openssl/crypto/rand/rand_unix.c

/*
 * Copyright 1995-2019 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the OpenSSL license (the "License").  You may not use
 * this file except in compliance with the License.  You can obtain a copy
 * in the file LICENSE in the source distribution or at
 * https://www.openssl.org/source/license.html
 */

#ifndef _GNU_SOURCE
# define _GNU_SOURCE
#endif
#include "e_os.h"
#include <stdio.h>
#include "internal/cryptlib.h"
#include <openssl/rand.h>
#include <openssl/crypto.h>
#include "rand_lcl.h"
#include "internal/rand_int.h"
#include <stdio.h>
#include "internal/dso.h"
#ifdef __linux
# include <sys/syscall.h>
# ifdef DEVRANDOM_WAIT
#  include <sys/shm.h>
#  include <sys/utsname.h>
# endif
#endif
#if defined(__FreeBSD__) && !defined(OPENSSL_SYS_UEFI)
# include <sys/types.h>
# include <sys/sysctl.h>
# include <sys/param.h>
#endif
#if defined(__OpenBSD__) || defined(__NetBSD__)
# include <sys/param.h>
#endif

#if defined(OPENSSL_SYS_UNIX) || defined(__DJGPP__)
# include <sys/types.h>
# include <sys/stat.h>
# include <fcntl.h>
# include <unistd.h>
# include <sys/time.h>

static uint64_t get_time_stamp(void);
static uint64_t get_timer_bits(void);

/* Macro to convert two thirty two bit values into a sixty four bit one */
# define TWO32TO64(a, b) ((((uint64_t)(a)) << 32) + (b))

/*
 * Check for the existence and support of POSIX timers.  The standard
 * says that the _POSIX_TIMERS macro will have a positive value if they
 * are available.
 *
 * However, we want an additional constraint: that the timer support does
 * not require an extra library dependency.  Early versions of glibc
 * require -lrt to be specified on the link line to access the timers,
 * so this needs to be checked for.
 *
 * It is worse because some libraries define __GLIBC__ but don't
 * support the version testing macro (e.g. uClibc).  This means
 * an extra check is needed.
 *
 * The final condition is:
 *      "have posix timers and either not glibc or glibc without -lrt"
 *
 * The nested #if sequences are required to avoid using a parameterised
 * macro that might be undefined.
 */
# undef OSSL_POSIX_TIMER_OKAY
# if defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0
#  if defined(__GLIBC__)
#   if defined(__GLIBC_PREREQ)
#    if __GLIBC_PREREQ(2, 17)
#     define OSSL_POSIX_TIMER_OKAY
#    endif
#   endif
#  else
#   define OSSL_POSIX_TIMER_OKAY
#  endif
# endif
#endif /* defined(OPENSSL_SYS_UNIX) || defined(__DJGPP__) */

#if defined(OPENSSL_RAND_SEED_NONE)
/* none means none. this simplifies the following logic */
# undef OPENSSL_RAND_SEED_OS
# undef OPENSSL_RAND_SEED_GETRANDOM
# undef OPENSSL_RAND_SEED_LIBRANDOM
# undef OPENSSL_RAND_SEED_DEVRANDOM
# undef OPENSSL_RAND_SEED_RDTSC
# undef OPENSSL_RAND_SEED_RDCPU
# undef OPENSSL_RAND_SEED_EGD
#endif

#if (defined(OPENSSL_SYS_VXWORKS) || defined(OPENSSL_SYS_UEFI)) && \
        !defined(OPENSSL_RAND_SEED_NONE)
# error "UEFI and VXWorks only support seeding NONE"
#endif

#if defined(OPENSSL_SYS_VXWORKS)
/* empty implementation */
int rand_pool_init(void)
{
    return 1;
}

void rand_pool_cleanup(void)
{
}

void rand_pool_keep_random_devices_open(int keep)
{
}

size_t rand_pool_acquire_entropy(RAND_POOL *pool)
{
    return rand_pool_entropy_available(pool);
}
#endif

#if !(defined(OPENSSL_SYS_WINDOWS) || defined(OPENSSL_SYS_WIN32) \
    || defined(OPENSSL_SYS_VMS) || defined(OPENSSL_SYS_VXWORKS) \
    || defined(OPENSSL_SYS_UEFI))

