asm: overwriting return point of the function/stack overflow

Stack overflow is a common attack in programming world.
To understand how it could be done we should be aware about the function's stack and how the function is being executed and how it passes the execution of the code in the parent function after its call.
The stack of the function, at least in *nix, should look like

|function parameters| <--top of the stack(higher memory addresses)
|---return  point---| <--%esp
|--local variables--|
|-------------------| <--bottom of the stack(lower memory addresses)

Let's examine simple program written in asm. It has a function pc that puts giver character onto stdout and adds '\n'. In main this function is called with argument which value is '*'.

 1 .text
 2 
 3 pc:
 4     pushl %ebp
 5     movl %esp, %ebp
 6     
 7     subl $4, %esp /*4 bytes for local variables*/
 8     
 9     pushl 8(%ebp)/*get value of the function parameter*/
10     call putchar
11     pushl $0x0a /*new line*/
12     call putchar
13     addl $8, %esp/*allign stack*/
14     
15     movl %ebp, %esp
16     popl %ebp
17     ret
18     
19 
20 .global main
21 
22 main:
23     pushl $0x0000002a /*character '*'*/
24     call pc
25     addl $4, %esp/*allign stack*/
26     
27     movl $1, %eax
28     movl $0, %ebx
29     int $0x80/*exit(0)*/

I set a breakpoint on line 4 in gdb and got the information about the registers

(gdb) i r
eax            0xbfae0a34 -1079113164
ecx            0x312f6668 825189992
edx            0x1 1
ebx            0xb7fa4ff4 -1208332300
esp            0xbfae09a4 0xbfae09a4
ebp            0xbfae0a08 0xbfae0a08
esi            0xb7fe2ca0 -1208079200
edi            0x0 0
eip            0x8048384 0x8048384 <pc>

Address of %esp is 0xbfae09a4, so here is the top of the stack of our function pc.
In *nix world stack of the process grows from the higher memory addresses to the lower ones. So to get function parameter we should add 4 bytes to %esp(the size of return point is 4 bytes)[Note, on line 9 I pushed the address of %ebp + 8 because after 'pushl %ebp' value of %esp increased with 4 bytes.]

(gdb) x/c 0xbfae09a4 + 4
0xbfae09a8: 42 '*'

Yes, here we have '*' _because_ we indeed pushed it onto the stack on line 23. In %esp we can find the address of the return point

(gdb) x/x 0xbfae09a4 
0xbfae09a4: 0x080483a7

0x080483a7 is the address of the next instruction after the call of pc in main. Let's check.
Going through instruction in gdb I got out from pc

(gdb) n
pc () at fcall.s:17
17  ret
(gdb) n
main () at fcall.s:25
25  addl $4, %esp/*allign stack*/
(gdb) i r
eax            0xa 10
ecx            0xffffffff -1
edx            0xb7fa60b0 -1208328016
ebx            0xb7fa4ff4 -1208332300
esp            0xbfae09a8 0xbfae09a8
ebp            0xbfae0a08 0xbfae0a08
esi            0xb7fe2ca0 -1208079200
edi            0x0 0
eip            0x80483a7 0x80483a7 <main+7>

You can see that value of %eip is 0x80483a7, so we were right. To make a program run any other code rather than return to the parent function the address of the return point has to be overwritten.
The following code attempts to do so.
It has function evil which address will be written to the return point of the function pc. Function evil writes '%\n' on the output and calls exit syscall with exit code 1.

 1 .text
 2 
 3 evil:
 4     pushl %ebp
 5     movl %esp, %ebp
 6 
 7     pushl $0x00000025 /*character '%'*/
 8     call putchar
 9     pushl $0x0a /*new line*/
10     call putchar
11 
12     movl $1, %eax
13     movl $1, %ebx
14     int $0x80/*exit(1)*/
15     
16 pc: 
17     pushl %ebp
18     movl %esp, %ebp
19     
20     subl $4, %esp /*4 bytes for local variables*/
21 
22     pushl 8(%ebp)
23     call putchar
24     pushl $0x0a /*new line*/
25     call putchar
26     addl $8, %esp/*allign stack*/
27     
28     movl %ebp, %esp
29     popl %ebp
30 
31     movl $evil, (%esp)
32 
33     ret
34 
35 
36 .global main
37 
38 main:
39     pushl $0x0000002a /*character '*'*/
40     call pc
41     addl $4, %esp/*allign stack*/
42     
43     movl $1, %eax
44     movl $0, %ebx
45     int $0x80/*exit(0)*/

