crypto: Add rand_bytes

This adds `rand_bytes(dst: []byte)` which fills the destination buffer
with entropy from the cryptographic random number generator.  This takes
the "simple is best" approach and just directly returns the OS CSPRNG
output instead of doing anything fancy (a la OpenBSD's arc4random).

core/crypto/crypto.odin

@@ -39,3 +39,14 @@ compare_byte_ptrs_constant_time :: proc "contextless" (a, b: ^byte, n: int) -> i
 	// iff v == 0, setting the sign-bit, which gets returned.
 	return int((u32(v)-1) >> 31)
 }
+
+// rand_bytes fills the dst buffer with cryptographic entropy taken from
+// the system entropy source.  This routine will block if the system entropy
+// source is not ready yet.  All system entropy source failures are treated
+// as catastrophic, resulting in a panic.
+rand_bytes :: proc (dst: []byte) {
+	// zero-fill the buffer first
+	mem.zero_explicit(raw_data(dst), len(dst))
+
+	_rand_bytes(dst)
+}

core/crypto/rand_generic.odin

@@ -0,0 +1,7 @@
+package crypto
+
+when ODIN_OS != "linux" {
+	_rand_bytes :: proc (dst: []byte) {
+		unimplemented("crypto: rand_bytes not supported on this OS")
+	}
+}

core/crypto/rand_linux.odin

@@ -0,0 +1,37 @@
+package crypto
+
+import "core:fmt"
+import "core:os"
+import "core:sys/unix"
+
+_MAX_PER_CALL_BYTES :: 33554431 // 2^25 - 1
+
+_rand_bytes :: proc (dst: []byte) {
+	dst := dst
+	l := len(dst)
+
+	for l > 0 {
+		to_read := min(l, _MAX_PER_CALL_BYTES)
+		ret := unix.sys_getrandom(raw_data(dst), to_read, 0)
+		if ret < 0 {
+			switch os.Errno(-ret) {
+			case os.EINTR:
+				// Call interupted by a signal handler, just retry the
+				// request.
+				continue
+			case os.ENOSYS:
+				// The kernel is apparently prehistoric (< 3.17 circa 2014)
+				// and does not support getrandom.
+				panic("crypto: getrandom not available in kernel")
+			case:
+				// All other failures are things that should NEVER happen
+				// unless the kernel interface changes (ie: the Linux
+				// developers break userland).
+				panic(fmt.tprintf("crypto: getrandom failed: %d", ret))
+			}
+		}
+
+		l -= ret
+		dst = dst[ret:]
+	}
+}