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 <stdint.h>
  21 #include <errno.h>
  22 #include <sys/types.h>
  23 #include <sys/stat.h>
  24 #include <fcntl.h>
  25 #include <time.h>
  26 #ifdef HAVE_SYS_AUXV_H
  27 #include <sys/auxv.h>
  28 #endif
  29 #include <linux/random.h>
  30
  31 #include "random-util.h"
  32 #include "time-util.h"
  33 #include "missing.h"
  34 #include "util.h"
  35
  36 int dev_urandom(void *p, size_t n) {
  37         static int have_syscall = -1;
  38
  39         _cleanup_close_ int fd = -1;
  40         int r;
  41
  42         /* Gathers some randomness from the kernel. This call will
  43          * never block, and will always return some data from the
  44          * kernel, regardless if the random pool is fully initialized
  45          * or not. It thus makes no guarantee for the quality of the
  46          * returned entropy, but is good enough for or usual usecases
  47          * of seeding the hash functions for hashtable */
  48
  49         /* Use the getrandom() syscall unless we know we don't have
  50          * it, or when the requested size is too large for it. */
  51         if (have_syscall != 0 || (size_t) (int) n != n) {
  52                 r = getrandom(p, n, GRND_NONBLOCK);
  53                 if (r == (int) n) {
  54                         have_syscall = true;
  55                         return 0;
  56                 }
  57
  58                 if (r < 0) {
  59                         if (errno == ENOSYS)
  60                                 /* we lack the syscall, continue with
  61                                  * reading from /dev/urandom */
  62                                 have_syscall = false;
  63                         else if (errno == EAGAIN)
  64                                 /* not enough entropy for now. Let's
  65                                  * remember to use the syscall the
  66                                  * next time, again, but also read
  67                                  * from /dev/urandom for now, which
  68                                  * doesn't care about the current
  69                                  * amount of entropy.  */
  70                                 have_syscall = true;
  71                         else
  72                                 return -errno;
  73                 } else
  74                         /* too short read? */
  75                         return -ENODATA;
  76         }
  77
  78         fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
  79         if (fd < 0)
  80                 return errno == ENOENT ? -ENOSYS : -errno;
  81
  82         return loop_read_exact(fd, p, n, true);
  83 }
  84
  85 void initialize_srand(void) {
  86         static bool srand_called = false;
  87         unsigned x;
  88 #ifdef HAVE_SYS_AUXV_H
  89         void *auxv;
  90 #endif
  91
  92         if (srand_called)
  93                 return;
  94
  95         x = 0;
  96
  97 #ifdef HAVE_SYS_AUXV_H
  98         /* The kernel provides us with a bit of entropy in auxv, so
  99          * let's try to make use of that to seed the pseudo-random
 100          * generator. It's better than nothing... */
 101
 102         auxv = (void*) getauxval(AT_RANDOM);
 103         if (auxv)
 104                 x ^= *(unsigned*) auxv;
 105 #endif
 106
 107         x ^= (unsigned) now(CLOCK_REALTIME);
 108         x ^= (unsigned) gettid();
 109
 110         srand(x);
 111         srand_called = true;
 112 }
 113
 114 void random_bytes(void *p, size_t n) {
 115         uint8_t *q;
 116         int r;
 117
 118         r = dev_urandom(p, n);
 119         if (r >= 0)
 120                 return;
 121
 122         /* If some idiot made /dev/urandom unavailable to us, he'll
 123          * get a PRNG instead. */
 124
 125         initialize_srand();
 126
 127         for (q = p; q < (uint8_t*) p + n; q ++)
 128                 *q = rand();
 129 }