[mothballed]

Watch some documentaries about Chinese borders (easy to find ones of peasants illegally entering Kashmir for instance) or even look at a map. There is absolutely no way to keep even broke peasants from leaving. China doesn't even know where many of their borders are, that's why a lot of them show up as dotted lines. China has about the most porous border for leaving as any place on earth but Russia or Brazil (entering illegally is a different issue -- if you want to go anywhere urban desirable you will be found out -- that they have good controls for). This compounded by the fact that many of China's neighboring countries have robust populations of undocumented / nationless people into which they can be absorbed rather than sticking out as a sore thumb wherever they end up.

Iran and Afghanistan are almost as laughable. When I was in Syria I met an Iranian guy with one eye that travelled through the Kurdish mountains into Iraq and then Syria. Iran doesn't even control the mountain passes out of the country, they're literally controlled by Kurdish rebels who have very little concept of immigration controls other than pay me a dollar.