Why editing .fini_array doesn't change course of program?

Question

I'm trying to see how editing of .fini_array will cause change in program flow but it seems that changing this entry cause nothing to change. my main is:

void cleanup() __attribute__((destructor));
int main() {
    printf("In main\n");
    exit(1);
}
void cleanup(){
    printf("Cleanning Up\n");
}
void fini_exec(){
    printf("FINI\n");
}

running nm test: 000000000000119e T fini_exec

while cleenup: 0000000000001187 T cleanup

running objdump -s -j .fini_array test :

Contents of section .fini_array:
 3db0 20110000 00000000 87110000 00000000   ...............

using a hex editor and going to byte of 2db0 which i obtained from readelf and editing 87110000 to 9e110000(indianese)after edit running objdump again

Contents of section .fini_array:
 3db0 20110000 00000000 9e110000 00000000   ...............

then saving and re-run still cause cleanup to run which i don't know why. isn't .fini_array is supposed to be run in backward followed by .fini but it seems that there is another reference for cleanup somewhere else that's causing fini_exec to not run.

Answer 1

I made two separate executables a.out.1 and a.out.2 respectively with void cleanup() __attribute__((destructor)); and void fini_exec() __attribute__((destructor));:

#include <stdio.h>
#include <stdlib.h>

void cleanup() __attribute__((destructor));
//void fini_exec() __attribute__((destructor));

int main() {
  printf("In main\n");
  exit(1);
}

void cleanup(){
  printf("Cleanning Up\n");
}

void fini_exec(){
  printf("FINI\n");
}

I generated the hexadecimal dumps with:

$ hexdump a.out.1 > a.out.1.hex
$ hexdump a.out.2 > a.out.2.hex

I compared the hexadecimal dumps and could see that the offsets (0x119e and 0x1187) of the functions tagged as destructors appear in 2 places (offsets 0x580 and 0x2db8) :

$ diff a.out.1.hex a.out.2.hex 
55,56c55,56
< 0000360 0003 0000 4e47 0055 5b7f a631 6798 4fc1
< 0000370 d865 85fc 1cc6 c187 eaab 06ab 0004 0000
---
> 0000360 0003 0000 4e47 0055 4bec d49a 80fc b24e
> 0000370 0d44 2dd5 81c9 82cb fe22 f498 0004 0000
89c89
< 0000580 119e 0000 0000 0000 4008 0000 0000 0000
---
> 0000580 1187 0000 0000 0000 4008 0000 0000 0000
173c173
< 0002db0 1120 0000 0000 0000 119e 0000 0000 0000
---
> 0002db0 1120 0000 0000 0000 1187 0000 0000 0000

So, I wrote a simple patcher tool which takes as parameters an executable name and a suite of offsets where to patch bytes (i.e. open(executable), lseek(offset), write(byte), close(executable)):

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(int ac, char *av[])
{
int            fd;
char          *fname = av[1];
unsigned char  byte;
int            offset;
int            i;
int            rc;

  if (!fname) {
    fprintf(stderr, "Usage: %s {offseth byteh...}\n", av[0]);
    return 1;
  }

  fd = open(fname, O_RDWR);
  if (fd < 0) {
    fprintf(stderr, "open(%s): '%m' (%d)\n", fname, errno);
    return 1;
  }

  i = 2;
  while (av[i] && av[i + 1]) {

    offset = strtol(av[i], NULL, 0);
    byte = strtol(av[i + 1], NULL, 0);

    printf("Patching @0x%x: 0x%02x\n", offset, byte);
    rc = lseek(fd, offset, SEEK_SET);
    if (rc != offset) {
      fprintf(stderr, "lseek(%s): '%m' (%d)\n", fname, errno);
      return 1;
    }

    rc = write(fd, &byte, 1);
    if (rc != 1) {
      fprintf(stderr, "write(%s): '%m' (%d)\n", fname, errno);
      return 1;
    }

    i += 2;

  } // End while

  close(fd);

  return 0;

} // main

When I use it to patch both places, the destructor is changed and works as expected:

$ gcc patch.c -o patch
$ patch ./a.out.1 0x580 0x87 0x2db8 0x87 # cleanup()'s offset (0x1187)
Patching @0x580: 0x87
Patching @0x2db8: 0x87
$ ./a.out.1
In main
Cleanning Up
$ ./patch ./a.out.1 0x580 0x9e 0x2db8 0x9e # fini_exec()'s offset (0x119e)
Patching @0x580: 0x9e
Patching @0x2db8: 0x9e
$ ./a.out.1
In main
FINI