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 [off-list ref] 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@gmail.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <redacted>

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?