Compilation fails when using musl

[dsprenkels]

@jaromil gets compilation errors when compiling with the musl libc:

randombytes.c: In function ‘randombytes_linux_get_entropy_avail’:
randombytes.c:134:12: error: ‘RNDGETENTCNT’ undeclared (first use in this function)
  ioctl(fd, RNDGETENTCNT, &ret);
            ^
randombytes.c:134:12: note: each undeclared identifier is reported only once for each function it appears in

Note that compilation also fails on getrandom, because linux/random.h cannot be found (header file redundant, because of sys/syscall.h?).

• Musl does not seem to be able to import linux symbols (because of portability issues). They say they will support getrandom() (not SYS_getrandom) here: http://www.openwall.com/lists/musl/2017/09/28/8. But in my case musl (1.1.19-1) hasn't yet added support.

• In these cases we should indeed fall back to using /dev/urandom. But RNDGETENTCNT is (probably) glib-specific. I hesitate to skip the entropy test. Maybe see what libsodium does here.

• [ ] getrandom syscall fix

• [ ] /dev/urandom fix

[dsprenkels]

Libsodium does not seem to actually check the entropy level of the pool. Instead, they block on /dev/random once and assume that after the kernel passes back control, the pool is sufficiently filled.

I am personally not sure about the entropy levels in this case (did not find any documentation), so to me it seems that we have two options:

1. Create a fallback to RNDGETENTCNT, which reads from /proc/sys/kernel/random/entropy_avail.
2. Do not use RNDGETENTCNT at all, but only read from /proc/sys/kernel/random/entropy_avail.
3. Block on /dev/random once (using a global value to make sure we don't block again).
4. Block on /dev/random on every call to randombytes.

~I would personally opt for 1., with a fallback that if /proc/sys/kernel/random/entropy_avail cannot be read, we will block on /dev/random (every time).~ This will not fix the include error. Linux header files are still needed in this case as build dependency. Also, including linux headers directly seems to be frowned upon.

[jaromil]

You may not like what you see :^) but this is how I solved it (for now?) https://github.com/DECODEproject/zenroom/blob/develop/src/randombytes.c this builds correctly on all zenroom targets (musl,osx,win,arm-gcc,arm-musl,emscripten)

[dsprenkels]

Yeah, bypassing the entropy check is not an option. :P

[dsprenkels]

5. Define RNDGETENTCNT inline.

[jaromil]

my favorites are 1. and 2. - thanks for taking care of details :^)

[dsprenkels]

Is fixed by #12. Used option 5.