# if defined(OPENSSL_SYS_VOS)

#  ifndef OPENSSL_RAND_SEED_OS
#   error "Unsupported seeding method configured; must be os"
#  endif

#  if defined(OPENSSL_SYS_VOS_HPPA) && defined(OPENSSL_SYS_VOS_IA32)
#   error "Unsupported HP-PA and IA32 at the same time."
#  endif
#  if !defined(OPENSSL_SYS_VOS_HPPA) && !defined(OPENSSL_SYS_VOS_IA32)
#   error "Must have one of HP-PA or IA32"
#  endif

/*
 * The following algorithm repeatedly samples the real-time clock (RTC) to
 * generate a sequence of unpredictable data.  The algorithm relies upon the
 * uneven execution speed of the code (due to factors such as cache misses,
 * interrupts, bus activity, and scheduling) and upon the rather large
 * relative difference between the speed of the clock and the rate at which
 * it can be read.  If it is ported to an environment where execution speed
 * is more constant or where the RTC ticks at a much slower rate, or the
 * clock can be read with fewer instructions, it is likely that the results
 * would be far more predictable.  This should only be used for legacy
 * platforms.
 *
 * As a precaution, we assume only 2 bits of entropy per byte.
 */
size_t rand_pool_acquire_entropy(RAND_POOL *pool)
{
    short int code;
    int i, k;
    size_t bytes_needed;
    struct timespec ts;
    unsigned char v;
#  ifdef OPENSSL_SYS_VOS_HPPA
    long duration;
    extern void s$sleep(long *_duration, short int *_code);
#  else
    long long duration;
    extern void s$sleep2(long long *_duration, short int *_code);
#  endif

    bytes_needed = rand_pool_bytes_needed(pool, 4 /*entropy_factor*/);

    for (i = 0; i < bytes_needed; i++) {
        /*
         * burn some cpu; hope for interrupts, cache collisions, bus
         * interference, etc.
         */
        for (k = 0; k < 99; k++)
            ts.tv_nsec = random();

#  ifdef OPENSSL_SYS_VOS_HPPA
        /* sleep for 1/1024 of a second (976 us).  */
        duration = 1;
        s$sleep(&duration, &code);
#  else
        /* sleep for 1/65536 of a second (15 us).  */
        duration = 1;
        s$sleep2(&duration, &code);
#  endif

        /* Get wall clock time, take 8 bits. */
        clock_gettime(CLOCK_REALTIME, &ts);
        v = (unsigned char)(ts.tv_nsec & 0xFF);
        rand_pool_add(pool, arg, &v, sizeof(v) , 2);
    }
    return rand_pool_entropy_available(pool);
}

void rand_pool_cleanup(void)
{
}

void rand_pool_keep_random_devices_open(int keep)
{
}

# else

#  if defined(OPENSSL_RAND_SEED_EGD) && \
        (defined(OPENSSL_NO_EGD) || !defined(DEVRANDOM_EGD))
#   error "Seeding uses EGD but EGD is turned off or no device given"
#  endif

#  if defined(OPENSSL_RAND_SEED_DEVRANDOM) && !defined(DEVRANDOM)
#   error "Seeding uses urandom but DEVRANDOM is not configured"
#  endif

#  if defined(OPENSSL_RAND_SEED_OS)
#   if !defined(DEVRANDOM)
#    error "OS seeding requires DEVRANDOM to be configured"
#   endif
#   define OPENSSL_RAND_SEED_GETRANDOM
#   define OPENSSL_RAND_SEED_DEVRANDOM
#  endif

#  if defined(OPENSSL_RAND_SEED_LIBRANDOM)
#   error "librandom not (yet) supported"
#  endif

#  if (defined(__FreeBSD__) || defined(__NetBSD__)) && defined(KERN_ARND)
/*
 * sysctl_random(): Use sysctl() to read a random number from the kernel
 * Returns the number of bytes returned in buf on success, -1 on failure.
 */
static ssize_t sysctl_random(char *buf, size_t buflen)
{
    int mib[2];
    size_t done = 0;
    size_t len;

    /*
     * Note: sign conversion between size_t and ssize_t is safe even
     * without a range check, see comment in syscall_random()
     */

    /*
     * On FreeBSD old implementations returned longs, newer versions support
     * variable sizes up to 256 byte. The code below would not work properly
     * when the sysctl returns long and we want to request something not a
     * multiple of longs, which should never be the case.
     */
    if (!ossl_assert(buflen % sizeof(long) == 0)) {
        errno = EINVAL;
        return -1;
    }

