If you’ve seen following error

>>> Emerging (1 of 1) ....
mount: /proc: cannot mount none read-only.
Unable to mark /proc slave: 32
 * The ebuild phase 'die_hooks' has been aborted since PORTAGE_BUILDDIR
 * does not exist: '/var/tmp/portage/ ...'

while emerging some package, than be sure this is a bug in portage. Just append

FEATURES="-pid-sandbox"

to make.conf, and it should work again.