Current:

void RandBytes(void* output, size_t output_length) {
  size_t remaining = output_length;
  unsigned char* cur = reinterpret_cast<unsigned char*>(output);
  while (remaining > 0) {
    // The syscall has a maximum number of bytes that can be read at once.
    size_t read_len =
        std::min(remaining, static_cast<size_t>(ZX_CPRNG_DRAW_MAX_LEN));
    zx_cprng_draw(cur, read_len);
    remaining -= read_len;
    cur += read_len;
  }
}

Better:

void RandBytes(void* output, size_t output_length) {
  zx_cprng_draw(output, output_length);
}