    /*
     * On NetBSD before 4.0 KERN_ARND was an alias for KERN_URND, and only
     * filled in an int, leaving the rest uninitialized. Since NetBSD 4.0
     * it returns a variable number of bytes with the current version supporting
     * up to 256 bytes.
     * Just return an error on older NetBSD versions.
     */
#if   defined(__NetBSD__) && __NetBSD_Version__ < 400000000
    errno = ENOSYS;
    return -1;
#endif

    mib[0] = CTL_KERN;
    mib[1] = KERN_ARND;

    do {
        len = buflen;
        if (sysctl(mib, 2, buf, &len, NULL, 0) == -1)
            return done > 0 ? done : -1;
        done += len;
        buf += len;
        buflen -= len;
    } while (buflen > 0);

    return done;
}
#  endif

#  if defined(OPENSSL_RAND_SEED_GETRANDOM)

#   if defined(__linux) && !defined(__NR_getrandom)
#    if defined(__arm__) && defined(__NR_SYSCALL_BASE)
#     define __NR_getrandom    (__NR_SYSCALL_BASE+384)
#    elif defined(__i386__)
#     define __NR_getrandom    355
#    elif defined(__x86_64__) && !defined(__ILP32__)
#     define __NR_getrandom    318
#    endif
#   endif

/*
 * syscall_random(): Try to get random data using a system call
 * returns the number of bytes returned in buf, or < 0 on error.
 */
static ssize_t syscall_random(void *buf, size_t buflen)
{
    /*
     * Note: 'buflen' equals the size of the buffer which is used by the
     * get_entropy() callback of the RAND_DRBG. It is roughly bounded by
     *
     *   2 * RAND_POOL_FACTOR * (RAND_DRBG_STRENGTH / 8) = 2^14
     *
     * which is way below the OSSL_SSIZE_MAX limit. Therefore sign conversion
     * between size_t and ssize_t is safe even without a range check.
     */

    /*
     * Do runtime detection to find getentropy().
     *
     * Known OSs that should support this:
     * - Darwin since 16 (OSX 10.12, IOS 10.0).
     * - Solaris since 11.3
     * - OpenBSD since 5.6
     * - Linux since 3.17 with glibc 2.25
     * - FreeBSD since 12.0 (1200061)
     */
#  if defined(__GNUC__) && __GNUC__>=2 && defined(__ELF__) && !defined(__hpux)
    extern int getentropy(void *buffer, size_t length) __attribute__((weak));

    if (getentropy != NULL)
        return getentropy(buf, buflen) == 0 ? (ssize_t)buflen : -1;
#  else
    union {
        void *p;
        int (*f)(void *buffer, size_t length);
    } p_getentropy;

    /*
     * We could cache the result of the lookup, but we normally don't
     * call this function often.
     */
    ERR_set_mark();
    p_getentropy.p = DSO_global_lookup("getentropy");
    ERR_pop_to_mark();
    if (p_getentropy.p != NULL)
        return p_getentropy.f(buf, buflen) == 0 ? (ssize_t)buflen : -1;
#  endif

    /* Linux supports this since version 3.17 */
#  if defined(__linux) && defined(__NR_getrandom)
    return syscall(__NR_getrandom, buf, buflen, 0);
#  elif (defined(__FreeBSD__) || defined(__NetBSD__)) && defined(KERN_ARND)
    return sysctl_random(buf, buflen);
#  else
    errno = ENOSYS;
    return -1;
#  endif
}
#  endif    /* defined(OPENSSL_RAND_SEED_GETRANDOM) */

#  if defined(OPENSSL_RAND_SEED_DEVRANDOM)
static const char *random_device_paths[] = { DEVRANDOM };
static struct random_device {
    int fd;
    dev_t dev;
    ino_t ino;
    mode_t mode;
    dev_t rdev;
} random_devices[OSSL_NELEM(random_device_paths)];
static int keep_random_devices_open = 1;

#   if defined(__linux) && defined(DEVRANDOM_WAIT)
static void *shm_addr;

static void cleanup_shm(void)
{
    shmdt(shm_addr);
}

/*
 * Ensure that the system randomness source has been adequately seeded.
 * This is done by having the first start of libcrypto, wait until the device
 * /dev/random becomes able to supply a byte of entropy.  Subsequent starts
 * of the library and later reseedings do not need to do this.
 */
static int wait_random_seeded(void)
{
    static int seeded = OPENSSL_RAND_SEED_DEVRANDOM_SHM_ID < 0;
    static const int kernel_version[] = { DEVRANDOM_SAFE_KERNEL };
    int kernel[2];
    int shm_id, fd, r;
    char c, *p;
    struct utsname un;
    fd_set fds;

