src/basic/random-util.c

   1 /***
   2   This file is part of systemd.
   3
   4   Copyright 2010 Lennart Poettering
   5
   6   systemd is free software; you can redistribute it and/or modify it
   7   under the terms of the GNU Lesser General Public License as published by
   8   the Free Software Foundation; either version 2.1 of the License, or
   9   (at your option) any later version.
  10
  11   systemd is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   Lesser General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  18 ***/
  19
  20 #include <errno.h>
  21 #include <fcntl.h>
  22 #include <linux/random.h>
  23 #include <stdint.h>
  24 #ifdef HAVE_SYS_AUXV_H
  25 #include <sys/auxv.h>
  26 #endif
  27 #include <sys/stat.h>
  28 #include <sys/types.h>
  29 #include <time.h>
  30
  31 #include "fd-util.h"
  32 #include "missing.h"
  33 #include "random-util.h"
  34 #include "time-util.h"
  35 #include "util.h"
  36
  37 int dev_urandom(void *p, size_t n) {
  38         static int have_syscall = -1;
  39
  40         _cleanup_close_ int fd = -1;
  41         int r;
  42
  43         /* Gathers some randomness from the kernel. This call will
  44          * never block, and will always return some data from the
  45          * kernel, regardless if the random pool is fully initialized
  46          * or not. It thus makes no guarantee for the quality of the
  47          * returned entropy, but is good enough for or usual usecases
  48          * of seeding the hash functions for hashtable */
  49
  50         /* Use the getrandom() syscall unless we know we don't have
  51          * it, or when the requested size is too large for it. */
  52         if (have_syscall != 0 || (size_t) (int) n != n) {
  53                 r = getrandom(p, n, GRND_NONBLOCK);
  54                 if (r == (int) n) {
  55                         have_syscall = true;
  56                         return 0;
  57                 }
  58
  59                 if (r < 0) {
  60                         if (errno == ENOSYS)
  61                                 /* we lack the syscall, continue with
  62                                  * reading from /dev/urandom */
  63                                 have_syscall = false;
  64                         else if (errno == EAGAIN)
  65                                 /* not enough entropy for now. Let's
  66                                  * remember to use the syscall the
  67                                  * next time, again, but also read
  68                                  * from /dev/urandom for now, which
  69                                  * doesn't care about the current
  70                                  * amount of entropy.  */
  71                                 have_syscall = true;
  72                         else
  73                                 return -errno;
  74                 } else
  75                         /* too short read? */
  76                         return -ENODATA;
  77         }
  78
  79         fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
  80         if (fd < 0)
  81                 return errno == ENOENT ? -ENOSYS : -errno;
  82
  83         return loop_read_exact(fd, p, n, true);
  84 }
  85
  86 void initialize_srand(void) {
  87         static bool srand_called = false;
  88         unsigned x;
  89 #ifdef HAVE_SYS_AUXV_H
  90         void *auxv;
  91 #endif
  92
  93         if (srand_called)
  94                 return;
  95
  96         x = 0;
  97
  98 #ifdef HAVE_SYS_AUXV_H
  99         /* The kernel provides us with a bit of entropy in auxv, so
 100          * let's try to make use of that to seed the pseudo-random
 101          * generator. It's better than nothing... */
 102
 103         auxv = (void*) getauxval(AT_RANDOM);
 104         if (auxv)
 105                 x ^= *(unsigned*) auxv;
 106 #endif
 107
 108         x ^= (unsigned) now(CLOCK_REALTIME);
 109         x ^= (unsigned) gettid();
 110
 111         srand(x);
 112         srand_called = true;
 113 }
 114
 115 void random_bytes(void *p, size_t n) {
 116         uint8_t *q;
 117         int r;
 118
 119         r = dev_urandom(p, n);
 120         if (r >= 0)
 121                 return;
 122
 123         /* If some idiot made /dev/urandom unavailable to us, he'll
 124          * get a PRNG instead. */
 125
 126         initialize_srand();
 127
 128         for (q = p; q < (uint8_t*) p + n; q ++)
 129                 *q = rand();
 130 }