Setting skb->len

Hi netfilter list,

This is my first post to this list, so if this question is
better 
handled by a different list, or offtopic, please kindly
inform me that 
is the case.  My C is a bit rusty also, so please forgive me
if I'm 
asking something that is not clear.  In addition, this is my
first 
netfilter module 

In a netfilter module I am working on I alter the packet
data such that 
the length of the packet is reduced by an amount diff.  I
then set 
skb->len via the following code:

   skb->len -= diff;

This worked fine with gcc 2.96 but when cross-compiling this
module for 
OpenWrt with gcc 4.1.2 I get the following error:

   error: assignment of read-only member 'len'

Is there a function I can use to set skb->len or do I
need to poke 
around the kernel code and modify this member to be
read-write.  I 
accept the fact that what I'm doing is probably a hack, but
I would like 
to get this module working first, then make it well coded
second.

Thanks in advance for any advice,

Fred