    if (!seeded) {
        /* See if anything has created the global seeded indication */
        if ((shm_id = shmget(OPENSSL_RAND_SEED_DEVRANDOM_SHM_ID, 1, 0)) == -1) {
            /*
             * Check the kernel's version and fail if it is too recent.
             *
             * Linux kernels from 4.8 onwards do not guarantee that
             * /dev/urandom is properly seeded when /dev/random becomes
             * readable.  However, such kernels support the getentropy(2)
             * system call and this should always succeed which renders
             * this alternative but essentially identical source moot.
             */
            if (uname(&un) == 0) {
                kernel[0] = atoi(un.release);
                p = strchr(un.release, '.');
                kernel[1] = p == NULL ? 0 : atoi(p + 1);
                if (kernel[0] > kernel_version[0]
                    || (kernel[0] == kernel_version[0]
                        && kernel[1] >= kernel_version[1])) {
                    return 0;
                }
            }
            /* Open /dev/random and wait for it to be readable */
            if ((fd = open(DEVRANDOM_WAIT, O_RDONLY)) != -1) {
                if (DEVRANDM_WAIT_USE_SELECT && fd < FD_SETSIZE) {
                    FD_ZERO(&fds);
                    FD_SET(fd, &fds);
                    while ((r = select(fd + 1, &fds, NULL, NULL, NULL)) < 0
                           && errno == EINTR);
                } else {
                    while ((r = read(fd, &c, 1)) < 0 && errno == EINTR);
                }
                close(fd);
                if (r == 1) {
                    seeded = 1;
                    /* Create the shared memory indicator */
                    shm_id = shmget(OPENSSL_RAND_SEED_DEVRANDOM_SHM_ID, 1,
                                    IPC_CREAT | S_IRUSR | S_IRGRP | S_IROTH);
                }
            }
        }
        if (shm_id != -1) {
            seeded = 1;
            /*
             * Map the shared memory to prevent its premature destruction.
             * If this call fails, it isn't a big problem.
             */
            shm_addr = shmat(shm_id, NULL, SHM_RDONLY);
            if (shm_addr != (void *)-1)
                OPENSSL_atexit(&cleanup_shm);
        }
    }
    return seeded;
}
#   else /* defined __linux */
static int wait_random_seeded(void)
{
    return 1;
}
#   endif

/*
 * Verify that the file descriptor associated with the random source is
 * still valid. The rationale for doing this is the fact that it is not
 * uncommon for daemons to close all open file handles when daemonizing.
 * So the handle might have been closed or even reused for opening
 * another file.
 */
static int check_random_device(struct random_device * rd)
{
    struct stat st;

    return rd->fd != -1
           && fstat(rd->fd, &st) != -1
           && rd->dev == st.st_dev
           && rd->ino == st.st_ino
           && ((rd->mode ^ st.st_mode) & ~(S_IRWXU | S_IRWXG | S_IRWXO)) == 0
           && rd->rdev == st.st_rdev;
}

/*
 * Open a random device if required and return its file descriptor or -1 on error
 */
static int get_random_device(size_t n)
{
    struct stat st;
    struct random_device * rd = &random_devices[n];

    /* reuse existing file descriptor if it is (still) valid */
    if (check_random_device(rd))
        return rd->fd;

    /* open the random device ... */
    if ((rd->fd = open(random_device_paths[n], O_RDONLY)) == -1)
        return rd->fd;

    /* ... and cache its relevant stat(2) data */
    if (fstat(rd->fd, &st) != -1) {
        rd->dev = st.st_dev;
        rd->ino = st.st_ino;
        rd->mode = st.st_mode;
        rd->rdev = st.st_rdev;
    } else {
        close(rd->fd);
        rd->fd = -1;
    }

    return rd->fd;
}

/*
 * Close a random device making sure it is a random device
 */
static void close_random_device(size_t n)
{
    struct random_device * rd = &random_devices[n];

    if (check_random_device(rd))
        close(rd->fd);
    rd->fd = -1;
}

int rand_pool_init(void)
{
    size_t i;

    for (i = 0; i < OSSL_NELEM(random_devices); i++)
        random_devices[i].fd = -1;

    return 1;
}

void rand_pool_cleanup(void)
{
    size_t i;

    for (i = 0; i < OSSL_NELEM(random_devices); i++)
        close_random_device(i);
}

void rand_pool_keep_random_devices_open(int keep)
{
    if (!keep)
        rand_pool_cleanup();

