Don't fall back to file I/O on Linux when Rust only supports kernels guaranteed to have it

[briansmith]

According to https://doc.rust-lang.org/beta/rustc/platform-support.html, there are only a handful of target architectures where Rust supports kernels that are so old that they would not have the getrandom syscall. Notably riscv64gc and aarch64 are guaranteed to have getrandom according to the Rust kernel support policy. For those architectures we should not have enable the fallback logic at all. I suggest in particular that we have an allowlist of target architectures where we allow the fallback, which would be i686, x86_64, arm, armv7, powerpc, powerpc64, powerpc64le. s390x, i586, according to the Rust documents.

[josephlr]

I like this idea, it also serves as a bit of a test run for removing such file-based support once rust increases its supported kernel version.

[newpavlov]

Sounds good to me!

[newpavlov]

What about Android targets?

In this issue @josephlr wrote

We need to have a /dev/urandom fallback and will need it at least until the 3.16 kernel ends LTS support in mid 2020.

So we probably can disable file fallback for Android targets as per our platform support policy?

[briansmith]

So we probably can disable file fallback for Android targets as per our platform support policy?

I have been assuming getrandom's platform support is that of its MSRV, not the latest stable Rust version. We we need to go back and look at what the kernel support policy of Rust 1.36 was.

[workingjubilee]

That documentation was introduced in 1.47.0: https://doc.rust-lang.org/1.47.0/rustc/platform-support.html