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Subject[PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header_len
In net/packet/af_packet.c, the function packet_snd first reserves a
headroom of length (dev->hard_header_len + dev->needed_headroom).
Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
which calls dev->header_ops->create, to create the link layer header.
If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
length (dev->hard_header_len), and assumes the user to provide the
appropriate link layer header.

So according to the logic of af_packet.c, dev->hard_header_len should
be the length of the header that would be created by

However, this driver doesn't provide dev->header_ops, so logically
dev->hard_header_len should be 0.

So we should use dev->needed_headroom instead of dev->hard_header_len
to request necessary headroom to be allocated.

Signed-off-by: Xie He <>

Patch v2 has no difference from v1.
I re-submitted it because I want to find new reviewers,
and I want to free new reviewers from the burden of reading the
lengthy discussion and explanations in the v1 email threads.

Summary of v1 discussion:
Cong Wang referred me to Brian Norris, who did a similar change before.
Brian Norris agreed with me on "hard_header_len vs needed_headroom",
but was unfamiliar with X.25 drivers.

drivers/net/wan/lapbether.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index b2868433718f..34cf6db89912 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -305,6 +305,7 @@ 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 = 0;
dev->mtu = 1000;
dev->addr_len = 0;
@@ -331,7 +332,8 @@ static int lapbeth_new_device(struct net_device *dev)
* 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;
+ ndev->needed_headroom = -1 + 3 + 2 + dev->hard_header_len
+ + dev->needed_headroom;

lapbeth = netdev_priv(ndev);
lapbeth->axdev = ndev;
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