    keep_random_devices_open = keep;
}

#  else     /* !defined(OPENSSL_RAND_SEED_DEVRANDOM) */

int rand_pool_init(void)
{
    return 1;
}

void rand_pool_cleanup(void)
{
}

void rand_pool_keep_random_devices_open(int keep)
{
}

#  endif    /* defined(OPENSSL_RAND_SEED_DEVRANDOM) */

/*
 * Try the various seeding methods in turn, exit when successful.
 *
 * TODO(DRBG): If more than one entropy source is available, is it
 * preferable to stop as soon as enough entropy has been collected
 * (as favored by @rsalz) or should one rather be defensive and add
 * more entropy than requested and/or from different sources?
 *
 * Currently, the user can select multiple entropy sources in the
 * configure step, yet in practice only the first available source
 * will be used. A more flexible solution has been requested, but
 * currently it is not clear how this can be achieved without
 * overengineering the problem. There are many parameters which
 * could be taken into account when selecting the order and amount
 * of input from the different entropy sources (trust, quality,
 * possibility of blocking).
 */
size_t rand_pool_acquire_entropy(RAND_POOL *pool)
{
#  if defined(OPENSSL_RAND_SEED_NONE)
    return rand_pool_entropy_available(pool);
#  else
    size_t entropy_available;

#   if defined(OPENSSL_RAND_SEED_GETRANDOM)
    {
        size_t bytes_needed;
        unsigned char *buffer;
        ssize_t bytes;
        /* Maximum allowed number of consecutive unsuccessful attempts */
        int attempts = 3;

        bytes_needed = rand_pool_bytes_needed(pool, 1 /*entropy_factor*/);
        while (bytes_needed != 0 && attempts-- > 0) {
            buffer = rand_pool_add_begin(pool, bytes_needed);
            bytes = syscall_random(buffer, bytes_needed);
            if (bytes > 0) {
                rand_pool_add_end(pool, bytes, 8 * bytes);
                bytes_needed -= bytes;
                attempts = 3; /* reset counter after successful attempt */
            } else if (bytes < 0 && errno != EINTR) {
                break;
            }
        }
    }
    entropy_available = rand_pool_entropy_available(pool);
    if (entropy_available > 0)
        return entropy_available;
#   endif

#   if defined(OPENSSL_RAND_SEED_LIBRANDOM)
    {
        /* Not yet implemented. */
    }
#   endif

#   if defined(OPENSSL_RAND_SEED_DEVRANDOM)
    if (wait_random_seeded()) {
        size_t bytes_needed;
        unsigned char *buffer;
        size_t i;

        bytes_needed = rand_pool_bytes_needed(pool, 1 /*entropy_factor*/);
        for (i = 0; bytes_needed > 0 && i < OSSL_NELEM(random_device_paths);
             i++) {
            ssize_t bytes = 0;
            /* Maximum number of consecutive unsuccessful attempts */
            int attempts = 3;
            const int fd = get_random_device(i);

            if (fd == -1)
                continue;

            while (bytes_needed != 0 && attempts-- > 0) {
                buffer = rand_pool_add_begin(pool, bytes_needed);
                bytes = read(fd, buffer, bytes_needed);

                if (bytes > 0) {
                    rand_pool_add_end(pool, bytes, 8 * bytes);
                    bytes_needed -= bytes;
                    attempts = 3; /* reset counter on successful attempt */
                } else if (bytes < 0 && errno != EINTR) {
                    break;
                }
            }
            if (bytes < 0 || !keep_random_devices_open)
                close_random_device(i);

            bytes_needed = rand_pool_bytes_needed(pool, 1);
        }
        entropy_available = rand_pool_entropy_available(pool);
        if (entropy_available > 0)
            return entropy_available;
    }
#   endif

#   if defined(OPENSSL_RAND_SEED_RDTSC)
    entropy_available = rand_acquire_entropy_from_tsc(pool);
    if (entropy_available > 0)
        return entropy_available;
#   endif

#   if defined(OPENSSL_RAND_SEED_RDCPU)
    entropy_available = rand_acquire_entropy_from_cpu(pool);
    if (entropy_available > 0)
        return entropy_available;
#   endif

