Restricting rsync over ssh

[Bob]

Hi
I have very special needs and i wanted to use rsync over ssh. I don't 
know if a solution already exists for what i want to do. I want to 
provide rsync over ssh to my users. Howevern i want to have the 
following limitations :
1. No shell access
2. Limitting users to their home directories

I was thinking to the folowing solution, but i don't know if it is 
secure enough :
Create a dummy-shell sor ssh login that only allow the rsync --server 
--sender command. Then i get the path of the wanted files, and i appened 
it to the home directory of the user. Ex : the user requests /test, i 
give him : /home/usrname/test
Then i uses the realpath function to canonicalize the path and i check 
that it really begins with /home/usrname to prevent users from getting 
files outside of their home directory.
I execute the rsync command with the new built path...

I think this should work but i would like to know what do you think 
about security.

If this is ok, i will code the dummy-shell and put it online for those 
who would like to do similar things...

Thanks in advance, Bob