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 <elf.h>
  21 #include <errno.h>
  22 #include <fcntl.h>
  23 #include <stdbool.h>
  24 #include <stdlib.h>
  25 #include <sys/time.h>
  26 #include <linux/random.h>
  27 #include <stdint.h>
  28
  29 #ifdef HAVE_SYS_AUXV_H
  30 #include <sys/auxv.h>
  31 #endif
  32
  33 #include "fd-util.h"
  34 #include "io-util.h"
  35 #include "missing.h"
  36 #include "random-util.h"
  37 #include "time-util.h"
  38
  39 int dev_urandom(void *p, size_t n) {
  40         static int have_syscall = -1;
  41
  42         _cleanup_close_ int fd = -1;
  43         int r;
  44
  45         /* Gathers some randomness from the kernel. This call will
  46          * never block, and will always return some data from the
  47          * kernel, regardless if the random pool is fully initialized
  48          * or not. It thus makes no guarantee for the quality of the
  49          * returned entropy, but is good enough for or usual usecases
  50          * of seeding the hash functions for hashtable */
  51
  52         /* Use the getrandom() syscall unless we know we don't have
  53          * it, or when the requested size is too large for it. */
  54         if (have_syscall != 0 || (size_t) (int) n != n) {
  55                 r = getrandom(p, n, GRND_NONBLOCK);
  56                 if (r == (int) n) {
  57                         have_syscall = true;
  58                         return 0;
  59                 }
  60
  61                 if (r < 0) {
  62                         if (errno == ENOSYS)
  63                                 /* we lack the syscall, continue with
  64                                  * reading from /dev/urandom */
  65                                 have_syscall = false;
  66                         else if (errno == EAGAIN)
  67                                 /* not enough entropy for now. Let's
  68                                  * remember to use the syscall the
  69                                  * next time, again, but also read
  70                                  * from /dev/urandom for now, which
  71                                  * doesn't care about the current
  72                                  * amount of entropy.  */
  73                                 have_syscall = true;
  74                         else
  75                                 return -errno;
  76                 } else
  77                         /* too short read? */
  78                         return -ENODATA;
  79         }
  80
  81         fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
  82         if (fd < 0)
  83                 return errno == ENOENT ? -ENOSYS : -errno;
  84
  85         return loop_read_exact(fd, p, n, true);
  86 }
  87
  88 void initialize_srand(void) {
  89         static bool srand_called = false;
  90         unsigned x;
  91 #ifdef HAVE_SYS_AUXV_H
  92         void *auxv;
  93 #endif
  94
  95         if (srand_called)
  96                 return;
  97
  98         x = 0;
  99
 100 #ifdef HAVE_SYS_AUXV_H
 101         /* The kernel provides us with a bit of entropy in auxv, so
 102          * let's try to make use of that to seed the pseudo-random
 103          * generator. It's better than nothing... */
 104
 105         auxv = (void*) getauxval(AT_RANDOM);
 106         if (auxv)
 107                 x ^= *(unsigned*) auxv;
 108 #endif
 109
 110         x ^= (unsigned) now(CLOCK_REALTIME);
 111         x ^= (unsigned) gettid();
 112
 113         srand(x);
 114         srand_called = true;
 115 }
 116
 117 void random_bytes(void *p, size_t n) {
 118         uint8_t *q;
 119         int r;
 120
 121         r = dev_urandom(p, n);
 122         if (r >= 0)
 123                 return;
 124
 125         /* If some idiot made /dev/urandom unavailable to us, he'll
 126          * get a PRNG instead. */
 127
 128         initialize_srand();
 129
 130         for (q = p; q < (uint8_t*) p + n; q ++)
 131                 *q = rand();
 132 }