Created attachment 182818 [details]
linux_getrandom.patch

In a Linux-based jail, running Ubuntu 16.04, `lsb_release` command (which is widely used in various scripts and during Factorio startup) hangs. `top` shows it forks a `python3.5` process which uses 100% CPU. `truss` shows this process repeatedly calls `linux_getrandom(…)`, which always returns 0.

`man 2 getrandom` on Ubuntu 16.04 specifies that this syscall should return number of random bytes written (http://man7.org/linux/man-pages/man2/getrandom.2.html). A short test program shows that this it returns positive value on Linux, and returns 0 on FreeBSD despite the fact that random bytes have been written to the buffer:

> $ cat test_getrandom.c
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <stdio.h>
> #include <linux/random.h>
> 
> int main(void) {
>         int rv;
>         int buf = 0;
>         rv = syscall(SYS_getrandom, &buf, sizeof(buf), GRND_NONBLOCK);
>         printf("getrandom(&buf, %d, 0) => %d buf=%d\n", (int)sizeof(buf), rv, buf);
>         return 0;
> }

When this program runs natively on Linux, `getrandom(2)` returns size of buffer:

> $ ./test_getrandom
> getrandom(&buf, 4, 0) => 4 buf=-707083248

On FreeBSD 12-CURRENT (possibly also on 11-STABLE, r315505 which introduces this implementation is marked for MFC after 1 month), the syscall always returns 0:

> $ ./test_getrandom
> getrandom(&buf, 4, 0) => 0 buf=-1643413282

After applying attached patch, return value reported from the test program is the same as on native Linux, and `lsb_release` no longer hangs:

> $ ./test_getrandom
> getrandom(&buf, 4, 0) => 4 buf=-943351330
> $ lsb_release -a
> No LSB modules are available.
> Distributor ID:	Ubuntu
> Description:	Ubuntu 16.04.2 LTS
> Release:	16.04
> Codename:	xenial