Protocol handler using dev_add_pack

[Jerry Yu]

I am working on a kernel module to monitor all TCP packets. I created a protocol
handler with protocol code ETH_P_ALL to handle all incoming and outgoing
TCP packets. The code worked fine on 2.6.14 kernel, but in current 3.2.0-26
kernel, I am no longer able to get the TCP payload for outgoing packets.
The data in TCP payload section of skb->data are mainly 0x00. I am still
able to get incoming TCP packets' payloads though.

Is there any change in 3.2.0 or other early kernel versions that will cause
this issue?

Thanks,

Jerry
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[Eric Dumazet]

On Wed, 2012-07-11 at 10:38 -0300, Jerry Yu wrote:
> I am working on a kernel module to monitor all TCP packets. I created a protocol
> handler with protocol code ETH_P_ALL to handle all incoming and outgoing
> TCP packets. The code worked fine on 2.6.14 kernel, but in current 3.2.0-26
> kernel, I am no longer able to get the TCP payload for outgoing packets.
> The data in TCP payload section of skb->data are mainly 0x00. I am still
> able to get incoming TCP packets' payloads though.
>
> Is there any change in 3.2.0 or other early kernel versions that will cause
> this issue?

Maybe you make wrong assumptions in your code.

skb->data doesnt exactly contains tcp payload of locally generated TCP
packets, unless you disabled scatter-gather on the NIC

ehtool -K eth0 sg off

[Jerry Yu]

Eric,

Thank you very much. Once I disabled scatter-gather on the NIC, the
issue disappeared. Now I just need to figure out how to access the
paged data if the scatter-gather I/O feature enabled. Seems that the
functions to map and unmap frag pages ( kmap_skb_frag and
kunmap_skb_frag) are removed from skbuff.h in 3.2.0-26. I may have to
create similar functions by myself.

Thanks again,

Jerry