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Why does LD_PRELOAD trick not work for librt?

I tried to apply the LD_PRELOAD trick to some native binary. I have done similar things before, but this time no luck. The call I was trying to intercept was timer_settime ().

Strace clearly shows that timer_settime () is called binary:

[pid 30500] timer_settime(0x2, 0, {it_interval={30, 0}, it_value={30, 0}}, {it_interval={0, 0}, it_value={0, 0}}) = 0

It has been called many times at different time intervals. I want to catch exactly the above with an interval of 30 seconds.

Here is my code, timerwrap.c:

#define _GNU_SOURCE
#include <dlfcn.h>
#include <time.h>
#include <stdio.h>
int timer_settime(timer_t timerid, int flags, const struct itimerspec *new_value, struct itimerspec *old_value)
{
    printf("Enter timer.\n");

    if((new_value->it_interval).tv_sec == 30) {
        printf("Catched!\n");
        return 0;
    }

    int (*real_timer_settime)(timer_t, int, const struct itimerspec *, struct itimerspec *);
    real_timer_settime = dlsym(RTLD_NEXT, "timer_settime");
    return real_timer_settime(timerid, flags, new_value, old_value);
}

Gcc command line:

gcc -Wall -g -shared -fPIC  -o timerwrap.so timerwrap.c -ldl -lrt

run the program:

export LD_PRELOAD=/home/Work/C/timerwrap.so
./the_program

But he could not intercept the challenge.

I ran it again with LD_DEBUG = all for further study. It turns out that for many other characters, the timerwrap.so function was looked at, for example, the dlsym lookup path looks like this:

2006:   symbol=dlsym;  lookup in file=/.../the_program [0]
2006:   symbol=dlsym;  lookup in file=/home/Work/C/timerwrap.so [0]
2006:   symbol=dlsym;  lookup in file=./lib/libssl.so.6 [0]
2006:   symbol=dlsym;  lookup in file=/lib/i686/cmov/libdl.so.2 [0]
2006:   binding file /.../the_program [0] to /lib/i686/cmov/libdl.so.2 [0]: normal symbol `dlsym' [GLIBC_2.0]

timer_settime /usr/lib/librt.so, timerwrap.so:

2006:   symbol=timer_settime;  lookup in file=/usr/lib/librt.so [0]
2006:   binding file /usr/lib/librt.so [0] to /usr/lib/librt.so [0]: normal symbol `timer_settime'

? librt.so ? ?

. !

+4
1

, , .

, , timer_settime dlsym.

dlsym(dlopen("librt.so"), "timer_settime")

, - :

extern void *_dl_sym (void *handle, const char *name, void *who);
void *dlsym(void *handle, const char *symbol) 
{ 
        printf("Enter dlsym.\n");
        void* result = _dl_sym(handle, symbol, 0); // the replacement we are going to use 
        if ((handle != RTLD_NEXT) || (handle != RTLD_DEFAULT))
        if(!strcmp(symbol, "timer_settime")) { 
               printf("Return our timer_settime");
               return timer_settime;
        } 
        return result; 
}
+2

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