> I'm playing with Gentoo in LXC running on ARM-based QNAP NAS, and when trying to compile Python 3.5.4, the build failed with "Fatal Python error: failed to get random numbers to initialize Python" (full details at https://forums.gentoo.org/viewtopic-p-8172124.html ).

I'm surprised. Python 3.5.4 does contain my latest fix: commit 035ba5da3e53e45c712b39fe1f6fb743e697c032 (bpo-29157).

> Looking at random.c, pyurandom returns -1 instead of falling back to dev_urandom when py_getrandom returns -1, so the attached fix is simple.

Ignoring failures is not a good idea.

What is the result of the getrandom() function?

> glibc in the environment is 2.25 (supports getrandom), while kernel is 3.10 (doesn't support getrandom).

I expect n < 0 with errno = ENOSYS: py_getrandom() should return 0 in this case, not -1.