The result of the exucution of this program should be

$gcc fcall.s -o fcall -g
$./fcall 
*
%
$echo $?
1

c++: multidimensional arrays in the (dynamic) memory

I know some solutions how to store multidimensional arrays in the (dynamic) memory.
I'd like to share this knowledge because I noticed that not all of the developers understand what is going on in this field.
Let's look at different ways how to create 2-dimension array of objects of the class A which code is below

class A
{
    public:
        void * operator new(size_t size)
        {
            void *p = malloc(size);
            cout << "new, size: " << size << "\n";
            return p;
        }

        void * operator new[](size_t size)
        {
            void *p = malloc(size);
            cout << "new[], size: " << size << "\n";
            return p;
        }

        A() 
        {   
            cout << "A()\n";
            id = ++counter;
        }

        ~A()
        {   
            cout << "~A()\n";
        }   

        void call()
        {   
            cout << "id #" << id << ", " << counter << " times constructor of A was called\n";
        }

        static int counter;
        int id; 
};

int A::counter = 0;

I added some code for tracing operator new, constructor and destructor calls.
Each time the constructor is called value of class static variable counter is incremented by 1 and its new value is assigned to class member variable id.

• The first method and the simplest.
  Simply to allocate 2x2 array of A on the stack.

  cout << "size of A: " << sizeof(A) << "\n";
  A z[2][2];
  z[1][1].call();
  (z[1]+1)->call();
  (*z+3)->call();

  This piece of code produces

  size of A: 4
  A()
  A()
  A()
  A()
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  ~A()
  ~A()
  ~A()
  ~A()

  4 times constructor was called, 4 times destructor, no calls of operator new.
• The second, a bit more complex.
  Allocate memory for 2x2 array of A in the heap.

  cout << "size of A: " << sizeof(A) << "\n";
  A (*z)[2] = new A[2][2];
  z[1][1].call();
  (z[1]+1)->call();
  (*z+3)->call();
  delete [] z;

  The output should be

  size of A: 4
  new[], size: 20
  A()
  A()
  A()
  A()
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  ~A()
  ~A()
  ~A()
  ~A()
  

  4 times constructor was called, 4 times destructor, 1 call of operator new[] to allocate memory for all 4 objects.
• The next method is used to allocate memory in the heap for one-dimension array of size 2 of pointers to A. Then allocate memory for one-dimension 'sub-arrays'.

  cout << "size of A: " << sizeof(A) << "\n";
  A **z = new A*[2];
  z[0] = new A[2];
  z[1] = new A[2];
  
  z[1][1].call();
  (z[1]+1)->call();
  (*z+3)->call();
  
  delete [] z[0];
  delete [] z[1];
  delete [] z;

  size of A: 4
  new[], size: 12
  A()
  A()
  new[], size: 12
  A()
  A()
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  ~A()
  ~A()
  ~A()
  ~A()

  2 times constructor was called after each call to operator new[] to allocate memory for 2 objects, 4 times destructor was called
• This method is tricky a little bit. We allocate one-dimension array of size 4. Using pointer arithmetics we can simulate two-dimension array.

  cout << "size of A: " << sizeof(A) << "\n";
  A *z = new A[2*2];
  z[2+1].call();
  (z+3)->call();
  delete [] z;

  size of A: 4
  new[], size: 20
  A()
  A()
  A()
  A()
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  ~A()
  ~A()
  ~A()
  ~A()
  

4 times constructor was called, 4 times destructor, 1 call of operator new[] to allocate memory for all 4 objects.
• This one is a combination of storing 2x2 array in the heap and in the stack. At first one-dimension array of pointers to A is put onto the stack and later memory from heap is used to allocate one-dimension 'sub-arrays'.

  cout << "size of A: " << sizeof(A) << "\n";
  A *z[2];
  z[0] = new A[2];
  z[1] = new A[2];
  
  z[1][1].call();
  (z[1]+1)->call();
  (*z+3)->call();
  
  delete [] z[0];
  delete [] z[1];

  size of A: 4
  new[], size: 12
  A()
  A()
  new[], size: 12
  A()
  A()
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  id #4, 4 times constructor of A was called
  ~A()
  ~A()
  ~A()
  ~A()
  

  2 times constructor was called after each call to operator new[] to allocate memory for 2 objects, 4 times destructor was called

