[PATCH] drivers/net/wan/lapbether: Fixed the value of hard_header_len

[PATCH] drivers/net/wan/lapbether: Fixed the value of hard_header_len

[Xie He]

When transmitting data from upper layers or from AF_PACKET sockets,
  this driver will first 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@gmail.com>
---
 drivers/net/wan/lapbether.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index e30d91a38cfb..3b5ed0928a4c 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 +323,8 @@ static int lapbeth_new_device(struct net_device *dev)
 	if (!ndev)
 		goto out;
 
+	ndev->hard_header_len = -1 + 3 + 2 + dev->hard_header_len;
+
 	lapbeth = netdev_priv(ndev);
 	lapbeth->axdev = ndev;
 
-- 
2.25.1

Re: [PATCH] drivers/net/wan/lapbether: Fixed the value of hard_header_len

[David Miller]

From: Xie He <xie.he.0141@gmail.com>
Date: Fri,  3 Jul 2020 22:12:46 -0700

> When transmitting data from upper layers or from AF_PACKET sockets,
>   this driver will first 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.

Please add something like this above text to a comment:

> @@ -324,6 +323,8 @@ static int lapbeth_new_device(struct net_device *dev)
>  	if (!ndev)
>  		goto out;
>  
> +	ndev->hard_header_len = -1 + 3 + 2 + dev->hard_header_len;
> +

Above this new line so that someone reading the code can understand
these magic numbers and what they mean.

Thank you.