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