All methods have their '+'s and '-'s. One can take more time but require less memory and the other one can take more memory but could be executed faster. That depends how many calls have been done to allocate memory, where memory was taken to allocate an array, etc. Also you should remember c++ restriction for arrays on the stack that their size must be known during the compile time. The dark side of memory from the heap is that it should be explicitly released when it become unused. Some of them are more expressive for understanding some of them not.
This is upon you.

autoconf: square brackets in AS_HELP_STRING

With autoconf(2.63) if I wanted to use square brackets for AS_HELP_STRING I didn't succeed. I have been trying to add extra [] around the helpstring according to manual:

Each time there can be a macro expansion, there is a quotation expansion, i.e., one level of quotes is stripped:

int tab[10];
     =>int tab10;
     [int tab[10];]
     =>int tab[10];

The solution I've found in configure.ac of qore programming language.
The quadrigraphs are used there.
'@<:@' for '[' and '@:>@' for ']' could be used in autoconf input file.
So now I have nice output of ./configure --help in my project:

....
  --with-mysql[=DIR]      use mysql
  --with-fcgi[=DIR]       use fast CGI
  --with-pcre[=DIR]       use pcre
....

The code in configure.in looks like:

....
AS_HELP_STRING([--with-sqlite@<:@=DIR@:>@], [use sqlite])...
....

emacs: the dark side of the force

Recently I've decided to try the dark side of the force - emacs.
I'm Vim user for a long time. Several times I wanted to try emacs but didn't have a good chance.
Now I'm working on project with huge amount of sources and I decided to try emacs for it.
It works! ;)

Playing with emacs I've found out that it's not so complex as some people say.

The thing to which I couldn't get used to for some time is that I don't have to press ESC to switch to the command mode, press i(INS) to switch to editor mode and so on.

I haven't found some Vim features(as visual mode) but I believe that just don't know how to make them work.

The main difference is that there is no distinct differences between editor mode and command mode. You are allowed to run commands while you are editing the text.

All commands(or better to say most of them) begin with control- or meta- character. control is usually Ctrl on your keyboard and meta is Alt.

For guys who want to try emacs here is the migration guide on vim-to-emacs commands.
The table of equivalence of vim and emacs commands.

split horizontalsplit vertical

VIM	EMACS	Description
:qa/:wqa/:xa/:qa!	C-x C-c	exit(emacs prompts whether to save buffers or not)
h	C-b	left
l	C-f	right
b/B	M-b	word backward
w/W	M-f	word forward
j	C-n	down
k	C-p	up
0	C-a	beginning of the line
$	C-e	end of the line
gg/1G	C-<	beginning of the buffer
G	C->	end of the buffer
x	C-d	delete under cursor
D	C-k	delete from cursor to EOL
dd	C-k C-k	delete line
dw/dW	M-d	delete word
db/dB	M-{BACKSPACE}	delete word backwards
:set ignorecase {ENTER} /	C-s {needle in lower case}	icase search forward
:set ignorecase {ENTER} ?	C-r {needle in lower case}	icase search backward
/	C-s	search forward
?	C-r	search backward
:set ignorecase {ENTER} /	M-C-s {regexp in lower case}	icase regexp search forward
:set ignorecase {ENTER} ?	M-C-r {regexp in lower case}	icase regexp search backward
:%s/{needle}/{replacement}/gc	M-% {needle} {ENTER} {replacement} {ENTER}	query replace
/	M-C-s	regexp search forward
?	M-C-r	regexp search backward
u	C-_/C-x u	undo
C-R	C-_/C-x u	redo(it's tricky for emacs*)
ESC	C-g	quit the running/entered command(switch to command mode in Vim)
:e file	C-x C-f	open file
:set ro	C-x C-q	set file as read-only
:w	C-x C-s	save buffer
:w file	C-x C-w file	save buffer as ...
:wa	C-x s	save all buffers
:buffers	C-x C-b	show buffers
:b [name]	C-x b [name]	switch to another buffer
:q/:q!/:wq/:x	C-x k	close buffer
C-w n/:split	C-x 2
C-w v/:vsplit	C-x 3
C-w C-w	C-x o	switch to another window
:q	C-x 0	close window
C-w o	C-x 1	close other windows
:! {cmd}	M-!	run shell command
m{a-zA-Z}	C-@/C-space	set mark
	C-x C-x	exchange mark and position
{visual}y	M-w	copy region**
{visual}d	C-w	delete region**
p	C-y	paste
C-V {key}	C-q {key}	insert special char, e.g. ^M:
{visual}SHIFT->	C-x TAB	indent region
C-]	M-.	find tag
C-t	M-*	return to previous location
:e!	M-x revert-buffer	reload buffer from disk

