netdev - [PATCH net] drivers/net/wan/lapbether: Make skb->protocol consistent with the header

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Date:   Wed, 16 Sep 2020 09:49:18 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Xie He <xie.he.0141@...il.com>, Martin Schiller <ms@....tdt.de>
Subject: [PATCH net] drivers/net/wan/lapbether: Make skb->protocol consistent with the header

This driver is a virtual driver stacked on top of Ethernet interfaces.

When this driver transmits data on the Ethernet device, the skb->protocol
setting is inconsistent with the Ethernet header prepended to the skb.

This causes a user listening on the Ethernet interface with an AF_PACKET
socket, to see different sll_protocol values for incoming and outgoing
frames, because incoming frames would have this value set by parsing the
Ethernet header.

This patch changes the skb->protocol value for outgoing Ethernet frames,
making it consistent with the Ethernet header prepended. This makes a
user listening on the Ethernet device with an AF_PACKET socket, to see
the same sll_protocol value for incoming and outgoing frames.

Cc: Martin Schiller <ms@....tdt.de>
Signed-off-by: Xie He <xie.he.0141@...il.com>
---
 drivers/net/wan/lapbether.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index 732a6c1851f5..b6be2454b8bd 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -198,8 +198,6 @@ static void lapbeth_data_transmit(struct net_device *ndev, struct sk_buff *skb)
 	struct net_device *dev;
 	int size = skb->len;

-	skb->protocol = htons(ETH_P_X25);
-
 	ptr = skb_push(skb, 2);

 	*ptr++ = size % 256;
@@ -210,6 +208,8 @@ static void lapbeth_data_transmit(struct net_device *ndev, struct sk_buff *skb)

 	skb->dev = dev = lapbeth->ethdev;

+	skb->protocol = htons(ETH_P_DEC);
+
 	skb_reset_network_header(skb);

 	dev_hard_header(skb, dev, ETH_P_DEC, bcast_addr, NULL, 0);
-- 
2.25.1