Can compiz use bash instead of sh?

On my Ubuntu 12.04 logging in is handled by the lightdm which starts a gnome-session for the user which starts a compiz instance which starts a login shell which is always sh, no matter what /etc/passwd states as the user's login shell.

This is problematic in my case. I want to set a bash shell function on login time and export it; so I create a ~/.profile:

l() {
  ls -la "$@"
}
export -f l

But this file now is not read by a bash, as I would expect when /etc/passwd states that my login shell is the bash. No, instead it is read by an sh (which is linked to dash in my case) and which cannot handle the export -f l statement.

Is there a way to make compiz honour my login shell as stated in /etc/passwd instead of using the sh? Is there a configuration for this somewhere?