sk_buff allocation

sk_buff allocation

[Emmanuel Varagnat]

I'm writing a module that is able to modify outgoing packets.
This is done by registering a new entry in ptype_all.
But my problem is that in dev_queue_xmit_nit the sk_buff is
cloned and that my function get this clone. So my modification
on skb->data isn't take into account by the ethernet driver.

My idea was to do my modifications and then copy all my datas
starting at skb->data so that nothing in the sk_buff is modified.

But what am I doing if the buffer doesn't have enough room to
support the new/modified data ?
skb_cow or skb_copy_expand, for example, will return me a new
sk_buff with a new buffer but how could I tell the system that
it must "replace" the old sk_buff by this one ?

Thanks

-Emmanuel Varagnat

Re: sk_buff allocation

[Alexey Vyskubov]

> I'm writing a module that is able to modify outgoing packets.
> This is done by registering a new entry in ptype_all.
> But my problem is that in dev_queue_xmit_nit the sk_buff is
> cloned and that my function get this clone. So my modification
> on skb->data isn't take into account by the ethernet driver.

Why don't you use netfilter hooks? Write your hook for NF_IP_POST_ROUTING; it
will receive sk_buff **pskb, so you can completely replace old skbuff with new.

-- 
Alexey

Re: sk_buff allocation

[Emmanuel Varagnat]

Alexey Vyskubov wrote:
> 
> Why don't you use netfilter hooks? Write your hook for NF_IP_POST_ROUTING; it
> will receive sk_buff **pskb, so you can completely replace old skbuff with new.

Because, it doesn't seems to fit my needs.
The hardware header must present.

But I could have missed something looking at the kernel code.

Thanks a lot

-Manu