#   if defined(OPENSSL_RAND_SEED_EGD)
    {
        static const char *paths[] = { DEVRANDOM_EGD, NULL };
        size_t bytes_needed;
        unsigned char *buffer;
        int i;

        bytes_needed = rand_pool_bytes_needed(pool, 1 /*entropy_factor*/);
        for (i = 0; bytes_needed > 0 && paths[i] != NULL; i++) {
            size_t bytes = 0;
            int num;

            buffer = rand_pool_add_begin(pool, bytes_needed);
            num = RAND_query_egd_bytes(paths[i],
                                       buffer, (int)bytes_needed);
            if (num == (int)bytes_needed)
                bytes = bytes_needed;

            rand_pool_add_end(pool, bytes, 8 * bytes);
            bytes_needed = rand_pool_bytes_needed(pool, 1);
        }
        entropy_available = rand_pool_entropy_available(pool);
        if (entropy_available > 0)
            return entropy_available;
    }
#   endif

    return rand_pool_entropy_available(pool);
#  endif
}
# endif
#endif

#if defined(OPENSSL_SYS_UNIX) || defined(__DJGPP__)
int rand_pool_add_nonce_data(RAND_POOL *pool)
{
    struct {
        pid_t pid;
        CRYPTO_THREAD_ID tid;
        uint64_t time;
    } data = { 0 };

    /*
     * Add process id, thread id, and a high resolution timestamp to
     * ensure that the nonce is unique with high probability for
     * different process instances.
     */
    data.pid = getpid();
    data.tid = CRYPTO_THREAD_get_current_id();
    data.time = get_time_stamp();

    return rand_pool_add(pool, (unsigned char *)&data, sizeof(data), 0);
}

int rand_pool_add_additional_data(RAND_POOL *pool)
{
    struct {
        int fork_id;
        CRYPTO_THREAD_ID tid;
        uint64_t time;
    } data = { 0 };

    /*
     * Add some noise from the thread id and a high resolution timer.
     * The fork_id adds some extra fork-safety.
     * The thread id adds a little randomness if the drbg is accessed
     * concurrently (which is the case for the <master> drbg).
     */
    data.fork_id = openssl_get_fork_id();
    data.tid = CRYPTO_THREAD_get_current_id();
    data.time = get_timer_bits();

    return rand_pool_add(pool, (unsigned char *)&data, sizeof(data), 0);
}


/*
 * Get the current time with the highest possible resolution
 *
 * The time stamp is added to the nonce, so it is optimized for not repeating.
 * The current time is ideal for this purpose, provided the computer's clock
 * is synchronized.
 */
static uint64_t get_time_stamp(void)
{
# if defined(OSSL_POSIX_TIMER_OKAY)
    {
        struct timespec ts;

        if (clock_gettime(CLOCK_REALTIME, &ts) == 0)
            return TWO32TO64(ts.tv_sec, ts.tv_nsec);
    }
# endif
# if defined(__unix__) \
     || (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L)
    {
        struct timeval tv;

        if (gettimeofday(&tv, NULL) == 0)
            return TWO32TO64(tv.tv_sec, tv.tv_usec);
    }
# endif
    return time(NULL);
}

/*
 * Get an arbitrary timer value of the highest possible resolution
 *
 * The timer value is added as random noise to the additional data,
 * which is not considered a trusted entropy sourec, so any result
 * is acceptable.
 */
static uint64_t get_timer_bits(void)
{
    uint64_t res = OPENSSL_rdtsc();

    if (res != 0)
        return res;

# if defined(__sun) || defined(__hpux)
    return gethrtime();
# elif defined(_AIX)
    {
        timebasestruct_t t;

        read_wall_time(&t, TIMEBASE_SZ);
        return TWO32TO64(t.tb_high, t.tb_low);
    }
# elif defined(OSSL_POSIX_TIMER_OKAY)
    {
        struct timespec ts;

#  ifdef CLOCK_BOOTTIME
#   define CLOCK_TYPE CLOCK_BOOTTIME
#  elif defined(_POSIX_MONOTONIC_CLOCK)
#   define CLOCK_TYPE CLOCK_MONOTONIC
#  else
#   define CLOCK_TYPE CLOCK_REALTIME
#  endif

        if (clock_gettime(CLOCK_TYPE, &ts) == 0)
            return TWO32TO64(ts.tv_sec, ts.tv_nsec);
    }
# endif
# if defined(__unix__) \
     || (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L)
    {
        struct timeval tv;

        if (gettimeofday(&tv, NULL) == 0)
            return TWO32TO64(tv.tv_sec, tv.tv_usec);
    }
# endif
    return time(NULL);
}
#endif /* defined(OPENSSL_SYS_UNIX) || defined(__DJGPP__) */