        DSR -	DTORS SECURITY RESEARCH


        Author: mercy
        Title: Reverse engineering - functions... functions! functions!? sheeeeeeeeeesh.
        Date: 23/04/2003

This paper is the second part to my reconstruction series. 
You will essentially need a basic understanding of ASM, C, and be able to navigate gdb. 
I recommend if you have not done anything along these lines before, you read my first article 
titled "Reconstructing Binaries to C for Beginners". 
This is a beginners article and no advanced techniques will be shown, so if you need something 
more advanced, branch out by yourself.

I will present the article in the following form:

  Briefing
  Disassembly dump.
  Run through of each line of the assembly.
  Main points in which to note.
  Reconstruction of the binary in C code.

This article presents three new features:
    Calculus.
    Strings.
    Functions.


Briefing:
In my first article, I did not explain the stack layout.
This is obviously important in the reconstruction of code, and should
indeed be talked about.
Note: The first article has been updated with this information in the beginning
for those of you that are interested.

To start with, we will talk about the allocation of local variables.
In our C programs we always use variables, any program does, and for this point
we must take carefull note as to the space allocated, and the data type
it may have been.

push %ebp
mov %esp,%ebp
sub $0x18,%esp

These three steps are known as the procedure prolog, and are essential in
the operations of your program.

push %ebp 
This is pushing your base pointer onto the stack, this is so we can 
return into the function that called our function.

mov %esp,%ebp
This is moving your current stack pointer to become your base pointer for
your program.

sub $0x18,%esp
This is actually increasing your stack pointer by 24 bytes. (Stack is growing down)
This is probably one of the most important things to note, because you will 
later see your local variables being accessed via a command such as:

movl   $0x0,0xfffffffc(%ebp)

It is important to note, the difference between:
mov
movb
movl
movw

These minor operations can effect which type of variable you will be using.
fffffffc(%ebp) is a negative 4 at %ebp.
If we think about it:
%ebp = %esp
%esp + 24
The stack grows down, so negative 4 ebp will be hitting the memory allocated
for a local variable, and it is a movl, long indicating an integer.
(An int is the same as type long on most systems)

Exit Status:
You will be able to determine programs return type by the value held in %eax.
A statement in C such as:
return(0);
Would result in the assembly equivelant to:
xor %eax,%eax
ret
(Our return value is obviously held in %eax)

Procedure epilog:
mov    %ebp,%esp
pop    %ebp
ret 

This is obviously moving our contents of ebp into esp. (The original %esp)
pop %ebp Is popping the pushed base pointer from the stack. (The original %ebp)
ret is obviously returning control.

   

Part one: Disassembly dump

mercy@hewey:~/examples/gdb > gcc 2.c -o 2 -ggdb3 -g
mercy@hewey:~/examples/gdb > gdb 2 -q
(gdb) disass main
Dump of assembler code for function main:
0x8048450 <main>:       push   %ebp
0x8048451 <main+1>:     mov    %esp,%ebp
0x8048453 <main+3>:     sub    $0x18,%esp
0x8048456 <main+6>:     movl   $0x0,0xfffffffc(%ebp)
0x804845d <main+13>:    movl   $0x0,0xfffffff8(%ebp)
0x8048464 <main+20>:    add    $0xfffffff4,%esp
0x8048467 <main+23>:    push   $0x8048594
0x804846c <main+28>:    call   0x8048334 <printf>
0x8048471 <main+33>:    add    $0x10,%esp
0x8048474 <main+36>:    add    $0xfffffff8,%esp
0x8048477 <main+39>:    lea    0xfffffffc(%ebp),%eax
0x804847a <main+42>:    push   %eax
0x804847b <main+43>:    push   $0x80485a6
0x8048480 <main+48>:    call   0x8048304 <scanf>
0x8048485 <main+53>:    add    $0x10,%esp
0x8048488 <main+56>:    add    $0xfffffff4,%esp
0x804848b <main+59>:    push   $0x80485a9
0x8048490 <main+64>:    call   0x8048334 <printf>
0x8048495 <main+69>:    add    $0x10,%esp
0x8048498 <main+72>:    add    $0xfffffff8,%esp
0x804849b <main+75>:    lea    0xfffffff8(%ebp),%eax
0x804849e <main+78>:    push   %eax
0x804849f <main+79>:    push   $0x80485a6
0x80484a4 <main+84>:    call   0x8048304 <scanf>
0x80484a9 <main+89>:    add    $0x10,%esp
0x80484ac <main+92>:    add    $0xfffffff8,%esp
0x80484af <main+95>:    mov    0xfffffff8(%ebp),%eax
0x80484b2 <main+98>:    push   %eax
0x80484b3 <main+99>:    mov    0xfffffffc(%ebp),%eax
0x80484b6 <main+102>:   push   %eax
0x80484b7 <main+103>:   call   0x80484d0 <calc>
0x80484bc <main+108>:   add    $0x10,%esp
0x80484bf <main+111>:   xor    %eax,%eax
0x80484c1 <main+113>:   jmp    0x80484c3 <main+115>
0x80484c3 <main+115>:   mov    %ebp,%esp
0x80484c5 <main+117>:   pop    %ebp
0x80484c6 <main+118>:   ret    

