Use libc::SYS_getrandom

[josephlr]

This has been in libc for a while, so we don't need to increment the
libc version in Cargo.toml. Note that we cannot use the getrandom()
function from libc, as it isn't available on Android.

Originally part of #744

[briansmith]

Can we just fix rust-lang/libc#659 (comment) and then use libc::getrandom() instead? I'd like to remove the direct use of syscall by ring completely.

[josephlr]

Can we just fix rust-lang/libc#659 (comment) and then use libc::getrandom() instead? I'd like to remove the direct use of syscall by ring completely.

One particular issue is that getrandom is only supported on fairly new glibc/musl versions (despite the syscall being around for awhile), and isn't supported on Android. If you're OK with:

• Requiring a recent glibc/musl
• Potentially going back to using the raw syscall if ring ever supports Android

Then I think going with rust-lang/libc#1399 is the best bet.

[josephlr]

Potentially going back to using the raw syscall if ring ever supports Android

@briansmith disregard the above, I forgot that ring currently supports Android. So it looks like using libc::getrandom is not an option, so we will have to stick with the raw syscall (i.e. this PR). 🙁

Edit: For context see rust-lang/libc#1399 (comment) and boostorg/uuid#76

[josephlr]

@briansmith this is ready to merge. rust-lang/libc#1399 was merged, but we can't use it because the bindings are only available for Linux, not Android.

[briansmith]

What's the specific goal that you are trying to achieve here? Is there something wrong with the constants that ring is currently using? Are you hoping to do another Linux port that isn't supported by ring yet? Or something else?

[josephlr]

What's the specific goal that you are trying to achieve here?

• Making ring easier to port (I was taking a shot at a writeup for Document how to port *ring* to a no_std-only platform #744). Also see: Add support for mips #562 and wip: ppc64/ppc support #819
  • Obviously many more steps are needed, but it's little things like this that increase complexity and the cost of a port.
• Ease of code review (I had to go check that these numbers matched the one's in libc).

[josephlr]

Closing for similar reasons to #856, this is trying to bring over chunks from the getrandom create which is the wrong approch.