1 /* SPDX-License-Identifier: LGPL-2.1+ */
6 //#include <linux/random.h>
14 # include <sys/auxv.h>
18 # include <sys/random.h>
20 # include <linux/random.h>
26 #include "random-util.h"
27 #include "time-util.h"
29 int acquire_random_bytes(void *p, size_t n, bool high_quality_required) {
30 static int have_syscall = -1;
32 _cleanup_close_ int fd = -1;
33 size_t already_done = 0;
36 /* Gathers some randomness from the kernel. This call will never block. If
37 * high_quality_required, it will always return some data from the kernel,
38 * regardless of whether the random pool is fully initialized or not.
39 * Otherwise, it will return success if at least some random bytes were
40 * successfully acquired, and an error if the kernel has no entropy whatsover
43 /* Use the getrandom() syscall unless we know we don't have it. */
44 if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
45 r = getrandom(p, n, GRND_NONBLOCK);
50 if (!high_quality_required) {
51 /* Fill in the remaining bytes using pseudorandom values */
52 pseudorandom_bytes((uint8_t*) p + r, n - r);
57 } else if (errno == ENOSYS)
58 /* We lack the syscall, continue with reading from /dev/urandom. */
60 else if (errno == EAGAIN) {
61 /* The kernel has no entropy whatsoever. Let's remember to
62 * use the syscall the next time again though.
64 * If high_quality_required is false, return an error so that
65 * random_bytes() can produce some pseudorandom
66 * bytes. Otherwise, fall back to /dev/urandom, which we know
67 * is empty, but the kernel will produce some bytes for us on
68 * a best-effort basis. */
71 if (!high_quality_required)
77 fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
79 return errno == ENOENT ? -ENOSYS : -errno;
81 return loop_read_exact(fd, (uint8_t*) p + already_done, n - already_done, true);
84 void initialize_srand(void) {
85 static bool srand_called = false;
95 /* The kernel provides us with 16 bytes of entropy in auxv, so let's
96 * try to make use of that to seed the pseudo-random generator. It's
97 * better than nothing... */
99 auxv = (void*) getauxval(AT_RANDOM);
101 assert_cc(sizeof(x) <= 16);
102 memcpy(&x, auxv, sizeof(x));
107 x ^= (unsigned) now(CLOCK_REALTIME);
108 x ^= (unsigned) gettid();
114 /* INT_MAX gives us only 31 bits, so use 24 out of that. */
115 #if RAND_MAX >= INT_MAX
118 /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
122 void pseudorandom_bytes(void *p, size_t n) {
127 for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
130 rr = (unsigned) rand();
133 if ((size_t) (q - (uint8_t*) p + 2) < n)
137 if ((size_t) (q - (uint8_t*) p + 1) < n)
144 void random_bytes(void *p, size_t n) {
147 r = acquire_random_bytes(p, n, false);
151 /* If some idiot made /dev/urandom unavailable to us, or the
152 * kernel has no entropy, use a PRNG instead. */
153 return pseudorandom_bytes(p, n);