+ +

Bomb Lab

+ +

Introduction

+ +

Lab 2 for CSCI 2400 - Computer Systems.

+ +

Phase 1

+ +

jovyan@jupyter-nach6988:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex 'break phase_1' -ex 'break explode_bomb' -ex 'run' ./bomb 
+GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1
+Copyright (C) 2022 Free Software Foundation, Inc.
+License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
+This is free software: you are free to change and redistribute it.
+There is NO WARRANTY, to the extent permitted by law.
+Type "show copying" and "show warranty" for details.
+This GDB was configured as "x86_64-linux-gnu".
+Type "show configuration" for configuration details.
+For bug reporting instructions, please see:
+<https://www.gnu.org/software/gdb/bugs/>.
+Find the GDB manual and other documentation resources online at:
+    <http://www.gnu.org/software/gdb/documentation/>.
+
+For help, type "help".
+Type "apropos word" to search for commands related to "word"...
+Reading symbols from ./bomb...
+Breakpoint 1 at 0x15c7
+Breakpoint 2 at 0x1d4a
+Starting program: /home/jovyan/lab2-bomblab-navanchauhan/bombbomb/bomb 
+[Thread debugging using libthread_db enabled]
+Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
+Welcome to my fiendish little bomb. You have 6 phases with
+which to blow yourself up. Have a nice day!
+test string
+
+Breakpoint 1, 0x00005555555555c7 in phase_1 ()
+(gdb) dias phase_1
+Undefined command: "dias".  Try "help".
+(gdb) disas phase_1
+Dump of assembler code for function phase_1:
+=> 0x00005555555555c7 <+0>:     endbr64 
+   0x00005555555555cb <+4>:     sub    $0x8,%rsp
+   0x00005555555555cf <+8>:     lea    0x1b7a(%rip),%rsi        # 0x555555557150
+   0x00005555555555d6 <+15>:    call   0x555555555b31 <strings_not_equal>
+   0x00005555555555db <+20>:    test   %eax,%eax
+   0x00005555555555dd <+22>:    jne    0x5555555555e4 <phase_1+29>
+   0x00005555555555df <+24>:    add    $0x8,%rsp
+   0x00005555555555e3 <+28>:    ret    
+   0x00005555555555e4 <+29>:    call   0x555555555d4a <explode_bomb>
+   0x00005555555555e9 <+34>:    jmp    0x5555555555df <phase_1+24>
+End of assembler dump.
+(gdb) print 0x555555557150
+$1 = 93824992244048
+(gdb) x/1s 0x555555557150
+0x555555557150: "Controlling complexity is the essence of computer programming."
+(gdb) 
+

+ +

Phase 2

+ +

Phase 1 defused. How about the next one?
+1 2 3 4 5 6
+
+Breakpoint 1, 0x00005555555555eb in phase_2 ()
+(gdb) disas
+Dump of assembler code for function phase_2:
+=> 0x00005555555555eb <+0>:     endbr64 
+   0x00005555555555ef <+4>:     push   %rbp
+   0x00005555555555f0 <+5>:     push   %rbx
+   0x00005555555555f1 <+6>:     sub    $0x28,%rsp
+   0x00005555555555f5 <+10>:    mov    %rsp,%rsi
+   0x00005555555555f8 <+13>:    call   0x555555555d97 <read_six_numbers>
+   0x00005555555555fd <+18>:    cmpl   $0x0,(%rsp)
+   0x0000555555555601 <+22>:    js     0x55555555560d <phase_2+34>
+   0x0000555555555603 <+24>:    mov    %rsp,%rbp
+   0x0000555555555606 <+27>:    mov    $0x1,%ebx
+   0x000055555555560b <+32>:    jmp    0x555555555620 <phase_2+53>
+   0x000055555555560d <+34>:    call   0x555555555d4a <explode_bomb>
+   0x0000555555555612 <+39>:    jmp    0x555555555603 <phase_2+24>
+   0x0000555555555614 <+41>:    add    $0x1,%ebx
+   0x0000555555555617 <+44>:    add    $0x4,%rbp
+   0x000055555555561b <+48>:    cmp    $0x6,%ebx
+   0x000055555555561e <+51>:    je     0x555555555631 <phase_2+70>
+   0x0000555555555620 <+53>:    mov    %ebx,%eax
+   0x0000555555555622 <+55>:    add    0x0(%rbp),%eax
+   0x0000555555555625 <+58>:    cmp    %eax,0x4(%rbp)
+   0x0000555555555628 <+61>:    je     0x555555555614 <phase_2+41>
+   0x000055555555562a <+63>:    call   0x555555555d4a <explode_bomb>
+   0x000055555555562f <+68>:    jmp    0x555555555614 <phase_2+41>
+   0x0000555555555631 <+70>:    add    $0x28,%rsp
+   0x0000555555555635 <+74>:    pop    %rbx
+   0x0000555555555636 <+75>:    pop    %rbp
+   0x0000555555555637 <+76>:    ret    
+End of assembler dump.
+(gdb) 
+