(gdb) disass calc
Dump of assembler code for function calc:
0x80484d0 <calc>:       push   %ebp
0x80484d1 <calc+1>:     mov    %esp,%ebp
0x80484d3 <calc+3>:     sub    $0x18,%esp
0x80484d6 <calc+6>:     mov    0x8(%ebp),%eax
0x80484d9 <calc+9>:     mov    0xc(%ebp),%edx
0x80484dc <calc+12>:    lea    (%edx,%eax,1),%ecx
0x80484df <calc+15>:    mov    %ecx,0xfffffffc(%ebp)
0x80484e2 <calc+18>:    add    $0xfffffff8,%esp
0x80484e5 <calc+21>:    push   $0x80485bb
0x80484ea <calc+26>:    mov    0xfffffffc(%ebp),%eax
0x80484ed <calc+29>:    push   %eax
0x80484ee <calc+30>:    call   0x8048500 <print>
0x80484f3 <calc+35>:    add    $0x10,%esp
0x80484f6 <calc+38>:    mov    %ebp,%esp
0x80484f8 <calc+40>:    pop    %ebp
0x80484f9 <calc+41>:    ret    

(gdb) disass print
Dump of assembler code for function print:
0x8048500 <print>:      push   %ebp
0x8048501 <print+1>:    mov    %esp,%ebp
0x8048503 <print+3>:    sub    $0x8,%esp
0x8048506 <print+6>:    add    $0xfffffffc,%esp
0x8048509 <print+9>:    mov    0xc(%ebp),%eax
0x804850c <print+12>:   push   %eax
0x804850d <print+13>:   mov    0x8(%ebp),%eax
0x8048510 <print+16>:   push   %eax
0x8048511 <print+17>:   push   $0x80485c9
0x8048516 <print+22>:   call   0x8048334 <printf>
0x804851b <print+27>:   add    $0x10,%esp
0x804851e <print+30>:   mov    %ebp,%esp
0x8048520 <print+32>:   pop    %ebp
0x8048521 <print+33>:   ret    



Part Two:  run through of each line of the assembly.