*To redo changes you have undone, type `C-f' or any other command that will harmlessly break the
sequence of undoing, then type more undo commands
**region is from current position to the mark

Other useful emacs commands:

M-g	go to line
C-x i	insert file
C-x h	mark whole buffer
C-x C-t	switch two lines
M-C-a	beginning of the function
M-C-e	end of the function
M-a	beginning of the statement
M-e	end of the statement
M-C-h	mark the function
M-/	autocompletion
M-C-\	indent region
C-c C-q	indent the whole function according to indention style
C-c C-c	comment out marked area
M-x uncomment-region	uncomment marked area
M-,	jumps to the next occurence for tags-search
M-;	insert comment in code
C-x w h	highlight the text by regexp
C-x w r	disable highlighting the text by regexp

To run emacs without X add -nw command line argument.

To run multiply commands 'C-u {number} {command}' or 'M-{digit} {command}'.

emacs has bookmarks that are close to Vim marks:

C-x r m	set a bookmark at current cursor position
C-x r b	jump to bookmark
C-x r l	list bookmarks
M-x bookmark-write	write all bookmarks in given file
M-x bookmark-load	load bookmarks from given file

My ~/.emacs looks like:

(setq load-path (cons "~/.emacs.d" load-path))

(auto-compression-mode t) ; uncompress files before displaying them

(global-font-lock-mode t) ; use colors to highlight commands, etc.
(setq font-lock-maximum-decoration t)
(custom-set-faces)
(transient-mark-mode t) ; highlight the region whenever it is active
(show-paren-mode t) ; highlight parent brace
(global-hi-lock-mode t) ; highlight region by regexp

(column-number-mode t) ; column-number in the mode line

(setq make-backup-files nil)

(setq scroll-conservatively most-positive-fixnum) ; scroll only one line when I move past the bottom of the screen

(add-hook 'text-mode-hook 'turn-on-auto-fill) ; break lines at space when they are too long

(fset 'yes-or-no-p 'y-or-n-p) ; make the y or n suffice for a yes or no question

(setq comment-style 'indent)

(global-set-key (kbd "C-x C-b") 'buffer-menu) ; buffers menu in the same window

(global-set-key (kbd "C-x 3") 'split-window-horizontally-other) ; open new window horisontally and switch to it
(defun split-window-horizontally-other ()
        (interactive)
        (split-window-horizontally)
        (other-window 1)
)

(global-set-key (kbd "C-x 2") 'split-window-vertically-other) ; open new window vertically and switch to it
(defun split-window-vertically-other ()
 (interactive)
 (split-window-vertically)
 (other-window 1)
)

(global-set-key (kbd "C-c c") 'comment-region) ; comment code block
(global-set-key (kbd "C-c u") 'uncomment-region) ; uncomment code block

(global-set-key (kbd "C-x TAB") 'tab-indent-region) ; indent region
(global-set-key (kbd "C-x <backtab>") 'unindent-region) ; unindent region
(defun tab-indent-region ()
    (interactive)
 (setq fill-prefix "\t")
    (indent-region (region-beginning) (region-end) 4)
)
(defun unindent-region ()
    (interactive)
    (indent-region (region-beginning) (region-end) -1)
)

(global-set-key (kbd "TAB") 'self-insert-command)
(global-set-key (kbd "RET") 'newline-and-indent)

(setq key-translation-map (make-sparse-keymap))
(define-key key-translation-map "\177" (kbd "C-="))
(define-key key-translation-map (kbd "C-=") "\177")
(global-set-key "\177" 'delete-backward-char)
(global-set-key (kbd "C-=") 'delete-backward-char)

(setq indent-tabs-mode t)
(setq tab-always-indent t)
(setq default-tab-width 4)

(setq inhibit-startup-message t) ; do not show startup message

(iswitchb-mode t)
(desktop-save-mode t)

(display-time)

Happy emacsing!

perl: manipulations with standart output stream

Builtin functions print and write put data to stdout if the filehandle is omitted.
This is really handy.
But since you want to write to some other stream each time you have to specify filehandle for them.
It's possible to point STDOUT to some other filehandle by reassigning it

open $nfh, '>test';
$ofh = *STDOUT;
*STDOUT = *$nfh;

print "test";

