gcc with electric fence library does not take effe

Question

I have written a snippet that has memory problems when dynamically allocating; when compiled with -lefence option, it seems that there is no effect. Here is the code segment:

int main(int argc, char *argv[])
{
  int *a = (int *)malloc(2*sizeof(int));
  for(int i = 0; i <=2; ++i){
    a[i] = i;
    printf ("%d\n",a[i]);
  }

  free(a);
  return 0;
}

And the compilation options:

gcc -g3 -Wall -std=c99 outOfBound.c -lefence

The expected result is that when a.out is executed there would be a core dump after i is assigned to 2 and a[i]=i is invoked.

So Why -lefence has no effect?

I have also increased the upper bound in the loop to 9, but there is still no core dump thatelectric-fence invoked. (Actually there is indeed a core dump by default, but this might due to the MALLOC_CHECK_ env virable since when I export MALLOC_CHECK_=0, there would be no more core dump).

UPDATE: the whole result of nm -A a.out is as below:

a.out:08049f28 d _DYNAMIC
a.out:08049ff4 d _GLOBAL_OFFSET_TABLE_
a.out:0804864c R _IO_stdin_used
a.out:         w _Jv_RegisterClasses
a.out:08049f18 d __CTOR_END__
a.out:08049f14 d __CTOR_LIST__
a.out:08049f20 d __DTOR_END__
a.out:08049f1c d __DTOR_LIST__
a.out:08048718 r __FRAME_END__
a.out:08049f24 d __JCR_END__
a.out:08049f24 d __JCR_LIST__
a.out:0804a01c A __bss_start
a.out:0804a014 D __data_start
a.out:08048600 t __do_global_ctors_aux
a.out:08048480 t __do_global_dtors_aux
a.out:0804a018 d __dso_handle
a.out:         w __gmon_start__
a.out:080485f2 t __i686.get_pc_thunk.bx
a.out:00000000 a __init_array_end
a.out:00000000 a __init_array_start
a.out:080485f0 T __libc_csu_fini
a.out:08048580 T __libc_csu_init
a.out:         U __libc_start_main
a.out:0804a01c A _edata
a.out:0804a024 A _end
a.out:0804862c T _fini
a.out:08048648 R _fp_hw
a.out:080483b4 T _init
a.out:08048450 T _start
a.out:0804a01c b completed.6159
a.out:0804a014 W data_start
a.out:0804a020 b dtor_idx.6161
a.out:080484e0 t frame_dummy
a.out:         U free
a.out:08048504 T main
a.out:         U malloc
a.out:         U printf

(I am using a debian package electric-fence on Ubuntu 12.04 32bit, gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3)

Update(20140801):

For electric-fence of version 2.2.4 packaged by debian(testing branch, i.e. jessie), it works.

Answer 1

It is possible you are running into this.

... it must increase the size of the allocation to a multiple of the word size. In addition, the functions memalign() and valloc() must honor explicit specifications on the alignment of the memory allocation, and this, as well can only be implemented by increasing the size of the allocation. Thus, there will be situations in which the end of a memory allocation contains some padding space, and accesses of that padding space will not be detected, even if they are overruns.

Try exceed the bounds a bit more, and see at what point the overrun detection kicks in.

Answer 2

Once you compile and execute above program without linking it with electric fence library, it may run without any segmentation fault.

So better to link it with electric fence library and then Run it by loading it in gdb giving the following command

$ gdb a.out
....
(gdb)run
Starting program: /home/arif/sysprog-2017/processmgmt/nonlocalgoto/a.out 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
  Electric Fence 2.2 Copyright (C) 1987-1999 Bruce Perens <[e-mail]>
0
1
Program received signal SIGSEGV, Segmentation fault.
0x000055555555484d in main (argc=1, argv=0x7fffffffe228) at temp.c:8
8       a[i] = i;

So from above output of gdb you can make out the source line number which is causing the problem if you print the value of i at this instant it will be 2 :)