Use getrandom on FreeBSD

[valpackett]

getentropy in libc uses sysctl only as a fallback and uses the getrandom syscall if available.

This crate should check for getrandom via dlopen and use it if available.

[josephlr]

Is there a good reason to add this? If we have to keep the sysctl approach around regardless, it just seems like more maintenance to maintain two equivalent implementations for FreeBSD.

[valpackett]

The sysctl is slower and does not block by itself (as in, it would occasionally return less bytes and your current code would return an error, it must be used in a loop)

[josephlr]

The sysctl is slower

So it looks like the only reason this is true is because the sysctl only allows up to 256 bytes per call, while the getrandom syscall allows very large reads per call. This only seems relevant if we are bottlenecked by the ring transition overhead. Are there any metrics on this?

and does not block by itself (as in, it would occasionally return less bytes and your current code would return an error, it must be used in a loop)

Good catch, fixed in: #36

Looking at FreeBSD's getrandom(2) man page, it would seem that we would have to do this loop regardless, as it does not guarantee the completion of large buffer requests.

@myfreeweb to do this we need to add libc related bindings (this was done for Linux in rust-lang/libc#1399). Is there a better way to detect if the syscall is supported? On Linux, this is indicated by the syscall returning ENOSYS, does FreeBSD do something similar?

[valpackett]

Do not call the syscall directly, FreeBSD's public ABI is the libc.

Just do what you're doing on Illumos:

getrandom/src/solaris_illumos.rs

Line 93 in 0b87a09

let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr() as *const _) as usize };

[newpavlov]

@myfreeweb
Can you prepare PR with suggested changes?