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
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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!
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