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Dario Rodriguez authored on 01/11/2025 11:49:23
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libyesterday.c
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+/*
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+ * libyesterday.c
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+ *
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+ * Captures gettimeofday(), time() and clock_gettime() to simulate yesterday's date
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+ *
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+ * History:
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+ *      01/11/2025 Creation.
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+ *
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+ * Documentation:
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+ *      https://rafalcieslak.wordpress.com/2013/04/02/dynamic-linker-tricks-using-ld_preload-to-cheat-inject-features-and-investigate-programs/
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+ *
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+ * Usage examples:
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+ *      # load date program with day set to yesterday
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+ *      LD_PRELOAD=/path/to/libyesterday.so /bin/date
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+ *      # load date program with day set to a week ago
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+ *      LIBYESTERDAY=$((3600*24*7)) LD_PRELOAD=/path/to/libyesterday.so /bin/date
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+ *
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+ * Author: Dario Rodriguez antartica@whereismybit.com
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+ * This program is licensed under the terms of the MIT/X license.
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+ */
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+
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+#define _GNU_SOURCE
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+#include <dlfcn.h>
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+#include <time.h>
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+#include <sys/time.h>
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+#include <stdlib.h>
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+
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+typedef time_t (*time_fn_type)(time_t *tloc);
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+typedef int (*gettimeofday_fn_type)(struct timeval *restrict tv, void *restrict tz);
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+typedef int (*clock_gettime_fn_type)(clockid_t clockid, struct timespec *tp);
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+
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+static int
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+libyesterdaydata(time_fn_type *paramtime_fn, gettimeofday_fn_type *paramgettimeofday_fn, clock_gettime_fn_type *paramclock_gettime_fn)
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+{
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+        static int init=0;
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+        static int seconds=3600*24;
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+        static time_fn_type realtimefn=(time_fn_type)NULL;
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+        static gettimeofday_fn_type realgettimeofdayfn=(gettimeofday_fn_type)NULL;
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+        static clock_gettime_fn_type realclock_gettimefn=(clock_gettime_fn_type)NULL;
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+        if(init==0) {
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+                char *ptr;
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+                realtimefn = (time_fn_type)dlsym(RTLD_NEXT,"time");
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+                realgettimeofdayfn = (gettimeofday_fn_type)dlsym(RTLD_NEXT,"gettimeofday");
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+                realclock_gettimefn = (clock_gettime_fn_type)dlsym(RTLD_NEXT,"clock_gettime");
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+                if((ptr=getenv("LIBYESTERDAY"))!=NULL)
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+                        seconds=atoi(ptr);
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+                init=1;
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+        }
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+        if(paramtime_fn!=NULL)
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+                *paramtime_fn=realtimefn;
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+        if(paramgettimeofday_fn!=NULL)
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+                *paramgettimeofday_fn=realgettimeofdayfn;
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+        if(paramclock_gettime_fn!=NULL)
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+                *paramclock_gettime_fn=realclock_gettimefn;
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+        return(seconds);
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+}
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+
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+extern time_t time(time_t *tloc)
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+{
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+        time_fn_type realtimefn=(time_fn_type)NULL;
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+        int seconds;
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+        int res;
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+        seconds=libyesterdaydata(&realtimefn,NULL,NULL);
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+        res=realtimefn(NULL);
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+        res-=seconds;
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+        if(tloc!=NULL)
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+                *tloc=res;
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+        return(res);
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+}
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+
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+extern int gettimeofday(struct timeval *restrict tv, void *restrict tz)
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+{
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+        static gettimeofday_fn_type realgettimeofdayfn=(gettimeofday_fn_type)NULL;
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+        int seconds;
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+        int res;
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+        seconds=libyesterdaydata(NULL,&realgettimeofdayfn,NULL);
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+        res=realgettimeofdayfn(tv,tz);
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+        tv->tv_sec-=seconds;
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+        return(res);
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+}
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+
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+extern int clock_gettime(clockid_t clockid, struct timespec *tp)
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+{
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+        static clock_gettime_fn_type realclock_gettimefn=(clock_gettime_fn_type)NULL;
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+        int seconds;
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+        int res;
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+        seconds=libyesterdaydata(NULL,NULL,&realclock_gettimefn);
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+        res=realclock_gettimefn(clockid,tp);
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+        tp->tv_sec-=seconds;
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+        return(res);
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+}
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+
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+