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Date:   Sun, 9 Aug 2020 11:12:33 +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>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Martin Schiller <ms@....tdt.de>
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 <xie.he.0141@...il.com> 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 <willemdebruijn.kernel@...il.com>
> Cc: Martin Schiller <ms@....tdt.de>
> Signed-off-by: Xie He <xie.he.0141@...il.com>

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

Seems to make sense based on call stack

  x25_asy_xmit               // skb_pull(skb, 1)
  lapb_data_request
  lapb_kick
  lapb_send_iframe        // skb_push(skb, 2)
  lapb_transmit_buffer    // skb_push(skb, 1)
  lapb_data_transmit
  x25_asy_data_transmit
  x25_asy_encaps

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
bug.

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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