ipp ipp - 1 month ago 126
C Question

Bomb lab phase 4 func4

I'm having trouble with the password for phase_4 of my binary bomb.

So far, I understand...
the inputs must be 2 integers (line 29)

and the second integer should <= than or equal to 2 when 2 is subtracted from it (lines 38-44). Which means it should be a number 2-4

Then, it calls func4, and compares the output of func4 to rsp (which I've done some testing and rsp is always 0) so i need to find a number that returns 0 when inputed into func4.

Whats confusing me is: if the result of func4 needs to be 0, that means it's input should be 0. Taking into account line 38 in phase_4 where it subtracts 2 from my input, that should mean my second # should be 2...? However I have tried that and it makes it explode in line 46 as it doesn't satisfy the first comparison.

I have tried numbers like (0 2) (2 0) (0 1) (0 3) and they all failed. Any suggestions to point me in the right direction? Thanks!

phase_4:
0x000000000040101e <+0>: sub $0x18,%rsp
0x0000000000401022 <+4>: lea 0xc(%rsp),%rcx
0x0000000000401027 <+9>: lea 0x8(%rsp),%rdx
0x000000000040102c <+14>: mov $0x4027cd,%esi
0x0000000000401031 <+19>: mov $0x0,%eax
0x0000000000401036 <+24>: callq 0x400c30 <__isoc99_sscanf@plt>
0x000000000040103b <+29>: cmp $0x2,%eax //check if 2 inputs
0x000000000040103e <+32>: jne 0x40104c <phase_4+46>
0x0000000000401040 <+34>: mov 0xc(%rsp),%eax
=> 0x0000000000401044 <+38>: sub $0x2,%eax
0x0000000000401047 <+41>: cmp $0x2,%eax
0x000000000040104a <+44>: jbe 0x401051 <phase_4+51>//if unsigned eax <= 2
0x000000000040104c <+46>: callq 0x401554 <explode_bomb>
0x0000000000401051 <+51>: mov 0xc(%rsp),%esi
0x0000000000401055 <+55>: mov $0x7,%edi
0x000000000040105a <+60>: callq 0x400fe6 <func4>
0x000000000040105f <+65>: cmp 0x8(%rsp),%eax //comparing eax to 0
0x0000000000401063 <+69>: je 0x40106a <phase_4+76>
0x0000000000401065 <+71>: callq 0x401554 <explode_bomb> //explode if output != 0
0x000000000040106a <+76>: add $0x18,%rsp
0x000000000040106e <+80>: retq





Func4
0x0000000000400fe6 <+0>: push %r12
0x0000000000400fe8 <+2>: push %rbp
0x0000000000400fe9 <+3>: push %rbx
0x0000000000400fea <+4>: mov %edi,%ebx
0x0000000000400fec <+6>: test %edi,%edi
0x0000000000400fee <+8>: jle 0x401014 <func4+46> //if input <= 0
0x0000000000400ff0 <+10>: mov %esi,%ebp
0x0000000000400ff2 <+12>: mov %esi,%eax
0x0000000000400ff4 <+14>: cmp $0x1,%edi
0x0000000000400ff7 <+17>: je 0x401019 <func4+51>
0x0000000000400ff9 <+19>: lea -0x1(%rdi),%edi
0x0000000000400ffc <+22>: callq 0x400fe6 <func4>
0x0000000000401001 <+27>: lea (%rax,%rbp,1),%r12d
0x0000000000401005 <+31>: lea -0x2(%rbx),%edi
0x0000000000401008 <+34>: mov %ebp,%esi
0x000000000040100a <+36>: callq 0x400fe6 <func4>
0x000000000040100f <+41>: add %r12d,%eax
0x0000000000401012 <+44>: jmp 0x401019 <func4+51>
0x0000000000401014 <+46>: mov $0x0,%eax //make return val 0
0x0000000000401019 <+51>: pop %rbx
0x000000000040101a <+52>: pop %rbp
0x000000000040101b <+53>: pop %r12
0x000000000040101d <+55>: retq

Answer

Then, it calls func4, and compares the output of func4 to rsp (which I've done some testing and rsp is always 0) so i need to find a number that returns 0 when inputed into func4.

This is incorrect. The output of func4 is compared with [rsp + 8], in which the first number was stored.

If we write the desired input as (a, b), then we have a = func4 (7, b) and 2 <= b <= 4.

Now you need to figure out what func4 (x, y) does. A hint: It depends on y and on the value of the Fibonacci series at an index that depends on x.

To understand func4 I recommend that you convert it to C. See my answer to this question for an illustration. Note that you would have understood phase_4 correctly if you had converted it to C as well.