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Computer Systems I Laboratory Exercise 5

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CSCI 247 Computer Systems I
Laboratory Exercise 5
Objectives
Practice reverse engineering an executable software
Submitting Your Work
Save your C program files as Exercise1.c, Exercise2.c and Exercise3.c and submit it via the Lab
Exercise 5 Submission item on the course web site. You must submit your program by 11:59pm
on the Friday following your scheduled lab session.
How to Reverse Engineer an Executable file
We start by writing and compiling a simple C program. We compile this program
with gcc and then run the program within the gdb debug environment. In gdb, we
use the disassemble command to convert the machine language of the program
back to assembly language. This will show us where the variables in the program are
stored and how the statements of the C program are implemented in assembly
language.
1. Save the following C program as sample.c
int main()
{
int x, y;
x = 10;
y = 15;
x = y-x;
y = x+y;
}
2. Compile the program:
gcc –g –o sample sample.c
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3. Run the program in gdb
gdb ./sample
4. At the (gdb) prompt, set a breakpoint at the start of the main() function of the program by using the b
command. This will make the program pause when it starts executing the main() function.
(gdb) b main
You will see the following output:
Breakpoint 1 at 0x4004da
5. A the (gdb) prompt, run the program by using the r command:
(gdb) r
You will see the following output:
Starting program: /home/gopalas/CSCI247/Lab5/Example
Breakpoint 1, 0x00000000004004da in main ()
(gdb)
6. Execution of the program has paused at the breakpoint. We can now disassemble the
program by entering the disassemble command at the (gdb) prompt:
(gdb) disassemble
You will see the following output (or something similar):
Dump of assembler code for function main:
0x00000000004004d6 <+0>: push %rbp
0x00000000004004d7 <+1>: mov %rsp,%rbp
=> 0x00000000004004da <+4>: movl $0xa,-0x8(%rbp)
0x00000000004004e1 <+11>: movl $0xf,-0x4(%rbp)
0x00000000004004e8 <+18>: mov -0x4(%rbp),%eax
0x00000000004004eb <+21>: sub -0x8(%rbp),%eax
0x00000000004004ee <+24>: mov %eax,-0x8(%rbp)
0x00000000004004f1 <+27>: mov -0x8(%rbp),%eax
0x00000000004004f4 <+30>: add %eax,-0x4(%rbp)
0x00000000004004f7 <+33>: mov $0x0,%eax
0x00000000004004fc <+38>: pop %rbp
0x00000000004004fd <+39>: retq
End of assembler dump.
SKG Active Minds Changing Lives Page 3 of 3
(gdb)
Note that %rbp (the base pointer) is set to the top of the stack. The local variables x and y are
stored on the stack at positions beyond the base pointer. Since x and y are both int, each
requires 4 bytes so their values are stored at addresses -0x8(%rbp) and -0x4(%rbp), respectively.
Here’s the assembly instructions again, with annotations:
push %rbp # save the old value of %rbp
mov %rsp,%rbp # set %rbp to the value of %rsp
movl $0xa,-0x8(%rbp) # x = 10
movl $0xf,-0x4(%rbp) # y = 15
mov -0x4(%rbp),%eax # load y into %eax
sub -0x8(%rbp),%eax # y – x is now in %eax
mov %eax,-0x8(%rbp) # x = y – x
mov -0x8(%rbp),%eax # load x into %eax
add %eax,-0x4(%rbp) # y = x + y
mov $0x0,%eax
pop %rbp # restore the old value of %rbp
retq # return from main()
Now see if you can reverse engineer the executable programs provided on the course web site:
Example1, Example2 and Example3.

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