  Modified from the standard half-space extension via reflection principle, we
construct a linear extension operator for the upper half space $\Bbb R^n_+$
that has the form $Ef(x)=\sum_{j=-\infty}^\infty a_jf(x',-b_jx_n)$ for $x_n<0$.
We prove that $E$ is bounded in all $C^k$-spaces, Sobolev and H\"older spaces,
Besov and Triebel-Lizorkin spaces, along with their Morrey generalizations. We
also give an analogous construction on bounded smooth domains.
