netdev - [PATCH v2] drivers/net/wan/lapbether: Fixed the value of hard_header

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Message-Id: <20200706004521.78091-1-xie.he.0141@gmail.com>
Date:   Sun,  5 Jul 2020 17:45:21 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>,
        Xie He <xie.he.0141@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-x25@...r.kernel.org
Subject: [PATCH v2] drivers/net/wan/lapbether: Fixed the value of hard_header_len

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,
  then this driver will prepend a length field of 2 bytes,
  then the underlying Ethernet device will prepend its own header.

So, the header length required should be:
  -1 + 3 + 2 + "the header length needed by the underlying device".

This patch fixes kernel panic when this driver is used with AF_PACKET
SOCK_DGRAM sockets.

Signed-off-by: Xie He <xie.he.0141@...il.com>
---
Change in v2: added a comment in the code

 drivers/net/wan/lapbether.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index e30d91a38cfb..284832314f31 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -303,7 +303,6 @@ static void lapbeth_setup(struct net_device *dev)
 	dev->netdev_ops	     = &lapbeth_netdev_ops;
 	dev->needs_free_netdev = true;
 	dev->type            = ARPHRD_X25;
-	dev->hard_header_len = 3;
 	dev->mtu             = 1000;
 	dev->addr_len        = 0;
 }
@@ -324,6 +323,14 @@ static int lapbeth_new_device(struct net_device *dev)
 	if (!ndev)
 		goto out;
 
+	/* When transmitting data:
+	 * first this driver removes a pseudo header of 1 byte,
+	 * then the lapb module prepends an LAPB header of at most 3 bytes,
+	 * then this driver prepends a length field of 2 bytes,
+	 * then the underlying Ethernet device prepends its own header.
+	 */
+	ndev->hard_header_len = -1 + 3 + 2 + dev->hard_header_len;
+
 	lapbeth = netdev_priv(ndev);
 	lapbeth->axdev = ndev;
 
-- 
2.25.1