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 #ifdef USE_SYS_RANDOM_H
  34 #  include <sys/random.h>
  35 #else
  36 #  include <linux/random.h>
  37 #endif
  38
  39 #include "fd-util.h"
  40 #include "io-util.h"
  41 #include "missing.h"
  42 #include "random-util.h"
  43 #include "time-util.h"
  44
  45 int dev_urandom(void *p, size_t n) {
  46         static int have_syscall = -1;
  47
  48         _cleanup_close_ int fd = -1;
  49         int r;
  50
  51         /* Gathers some randomness from the kernel. This call will
  52          * never block, and will always return some data from the
  53          * kernel, regardless if the random pool is fully initialized
  54          * or not. It thus makes no guarantee for the quality of the
  55          * returned entropy, but is good enough for our usual usecases
  56          * of seeding the hash functions for hashtable */
  57
  58         /* Use the getrandom() syscall unless we know we don't have
  59          * it, or when the requested size is too large for it. */
  60         if (have_syscall != 0 || (size_t) (int) n != n) {
  61                 r = getrandom(p, n, GRND_NONBLOCK);
  62                 if (r == (int) n) {
  63                         have_syscall = true;
  64                         return 0;
  65                 }
  66
  67                 if (r < 0) {
  68                         if (errno == ENOSYS)
  69                                 /* we lack the syscall, continue with
  70                                  * reading from /dev/urandom */
  71                                 have_syscall = false;
  72                         else if (errno == EAGAIN)
  73                                 /* not enough entropy for now. Let's
  74                                  * remember to use the syscall the
  75                                  * next time, again, but also read
  76                                  * from /dev/urandom for now, which
  77                                  * doesn't care about the current
  78                                  * amount of entropy.  */
  79                                 have_syscall = true;
  80                         else
  81                                 return -errno;
  82                 } else
  83                         /* too short read? */
  84                         return -ENODATA;
  85         }
  86
  87         fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
  88         if (fd < 0)
  89                 return errno == ENOENT ? -ENOSYS : -errno;
  90
  91         return loop_read_exact(fd, p, n, true);
  92 }
  93
  94 void initialize_srand(void) {
  95         static bool srand_called = false;
  96         unsigned x;
  97 #ifdef HAVE_SYS_AUXV_H
  98         void *auxv;
  99 #endif
 100
 101         if (srand_called)
 102                 return;
 103
 104 #ifdef HAVE_SYS_AUXV_H
 105         /* The kernel provides us with 16 bytes of entropy in auxv, so let's try to make use of that to seed the
 106          * pseudo-random generator. It's better than nothing... */
 107
 108         auxv = (void*) getauxval(AT_RANDOM);
 109         if (auxv) {
 110                 assert_cc(sizeof(x) < 16);
 111                 memcpy(&x, auxv, sizeof(x));
 112         } else
 113 #endif
 114                 x = 0;
 115
 116
 117         x ^= (unsigned) now(CLOCK_REALTIME);
 118         x ^= (unsigned) gettid();
 119
 120         srand(x);
 121         srand_called = true;
 122 }
 123
 124 void random_bytes(void *p, size_t n) {
 125         uint8_t *q;
 126         int r;
 127
 128         r = dev_urandom(p, n);
 129         if (r >= 0)
 130                 return;
 131
 132         /* If some idiot made /dev/urandom unavailable to us, he'll
 133          * get a PRNG instead. */
 134
 135         initialize_srand();
 136
 137         for (q = p; q < (uint8_t*) p + n; q ++)
 138                 *q = rand();
 139 }