Dump of assembler code for function main:
0x8048450 <main>:       push   %ebp
Push the ebp (extended base pointer) onto the stack.
0x8048451 <main+1>:     mov    %esp,%ebp
Move the current esp into the old ebp register, in turn setting up a new bp.
0x8048453 <main+3>:     sub    $0x18,%esp
Set aside 24 bytes for local variables.
(Those three steps are known as setting up the procedure prolog.)
0x8048456 <main+6>:     movl   $0x0,0xfffffffc(%ebp)
put 0 at -4 ebp, or four bytes from the ebp. (long)
0x804845d <main+13>:    movl   $0x0,0xfffffff8(%ebp)
put 0 at -8 ebp, or eight bytes from the ebp. (long)
0x8048464 <main+20>:    add    $0xfffffff4,%esp
subtract 12 from esp 
0x8048467 <main+23>:    push   $0x8048594
(gdb) x/s 0x8048594
0x8048594 <_IO_stdin_used+4>:    "Enter value one: "
so the command push   $0x8048594 equates to pushing the string "Enter value one: "
0x804846c <main+28>:    call   0x8048334 <printf>
call the function printf with the value pushed on the stack.
0x8048471 <main+33>:    add    $0x10,%esp
change the stack pointer
0x8048474 <main+36>:    add    $0xfffffff8,%esp
subtract 8 from esp (stack setup)
0x8048477 <main+39>:    lea    0xfffffffc(%ebp),%eax
load what is held at -4 byte from ebp, into eax register.
0x804847a <main+42>:    push   %eax
push eax onto the stack
0x804847b <main+43>:    push   $0x80485a6
(gdb) x/s 0x80485a6
0x80485a6 <_IO_stdin_used+22>:   "%d"
so push %d onto the stack for the function scanf to use.
0x8048480 <main+48>:    call   0x8048304 <scanf>
call the function scanf.
0x8048485 <main+53>:    add    $0x10,%esp
Add 16 to sp
0x8048488 <main+56>:    add    $0xfffffff4,%esp
subtract -12 from esp
0x804848b <main+59>:    push   $0x80485a9
(gdb) x/s 0x8048479
0x8048479 <_IO_stdin_used+25>:   "Enter value two: "
So push "Enter value two: " onto the stack.
0x8048490 <main+64>:    call   0x8048334 <printf>
Call printf
0x8048495 <main+69>:    add    $0x10,%esp
change stack pointer.
0x8048498 <main+72>:    add    $0xfffffff8,%esp
Subtract 8 from esp.
0x804849b <main+75>:    lea    0xfffffff8(%ebp),%eax
load the address ebp - 8 into eax
0x804849e <main+78>:    push   %eax
push eax onto the stack
0x804849f <main+79>:    push   $0x80485a6
0x80485a6 <_IO_stdin_used+22>:   "%d"
so push %d onto the stack for the function scanf to use.
0x80484a4 <main+84>:    call   0x8048304 <scanf>
Call scanf
0x80484a9 <main+89>:    add    $0x10,%esp
change stack ptr.
0x80484ac <main+92>:    add    $0xfffffff8,%esp
subtract 8 from sp
0x80484af <main+95>:    mov    0xfffffff8(%ebp),%eax
move -8 ebp into eax
0x80484b2 <main+98>:    push   %eax
push onto stack
0x80484b3 <main+99>:    mov    0xfffffffc(%ebp),%eax
move -4 ebp, to eax
0x80484b6 <main+102>:   push   %eax
push eax onto the stack
0x80484b7 <main+103>:   call   0x80484d0 <calc>
call function calc
0x80484bc <main+108>:   add    $0x10,%esp
change stack ptr.
0x80484bf <main+111>:   xor    %eax,%eax
xor eax with eax in turn making eax == 0 (exit status)
0x80484c1 <main+113>:   jmp    0x80484c3 <main+115>
go to 0x80484c3 (next line)
0x80484c3 <main+115>:   mov    %ebp,%esp
move base pointer into stackpointer
0x80484c5 <main+117>:   pop    %ebp
pop the original bp from the stack
0x80484c6 <main+118>:   ret    
Return
Upon procedure exit, the stack must be cleaned up again, 
something called the procedure epilog. That is the mov pop ret statements.
End of assembler dump.

Dump of assembler code for function calc:
0x80484d0 <calc>:       push   %ebp
push base pointer onto the stack
0x80484d1 <calc+1>:     mov    %esp,%ebp
move new stack pointer into bp.
0x80484d3 <calc+3>:     sub    $0x18,%esp
Set aside 24 bytes 
0x80484d6 <calc+6>:     mov    0x8(%ebp),%eax
mov whats at 8 bytes from ebp, into eax
0x80484d9 <calc+9>:     mov    0xc(%ebp),%edx
move whats at 4 bytes from ebp, into edx
0x80484dc <calc+12>:    lea    (%edx,%eax,1),%ecx
storing the contents of the address edx+eax*1 in ecx
0x80484df <calc+15>:    mov    %ecx,0xfffffffc(%ebp)
mov ecx into -4 ebp
0x80484e2 <calc+18>:    add    $0xfffffff8,%esp
subtract -8 bytes from sp
0x80484e5 <calc+21>:    push   $0x80485bb
(gdb) x/s 0x80485bb
0x80485bb <_IO_stdin_used+43>:   "correct value"
Push string "correct value" onto the stack.
0x80484ea <calc+26>:    mov    0xfffffffc(%ebp),%eax
move whats -4 bytes from ebp into eax.
0x80484ed <calc+29>:    push   %eax
push eax onto the stack
0x80484ee <calc+30>:    call   0x8048500 <print>
call function print
0x80484f3 <calc+35>:    add    $0x10,%esp
change sp
0x80484f6 <calc+38>:    mov    %ebp,%esp
move bp into sp
0x80484f8 <calc+40>:    pop    %ebp
pop the original bp
0x80484f9 <calc+41>:    ret    
return

