linux-kernel - Re: [PATCH net] drivers/net/wan/hdlc: Change the default of hard_header

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Date:   Wed, 02 Sep 2020 15:43:01 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     xie.he.0141@...il.com
Cc:     khc@...waw.pl, kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ms@....tdt.de
Subject: Re: [PATCH net] drivers/net/wan/hdlc: Change the default of
 hard_header_len to 0

From: Xie He <xie.he.0141@...il.com>
Date: Wed,  2 Sep 2020 05:07:06 -0700

> Change the default value of hard_header_len in hdlc.c from 16 to 0.
> 
> Currently there are 6 HDLC protocol drivers, among them:
> 
> hdlc_raw_eth, hdlc_cisco, hdlc_ppp, hdlc_x25 set hard_header_len when
> attaching the protocol, overriding the default. So this patch does not
> affect them.
> 
> hdlc_raw and hdlc_fr don't set hard_header_len when attaching the
> protocol. So this patch will change the hard_header_len of the HDLC
> device for them from 16 to 0.
> 
> This is the correct change because both hdlc_raw and hdlc_fr don't have
> header_ops, and the code in net/packet/af_packet.c expects the value of
> hard_header_len to be consistent with header_ops.
> 
> In net/packet/af_packet.c, in the packet_snd function,
> for AF_PACKET/DGRAM sockets it would reserve a headroom of
> hard_header_len and call dev_hard_header to fill in that headroom,
> and for AF_PACKET/RAW sockets, it does not reserve the headroom and
> does not call dev_hard_header, but checks if the user has provided a
> header of length hard_header_len (in function dev_validate_header).
> 
> Cc: Krzysztof Halasa <khc@...waw.pl>
> Cc: Martin Schiller <ms@....tdt.de>
> Signed-off-by: Xie He <xie.he.0141@...il.com>

Applied, thanks.