PoC-time-jumps.sh -- proof of concept to detect FPU/hardware related issues causing time jump in python (fail2ban/fail2ban#3187)

PoC-time-jumps.sh

#! /bin/sh
printf '%s' '

#include <inttypes.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <time.h>
#include <unistd.h>

#define SEC_TO_NS (1000 * 1000 * 1000)

double
_Time_AsSecondsDouble(int64_t t)
{
    /* volatile avoids optimization changing how numbers are rounded */
    volatile double d;

    if (t % SEC_TO_NS == 0) {
        /* Divide using integers to avoid rounding issues on the integer part.
           1e-9 cannot be stored exactly in IEEE 64-bit. */
        t /= SEC_TO_NS;
        d = (double)t;
    }
    else {
        d = (double)t;
        d /= 1e9;
    }
    return d;
}

int64_t
_Time_GetTime()
{
    int64_t t;

    struct timespec ts;
    if (clock_gettime(CLOCK_REALTIME, &ts)) {
        printf("ERROR: clock_gettime fails\n");
        exit(-1);
    }

    t = ts.tv_sec;
    t *= SEC_TO_NS;
    t += ts.tv_nsec;

    return t;
}

int
main()
{
    int64_t t, t0 = _Time_GetTime();
    double  d, d0 = _Time_AsSecondsDouble(t0);
    printf("%"PRId64"\t%.3f \t ...\n", t0, d0);
    while (1) {
        usleep(500000);
        t = _Time_GetTime();
        d = _Time_AsSecondsDouble(t);
        printf("%"PRId64"\t%.3f \t +%"PRId64"\t+%.3f\n", t, d, t - t0, d - d0);
        if (t - t0 > SEC_TO_NS || t0 - t > SEC_TO_NS || d - d0 > 1 || d0 - d > 1) break;
        t0 = t;
        d0 = d;
    }
    return 0;
}

// ' > ./poc-time-jumps.c; gcc -O2 -Wall -Wextra ./poc-time-jumps.c -o ./poc-time-jumps && ./poc-time-jumps