Dump of assembler code for function print:
0x8048500 <print>:      push   %ebp
push the old bp onto the stack
0x8048501 <print+1>:    mov    %esp,%ebp
create a new bp
0x8048503 <print+3>:    sub    $0x8,%esp
Set aside 8 bytes
0x8048506 <print+6>:    add    $0xfffffffc,%esp
subtract -4 bytes from sp
0x8048509 <print+9>:    mov    0xc(%ebp),%eax
move what is 12 bytes from bp into the eax register
0x804850c <print+12>:   push   %eax
push eax onto the stack
0x804850d <print+13>:   mov    0x8(%ebp),%eax
mov what is 8 bytes from ebp into the eax register
0x8048510 <print+16>:   push   %eax
Push %eax onto the stack.
0x8048511 <print+17>:   push   $0x80485c9
(gdb) x/s $0x80485c9
$0x80485c9 <_IO_stdin_used+57>:   "Number %d, %s\n"
push that string onto the stack
0x8048516 <print+22>:   call   0x8048334 <printf>
call function printf
0x804851b <print+27>:   add    $0x10,%esp
change sp
0x804851e <print+30>:   mov    %ebp,%esp
mov the bp into the sp
0x8048520 <print+32>:   pop    %ebp
pop the old bp
0x8048521 <print+33>:   ret    
return



Part Three: Main points to note.

If your eyes are a lil sore, thats ok. I'm the one writing this, so I can tell you now mine 
have got to be 100x more painfull. 

This is the part in which we will depict each function's main lines and work out an order 
in which we can put together an accurate C program. Take careful note to compare what is 
presented with the assembly dump.

MAIN:
put 0 at -4 and -8 ebp (long), this is obviously putting 0 at two long variables.
push "Enter value one: " onto the stack, is putting a string on to print out.
call printf
load what is stored in -4 ebp into eax and push onto stack, mov %d into eax and push
call scanf (scanf("%d", -4(ebp));)
push "Enter value two: " onto the stack.
call printf
load what is stored in -8 ebp into eax onto the stack, mov %d into eax and push.
call scanf (scanf("%d", -8(ebp));)
mov -8 ebp into eax and push, mov -4 ebp into eax and push. 
(order is important for exact reconstruction)
call calc
xor eax with itself making it 0, this is the return status.
pop the original bp and return.

CALC:
mov what is at 8 ebp into eax, mov what is at 4 ebp into edx, calculate and store in ecx.
mov ecx into -4 ebp (local var)
push "correct value" onto the stack.
mov -4 ebp into eax and push onto the stack.
call print
move base pointer into esp and pop the original, return.

PRINT:
Set aside 8 bytes.
mov -4 and -8 bytes from ebp into eax and push
push "Number %d, %s\n" onto the stack.
call printf
mov ebp into esp and pop original ebp.
return.

With all these important factors to note, you are prolly hoping I dont go into writing a lil
more about each line.. or IS that what you're hoping? I just want you to recognise that in dumps
it is necessary to reconstruct the binary in the exact order it is shown in the ASM.
I will now move onto writing the C code to complement this dump, and it will have the ASM 
equivalent next to it.



Part Four: Reconstruction of the binary in C code.

