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SubjectRe: [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header_len
I'm really sorry to have re-sent the patch when the patch is still in
review. I don't intend to be disrespectful to anyone. And I apologize
for any disrespectfulness this might appear. Sorry.

I'm also sorry for not having sent the patch with the proper subject
prefixed with "net" or "net-next". If anyone requests I can re-send
this patch with the proper subject "PATCH net".

This patch actually fixes a kernel panic when this driver is used with
a AF_PACKET/RAW socket. This driver runs on top of Ethernet
interfaces. So I created a pair of virtual Ethernet (veth) interfaces,
loaded this driver to create a pair of X.25 interfaces on top of them,
and wrote C programs to use AF_PACKET sockets to send/receive data
through them.

At first I used AF_PACKET/DGRAM sockets. I prepared packet data
according to the requirements of X.25 drivers. I first sent an
one-byte packet ("\x01") to instruct the driver to connect, then I
sent data prefixed with an one-byte pseudo header ("\x00") to instruct
the driver to send the data, and then I sent another one-byte packet
("\x02") to instruct the driver to disconnect.

This works fine with AF_PACKET/DGRAM sockets. However, when I change
it to AF_PACKET/RAW sockets, kernel panic occurs. The stack trace is
as follows. We can see the kernel panicked because of insufficient
header space when pushing the Ethernet header.

[ 168.399197] skbuff: skb_under_panic: text:ffffffff819d95fb len:20
put:14 head:ffff8882704c0a00 data:ffff8882704c09fd tail:0x11 end:0xc0
dev:veth0
...
[ 168.399255] Call Trace:
[ 168.399259] skb_push.cold+0x14/0x24
[ 168.399262] eth_header+0x2b/0xc0
[ 168.399267] lapbeth_data_transmit+0x9a/0xb0 [lapbether]
[ 168.399275] lapb_data_transmit+0x22/0x2c [lapb]
[ 168.399277] lapb_transmit_buffer+0x71/0xb0 [lapb]
[ 168.399279] lapb_kick+0xe3/0x1c0 [lapb]
[ 168.399281] lapb_data_request+0x76/0xc0 [lapb]
[ 168.399283] lapbeth_xmit+0x56/0x90 [lapbether]
[ 168.399286] dev_hard_start_xmit+0x91/0x1f0
[ 168.399289] ? irq_init_percpu_irqstack+0xc0/0x100
[ 168.399291] __dev_queue_xmit+0x721/0x8e0
[ 168.399295] ? packet_parse_headers.isra.0+0xd2/0x110
[ 168.399297] dev_queue_xmit+0x10/0x20
[ 168.399298] packet_sendmsg+0xbf0/0x19b0
......

After applying this patch, the kernel panic no longer appears, and
AF_PACKET/RAW sockets would then behave the same as AF_PACKET/DGRAM
sockets.

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