North Korea will not attack the U.S., directly anyway, ever. Never ever will they do that. South Korea, likely, but the U.S.? Hahahahahahahahahahahahahaha no.
A few things about attacking us. First, if they did attack, they would be blown off the face of the Earth (figuratively - we learned a lot from Hiroshima and Nagasaki). If they did attack us, we would invade and they would be overthrown. I think it would resemble the Iraq war, but with more care, precision, drones, less ground force, and perhaps with a purpose (yeah I said it).
Second, if they attacked, China would be drawn into the fight. China does not want to fight the U.S. for economic reasons - if they attacked us, they would have to ramp up their military spending considerably to play catch up and axe their economic operations. Even if they did actually catch up, we would have won the war long before that.
An attack on the U.S. would not bode well for North Korea's permanence as a state. China knows this, and they've even told North Korea to stop it before they get annihilated.