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SubjectRe: [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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