Hi. Incidentally, my current workaround for this and related issues has been (for the last months) to bind-mount /dev/urandom on /dev/random inside the chroot.
This has been working like a charm in most cases, the exception being packages who insist on testing the hardware or the operating system instead of the software which has just been built: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907191 I'll take a look at the kernel feature to see if it's better than this. Thanks a lot.