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Subject[PATCH 5.8 104/177] drivers/net/wan/hdlc: Change the default of hard_header_len to 0
From: Xie He <>

[ Upstream commit 2b7bcd967a0f5b7ac9bb0c37b92de36e073dd119 ]

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 <>
Cc: Martin Schiller <>
Signed-off-by: Xie He <>
Signed-off-by: David S. Miller <>
Signed-off-by: Sasha Levin <>
drivers/net/wan/hdlc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wan/hdlc.c b/drivers/net/wan/hdlc.c
index 386ed2aa31fd9..9b00708676cf7 100644
--- a/drivers/net/wan/hdlc.c
+++ b/drivers/net/wan/hdlc.c
@@ -229,7 +229,7 @@ static void hdlc_setup_dev(struct net_device *dev)
dev->min_mtu = 68;
dev->max_mtu = HDLC_MAX_MTU;
dev->type = ARPHRD_RAWHDLC;
- dev->hard_header_len = 16;
+ dev->hard_header_len = 0;
dev->needed_headroom = 0;
dev->addr_len = 0;
dev->header_ops = &hdlc_null_ops;

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