text: /* hwrng.c - access to system CRNG
   *
   * Copyright 2013-2018 Nicholas J. Kain <njkain at gmail dot com>
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   * - Redistributions of source code must retain the above copyright notice,
   *   this list of conditions and the following disclaimer.
   *
   * - Redistributions in binary form must reproduce the above copyright notice,
   *   this list of conditions and the following disclaimer in the documentation
   *   and/or other materials provided with the distribution.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
   * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
   * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   * POSSIBILITY OF SUCH DAMAGE.
   */
  
  #include <stdint.h>
  #include <string.h>
  #include <stdbool.h>
  #include <unistd.h>
  #include <errno.h>
  #include <time.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include "nk/hwrng.h"
  #include "nk/log.h"
  #include "nk/io.h"
  
  #ifdef NK_USE_GETRANDOM_SYSCALL
  #include <sys/syscall.h>
  #include <linux/random.h>
  static bool nk_getrandom(char *seed, size_t len)
  {
      size_t fetched = 0;
      while (fetched < len) {
          int r = syscall(SYS_getrandom, seed + fetched, len - fetched, 0);
          if (r <= 0) {
              if (r == 0) {
                  // Failsafe to guard against infinite loops.
                  log_line("%s: getrandom() returned no entropy", __func__);
                  return false;
              }
              if (errno == EINTR)
                  continue;
              log_line("%s: getrandom() failed: %s", __func__, strerror(errno));
              return false;
          }
          fetched += (size_t)r;
      }
      return true;
  }
  #else
  static bool nk_getrandom(char *seed, size_t len)
  {
      (void)seed;
      (void)len;
      return false;
  }
  #endif
  static bool nk_get_rnd_clk(char *seed, size_t len)
  {
      struct timespec ts;
      for (size_t i = 0; i < len; ++i) {
          int r = clock_gettime(CLOCK_REALTIME, &ts);
          if (r < 0) {
              log_line("%s: Could not call clock_gettime(CLOCK_REALTIME): %s",
                       __func__, strerror(errno));
              return false;
          }
          char *p = (char *)&ts.tv_sec;
          char *q = (char *)&ts.tv_nsec;
          for (size_t j = 0; j < sizeof ts.tv_sec; ++j)
              seed[i] ^= p[j];
          for (size_t j = 0; j < sizeof ts.tv_nsec; ++j)
              seed[i] ^= q[j];
          // Force some scheduler jitter.
          static const struct timespec st = { .tv_sec=0, .tv_nsec=1 };
          nanosleep(&st, (struct timespec *)0);
      }
      return true;
  }
  
  static bool nk_get_urandom(char *seed, size_t len)
  {
      int fd = open("/dev/urandom", O_RDONLY);
      if (fd < 0) {
          log_line("%s: Could not open /dev/urandom: %s", __func__,
                   strerror(errno));
          return false;
      }
      bool ret = true;
      int r = safe_read(fd, seed, len);
      if (r < 0) {
          ret = false;
          log_line("%s: Could not read /dev/urandom: %s",
                   __func__, strerror(errno));
      }
      close(fd);
      return ret;
  }
  
  void nk_get_hwrng(void *seed, size_t len)
  {
      char *s = (char *)seed;
      if (nk_getrandom(s, len))
          return;
      if (nk_get_urandom(s, len))
          return;
      log_line("%s: Seeding PRNG via system clock.  May be predictable.",
               __func__);
      if (nk_get_rnd_clk(s, len))
          return;
      suicide("%s: All methods to seed PRNG failed.  Exiting.", __func__);
  }
  
  