*STDOUT = $ofh;
close $nfh;

or by using select function

open $nfh, '>test';
$ofh = select $nfh;

print "test";

select $ofh;
close $nfh;

and still use print and write w/o specifying the filehandle. The second method with select looks better for me.
If you use a lot of ways for output the LIFO(Last-In, First-Out) of filehandles can be used to implement an easy way to walk through them like pushd/popd in bash

#!/usr/bin/perl
@fh = ();

open my $nfh, '>test-0';
push @fh, select $nfh;

print "test\n";

open my $nfh, '>test-1';
push @fh, select $nfh;

print "test\n";

close select pop @fh;
print "test\n";
close select pop @fh;

print "test\n";

The result of executing this script

$./test.pl 
test
$cat test-*
test
test
test

Note, I used my with filehandle in open to give it an undefined scalar variable. Otherwise open will associate the stream with this variable. And the old stream will be lost. The following code does the same work w/o using my expression

#!/usr/bin/perl
@fh = ();

open $nfh, '>test-0';
push @fh, select $nfh;

print "test\n";

$nfh = undef;
open $nfh, '>test-1';
push @fh, select $nfh;

print "test\n";

close select pop @fh;
print "test\n";
close select pop @fh;

print "test\n";

The result of executing this script

$./test.pl 
test
$cat test-*
test
test
test

intel VT: how to enable

My Intel(R) Core(TM)2 Duo CPU supports intel VT but it's disabled by default in BIOS.

Currently I'm hardly testing libdodo in different environments. As I'm running IA32 kernel I was testing it in x86 environments. Lately I decided to test it in x86_64 with VMWare. I went to BIOS, turned on VT support, rebooted ... and VMWare player told me that my platform supports VT but it's disabled. What the heck? I checked that the option is enabled in BIOS several times but still couldn't run x86_64 guest OS.

Googling didn't help. Running guest OS under qemu x86_64 simulation was the last option. It's extremely slow.

When I almost gave up I reached a very interesting article.
The author is telling that after the VT option was enabled in BIOS, the processor must be plugged off from the electricity circuit.
I turned off my laptop removed power supply and battery, waited for a few seconds, plugged the stuff back, booted and started VMWare with x86_64 guest. BINGO.

Now I'm able to run x86_64 guests on x86 host.

linux: zombies

New processes are being created with fork system call. After the fork is completed there are two processes available - parent and child.

Zombie is a process that was finished before the parent and the parent hadn't made any attempts to reap it.

When the child process was stopped or terminated the kernel still holds some minimal information about it in case the parent process will attempt to get it. This information is a task_struct where pid, exit code, resource usage information, etc. could be found. All other resources(memory, file descriptors, etc.) should be released. For details you can dig into the code of the do_exit function in kernel/exit.c in the linux kernel sources.

The parent should be notified about the child's death with SIGCHLD signal.

When the parent receives SIGCHLD it can get the information about the child by calling wait/waitpid/... system call. In the interval between the child terminating and the parent calling wait, the child is said to be a zombie. Even it's can't be in running or idle states it still takes place in the process table.
As soon as parent process receives SIGCHLD signal and it's ready to get information about the dead child this information is passed to it and all the information about the child is being removed from the kernel. In wait_task_zombie function(kernel/exit.c) you should find the details.

In fact the memory that is hold by a zombie is really small but it still in the process table and processed by the scheduler and also as the process table has a fixed number of entries it is possible for the system to run out of them.

When the parent terminates without waiting for the child zombie process is adopted by 'init' process which calls wait to clean up after it.

Let's look at the common zombie and its code.

#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
    if (fork() == 0)
    {   
        printf("%d\n", getpid());
        fflush(stdout);
        _exit(0);
    }   
    else
    {   
        printf("%d\n", getpid());
        fflush(stdout);
        while (1) 
            sleep(10);
    }   
    
    return 0;
}

The output should be something like

$./zombie 
4090
4091

Grepping ps's output I got

$ps aux | grep -E "(4090|4091)"
niam      4090  0.0  0.0   1496   340 pts/2    S+   12:24   0:00 ./zombie
niam      4091  0.0  0.0      0     0 pts/2    Z+   12:24   0:00 [zombie] 

You can see that child process became a zombie.

Knowing that zombies if are not a complete evil but are very close to it, there existence should be prevented.

There are some possibilities to do that.

First of all if you want to care why the child was finished you should call wait. This is an only way I know to do that.

I know two modes of wait: blocking and non-blocking. Both methods are listed below.

