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Date:   Fri, 14 Aug 2020 20:41:21 -0700 (PDT)
From:   David Miller <>
Subject: Re: [PATCH net] drivers/net/wan/hdlc_x25: Added needed_headroom
 and a skb->len check

From: Xie He <>
Date: Thu, 13 Aug 2020 11:17:04 -0700

> 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 and set hard_header_len to 0
> 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.
> Because this driver has no header_ops, according to the logic of
> af_packet.c, the value of hard_header_len should be 0.
> Reason of setting needed_headroom and hard_header_len at this place:
> This driver is written using the API of the hdlc module, the hdlc
> module enables this driver (the protocol driver) to run on any hardware
> that has a driver (the hardware driver) written using the API of the
> hdlc module.
> Two other hdlc protocol drivers - hdlc_ppp and hdlc_raw_eth, also set
> things like hard_header_len at this place. In hdlc_ppp, it sets
> hard_header_len after attach_hdlc_protocol and before setting dev->type.
> In hdlc_raw_eth, it sets hard_header_len by calling ether_setup after
> attach_hdlc_protocol and after memcpy the settings.
> 3. Reset needed_headroom when detaching protocols (in hdlc.c)
> When detaching a protocol from a hardware device, the hdlc module will
> reset various parameters of the device (including hard_header_len) to
> the default values. We add needed_headroom here so that needed_headroom
> will also be reset.
> Cc: Willem de Bruijn <>
> Cc: Martin Schiller <>
> Cc: Andrew Hendry <>
> Signed-off-by: Xie He <>

Applied, thanks.

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