I tried to implement the system call, the way you did. That is
passing 2 strings s1, s2 and finding out their lengths. It did 
give me segmentation fault.
     I even tried to just print the strings within sys_print_data () 
using printk() as well as sys_write(), it did the same thing. 
     The message it gave was that the kernel was unable to
do paging at virtual address xxxxx.. 
     I suppose there must be another way to pass strings to 
the system call, but I don't know at this point. If I do in the
future, I will let you know.

saurabh desai.