• Blocking method that will suspend parent until the SIGCHLD is received.

  #include <unistd.h>
  #include <stdio.h>
  #include <signal.h>
  
  int main(int argc, char **argv)
  {
      if (fork() == 0)
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          _exit(0);
      }   
      else
      {   
          int status;
          wait(&status);
          printf("%d\n", getpid());
          fflush(stdout);
          while (1) 
              sleep(10);
      }   
      
      return 0;
  }

  And the resulting output for this code was

  $./zombie 
  4949
  4950
  
  $ps aux | grep -E '(4949|4950)'
  niam      4949  0.0  0.0   1496   340 pts/2    S+   13:12   0:00 ./zombie

• Non-blocking which won't put the parent into the sleep state but requires multiply calls of waitpid.

  #include <unistd.h>
  #include <stdio.h>
  #include <signal.h>
  #include <sys/wait.h>
  
  int main(int argc, char **argv)
  {
      if (fork() == 0)
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          _exit(0);
      }   
      else
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          int status;
          while (1) 
          {
              waitpid(-1, &status, WNOHANG);
              sleep(10);
          }
      }   
      
      return 0;
  }

  The output was

  $./zombie 
  4932
  4931
  
  $ps aux | grep -E '(4931|4932)'
  niam      4931  0.0  0.0   1496   336 pts/2    S+   13:07   0:00 ./zombie

Another approach is to disregard child's exit status and detach it.

• Redefine SIGCHLD signal handler to specify SA_NOCLDSTOP flag for it.

  #include <unistd.h>
  #include <stdio.h>
  #include <signal.h>
  
  int main(int argc, char **argv)
  {
      struct sigaction sa;
      sigaction(SIGCHLD, NULL, &sa);
      sa.sa_flags |= SA_NOCLDWAIT;//(since POSIX.1-2001 and Linux 2.6 and later)
      sigaction(SIGCHLD, &sa, NULL);
  
      if (fork() == 0)
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          _exit(0);
      }   
      else
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          while (1) 
              sleep(10);
      }   
      
      return 0;
  }

  The output should be something like

  $./zombie 
  4416
  4417
  
  $ps aux | grep -E '(4416|4417)'
  niam      4416  0.0  0.0   1496   340 pts/2    S+   12:41   0:00 ./zombie

• Set SIGCHLD signal handler to SIG_IGN(ignore this signal).

  #include <unistd.h>
  #include <stdio.h>
  #include <signal.h>
  
  int main(int argc, char **argv)
  {
      struct sigaction sa;
      sigaction(SIGCHLD, NULL, &sa);
      sa.sa_handler = SIG_IGN;
      sigaction(SIGCHLD, &sa, NULL);
  
      if (fork() == 0)
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          _exit(0);
      }   
      else
      {   
          printf("%d\n", getpid());
          fflush(stdout);
          while (1) 
              sleep(10);
      }   
      
      return 0;
  }

  This code should produce the following output.

  $./zombie 
  4458
  4459
  
  $ps aux | grep -E '(4459|4458)'
  niam      4458  0.0  0.0   1496   340 pts/2    S+   12:45   0:00 ./zombie

  Note that POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN. POSIX.1-2001 allows this possibility, so that ignoring SIGCHLD can be used to prevent the creation of zombies

According to the linux-2.6.27 sources setting signal handler to SIG_IGN might give a small benefit in performance. Here is a piece of code from kernel/exit.c

static int ignoring_children(struct task_struct *parent)     
{                               
    int ret;                                                        
    struct sighand_struct *psig = parent->sighand;     
    unsigned long flags;        
    spin_lock_irqsave(&psig->siglock, flags);            
    ret = (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||     
           (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT));     
    spin_unlock_irqrestore(&psig->siglock, flags);     
    return ret;                
}

The kernel checks signal handler first.

There is other problem when the software wasn't developed by you but it produces zombies during the execution. There is a trick with gdb to kill process' zombies. You can attach to the parent process and manually call wait.

$./zombie 
4980
4981

$ps aux | grep -E '(4980|4981)'
niam      4980  0.0  0.0   1496   336 pts/2    S+   13:19   0:00 ./zombie
niam      4981  0.0  0.0      0     0 pts/2    Z+   13:19   0:00 [zombie] 

$gdb -p 4980
....
(gdb) call wait()
$1 = 4981

$ps aux | grep -E '(4980|4981)'
niam      4980  0.0  0.0   1496   336 pts/2    S+   13:19   0:00 ./zombie