wrote:
> 1. Added a skb->len check
> This driver expects upper layers to include a pseudo header of 1 byte
> when passing down a skb for transmission. This driver will read this
> 1-byte header. This patch added a skb->len check before reading the
> header to make sure the header exists.
> 2. Added needed_headroom
> When this driver transmits data,
>   first this driver will remove a pseudo header of 1 byte,
>   then the lapb module will prepend the LAPB header of 2 or 3 bytes.
> So the value of needed_headroom in this driver should be 3 - 1.
> Cc: Willem de Bruijn <>
> Cc: Martin Schiller <>
> Signed-off-by: Xie He <>

The patch is analogous to commit c7ca03c216ac
("drivers/net/wan/lapbether: Added needed_headroom and a skb->len

Seems to make sense based on call stack

  x25_asy_xmit               // skb_pull(skb, 1)
  lapb_send_iframe        // skb_push(skb, 2)
  lapb_transmit_buffer    // skb_push(skb, 1)

But I frankly don't know this code and would not modify logic that no
one has complained about for many years without evidence of a real

Were you able to actually exercise this path, similar to lapb_ether:
configure the device, send data from a packet socket? If so, can you
share the configuration steps?

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