-----------------------SNIP------------------------------

#include <stdio.h>

void calc(int a, int b) // we know this function is void because it has no return status.
{ 			// we also know it accepts two integer arguments.
   int c; // we know it stores the result of a sum into a local var (ecx register)

/* 0x80484d6 <calc+6>:     mov    0x8(%ebp),%eax
 * 0x80484d9 <calc+9>:     mov    0xc(%ebp),%edx 
 * This is moving a into eax, b into edx 
*/

   c = a + b; // we know the calculation is in order of:lea    (%edx,%eax,1),%ecx
              // ecx = eax + edx * 1 

/* 0x80484e5 <calc+21>:    push   $0x80485bb  # "correct value"
 * 0x80484ea <calc+26>:    mov    0xfffffffc(%ebp),%eax
 * 0x80484ed <calc+29>:    push   %eax
 * 0x80484ee <calc+30>:    call   0x8048500 <print>
*/
   print(c, "correct value");
}

void print(int n, char *str)  // this accepts two arguments, a string and an int.
{			// no return status.
/*
 * 0x8048509 <print+9>:    mov    0xc(%ebp),%eax
 * 0x804850c <print+12>:   push   %eax
 * 0x804850d <print+13>:   mov    0x8(%ebp),%eax
 * 0x8048510 <print+16>:   push   %eax
 * 0x8048511 <print+17>:   push   $0x80485c9 # Number %d, %s\n
 * 0x8048516 <print+22>:   call   0x8048334 <printf>
*/
   printf("Number %d, %s\n", n, str);
}

int main(void)  // we know this functions returns an int.
{
   int num1 = 0, num2 = 0; // we know it has two ints initialized to 0.
   printf("Enter value one: "); // pushes this onto the stack and calls printf.
/* 0x8048477 <main+39>:    lea    0xfffffffc(%ebp),%eax
 * 0x804847a <main+42>:    push   %eax
 * 0x804847b <main+43>:    push   $0x80485a6
 * 0x8048480 <main+48>:    call   0x8048304 <scanf>
*/
   scanf("%d", &num1);
   printf("Enter vaulue two: "); // pushes this onto the stack and calls printf.
/* 0x804849b <main+75>:    lea    0xfffffff8(%ebp),%eax
 * 0x804849e <main+78>:    push   %eax
 * 0x804849f <main+79>:    push   $0x80485a6
 * 0x80484a4 <main+84>:    call   0x8048304 <scanf>
*/
   scanf("%d", &num2);

/* 0x80484af <main+95>:    mov    0xfffffff8(%ebp),%eax
 * 0x80484b2 <main+98>:    push   %eax
 * 0x80484b3 <main+99>:    mov    0xfffffffc(%ebp),%eax
 * 0x80484b6 <main+102>:   push   %eax
 * 0x80484b7 <main+103>:   call   0x80484d0 <calc>
*/
   calc(num1, num2);
   return(0); // xor    %eax,%eax
}
   
---------------------</SNIP>-----------------------------

If we have a look at the original program, we can see we have
constructed the code near perfection. 
Points to note in future reconstructions is ordering of how things
get pushed onto the stack with a call to a function etc. This sort of
small detail can influence a programs control.

#include <stdio.h>

void calc(int x, int y);
void print(int z, char *str);

int main(int argc, char **argv)
{
     int val1 = 0, val2 = 0;
     printf("Enter value one: ");
     scanf("%d", &val1);
     printf("Enter value two: ");
     scanf("%d", &val2);
     calc(val1, val2);
     return(0);
}

void calc(int x, int y)
{
      int a;
      a = x + y;
      print(a, "correct value");
}

void print(int x, char *str)
{
     printf("Number %d, %s\n", x, str);
}


Contact me at:
mercy@dtors.net
http://mercy.dtors.net



---------SNIP-----------------------------
------------------------------------------
/* Just something I use to find out hex representations of characters
 * quickly and efficiently.
*/
#include <stdio.h>

int main(int argc, char **argv)
{
   int i;
   if(argc <= 1){
     printf("Usage: %s <string>\n", argv[0]);
     exit(-1);}

   i = strlen(argv[1]);
   while(i > -1)
   {
    printf("%c: %x\n", argv[1][i], argv[1][i]);
    i--;
   }
   return 0;
}
------------------------------------------
------------------------------------------

/* Thanks AndrewG from PTP
 * Decimal representations etc. of hex numbers.
*/
#include <stdio.h>
int main(int argc, char **argv)
{
   unsigned long x;
   x = strtoul(argv[1], NULL, 0);
   printf("hex: %p\n", x);
   printf("dec: %d\n", x);
   printf("unsigned dec: %u\n", x);
   return 0;
}
------------------------------------------
</end>