+ +

   0x00005555555555fd <+18>:    cmpl   $0x0,(%rsp)
+   0x0000555555555601 <+22>:    js     0x55555555560d <phase_2+34>
+...
+   0x000055555555560d <+34>:    call   0x555555555d4a <explode_bomb>
+

+ +

The program first compares if the first number is not 0. If the number is not 0, then the cmpl instruction returns a negative value. The js instruction stands for jump if sign -> causing a jump to the specified address if the sign bit is set. This would result in the explode_bomb function being called.

+ +

   0x0000555555555603 <+24>:    mov    %rsp,%rbp
+   0x0000555555555606 <+27>:    mov    $0x1,%ebx
+

+ +

%rsp in x86-64 asm, is the stack pointer i.e. it points to the top of the current stack frame. Since the program just read six numbers, the top of the stack (%rsp) contains the address of the first number.

+ +

By executing mov %rsp,%rbp we are setting the base pointer (%rbp) to point to this address.

+ +

Now, for the second instruction mov $0x1,%ebx, we are initalising the %ebx register with the value 1. Based on the assembly code, you can see that this is being used as a counter/index for the loop.

+ +

   0x000055555555560b <+32>:    jmp    0x555555555620 <phase_2+53>
+

+ +

The program now jumps to

+ +

   0x0000555555555620 <+53>:    mov    %ebx,%eax
+   0x0000555555555622 <+55>:    add    0x0(%rbp),%eax
+   0x0000555555555625 <+58>:    cmp    %eax,0x4(%rbp)
+   0x0000555555555628 <+61>:    je     0x555555555614 <phase_2+41>
+

+ +

Here, the value from %ebx is copied to the %eax register. For this iteration, the value should be 1.

+ +

Then, the value at the memory location pointed by %rbp is added to the value in %eax. For now, 0 is added (the first number that we read).

+ +

cmp %eax,0x4(%rbp) - The instruction compares the value in %eax to the value at the memory address %rbp + 4. Since Integers in this context are stored using a word of memory of 4 bytes, this indicates it checks against the second number in the sequence.

+ +

je 0x555555555614 <phase_2+41> - The program will jump to phase_2+41 if the previous cmp instruction determined the values as equal.

+ +

   0x0000555555555614 <+41>:    add    $0x1,%ebx
+   0x0000555555555617 <+44>:    add    $0x4,%rbp
+   0x000055555555561b <+48>:    cmp    $0x6,%ebx
+   0x000055555555561e <+51>:    je     0x555555555631 <phase_2+70>
+   0x0000555555555620 <+53>:    mov    %ebx,%eax
+   0x0000555555555622 <+55>:    add    0x0(%rbp),%eax
+   0x0000555555555625 <+58>:    cmp    %eax,0x4(%rbp)
+   0x0000555555555628 <+61>:    je     0x555555555614 <phase_2+41>
+

+ +

Here, we can see that the program increments %ebx by 1, adds a 4 byte offset to %rbp (the number we will be matching now), and checks if %ebx is equal to 6. If it is, it breaks the loop and jumps to <phase_2+70> succesfully finishing this stage.

+ +

Now, given that we know the first two numbers in the sequence are 0 1, we can calculate the other numbers by following the pattern of adding the counter and the value of the previous number.

+ +

Thus,

+ +

3rd number = 1 (previous value) + 2 = 3
4th number = 3 (prev value) + 3 = 6
5th number = 6 (prev value) + 4 = 10
6th number = 10 (prev value) + 5 = 15

+ +

...
+Phase 1 defused. How about the next one?
+0 1 3 6 10 15
+
+Breakpoint 1, 0x00005555555555eb in phase_2 ()
+(gdb) continue
+Continuing.
+That's number 2.  Keep going!
+

+ +

If you have scrolled this far, consider subscribing to my mailing list here. You can subscribe to either a specific type of post you are interested in, or subscribe to everything with the "Everything" list.

+ +