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<main>
<h1>Bomb Lab</h1>
<h2>Introduction</h2>
<p>Lab 2 for CSCI 2400 @ CU Boulder - Computer Systems</p>
<blockquote>
<p>The nefarious Dr. Evil has planted a slew of “binary bombs” on our class machines. A binary bomb is a program that consists of a sequence of phases. Each phase expects you to type a particular string on stdin. If you type the correct string, then the phase is defused and the bomb proceeds to the next phase. Otherwise, the bomb explodes by printing "BOOM!!!" and then terminating. The bomb is defused when every phase has been defused.</p>
</blockquote>
<blockquote>
<p>There are too many bombs for us to deal with, so we are giving each student a bomb to defuse. Your mission, which you have no choice but to accept, is to defuse your bomb before the due date. Good luck, and welcome to the bomb squad!</p>
</blockquote>
<p>I like using objdump to disassemble the code and get a broad overview of what is happening before I start. </p>
<p><code>objdump -d bomb > dis.txt</code></p>
<p><em>Note: I am not sure about the history of the bomb lab. I think it started at CMU.</em></p>
<h2>Phase 1</h2>
<div class="codehilite">
<pre><span></span><code>joxxxn@jupyter-nxxh6xx8:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex <span class="s1">'break phase_1'</span> -ex <span class="s1">'break explode_bomb'</span> -ex <span class="s1">'run'</span> ./bomb
GNU gdb <span class="o">(</span>Ubuntu <span class="m">12</span>.1-0ubuntu1~22.04<span class="o">)</span> <span class="m">12</span>.1
Copyright <span class="o">(</span>C<span class="o">)</span> <span class="m">2022</span> Free Software Foundation, Inc.
License GPLv3+: GNU GPL version <span class="m">3</span> 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 <span class="s2">"show copying"</span> and <span class="s2">"show warranty"</span> <span class="k">for</span> details.
This GDB was configured as <span class="s2">"x86_64-linux-gnu"</span>.
Type <span class="s2">"show configuration"</span> <span class="k">for</span> 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, <span class="nb">type</span> <span class="s2">"help"</span>.
Type <span class="s2">"apropos word"</span> to search <span class="k">for</span> commands related to <span class="s2">"word"</span>...
Reading symbols from ./bomb...
Breakpoint <span class="m">1</span> at 0x15c7
Breakpoint <span class="m">2</span> at 0x1d4a
Starting program: /home/joxxxn/lab2-bomblab-navanchauhan/bombbomb/bomb
<span class="o">[</span>Thread debugging using libthread_db enabled<span class="o">]</span>
Using host libthread_db library <span class="s2">"/lib/x86_64-linux-gnu/libthread_db.so.1"</span>.
Welcome to my fiendish little bomb. You have <span class="m">6</span> phases with
which to blow yourself up. Have a nice day!
<span class="nb">test</span> string
Breakpoint <span class="m">1</span>, 0x00005555555555c7 <span class="k">in</span> phase_1 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> dias phase_1
Undefined command: <span class="s2">"dias"</span>. Try <span class="s2">"help"</span>.
<span class="o">(</span>gdb<span class="o">)</span> disas phase_1
Dump of assembler code <span class="k">for</span> <span class="k">function</span> phase_1:
<span class="o">=</span>> 0x00005555555555c7 <+0>: endbr64
0x00005555555555cb <+4>: sub <span class="nv">$0</span>x8,%rsp
0x00005555555555cf <+8>: lea 0x1b7a<span class="o">(</span>%rip<span class="o">)</span>,%rsi <span class="c1"># 0x555555557150</span>
0x00005555555555d6 <+15>: call 0x555555555b31 <strings_not_equal>
0x00005555555555db <+20>: <span class="nb">test</span> %eax,%eax
0x00005555555555dd <+22>: jne 0x5555555555e4 <phase_1+29>
0x00005555555555df <+24>: add <span class="nv">$0</span>x8,%rsp
0x00005555555555e3 <+28>: ret
0x00005555555555e4 <+29>: call 0x555555555d4a <explode_bomb>
0x00005555555555e9 <+34>: jmp 0x5555555555df <phase_1+24>
End of assembler dump.
<span class="o">(</span>gdb<span class="o">)</span> print 0x555555557150
<span class="nv">$1</span> <span class="o">=</span> <span class="m">93824992244048</span>
<span class="o">(</span>gdb<span class="o">)</span> x/1s 0x555555557150
0x555555557150: <span class="s2">"Controlling complexity is the essence of computer programming."</span>
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<h2>Phase 2</h2>
<div class="codehilite">
<pre><span></span><code>Phase <span class="m">1</span> defused. How about the next one?
<span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> <span class="m">5</span> <span class="m">6</span>
Breakpoint <span class="m">1</span>, 0x00005555555555eb <span class="k">in</span> phase_2 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> disas
Dump of assembler code <span class="k">for</span> <span class="k">function</span> phase_2:
<span class="o">=</span>> 0x00005555555555eb <+0>: endbr64
0x00005555555555ef <+4>: push %rbp
0x00005555555555f0 <+5>: push %rbx
0x00005555555555f1 <+6>: sub <span class="nv">$0</span>x28,%rsp
0x00005555555555f5 <+10>: mov %rsp,%rsi
0x00005555555555f8 <+13>: call 0x555555555d97 <read_six_numbers>
0x00005555555555fd <+18>: cmpl <span class="nv">$0</span>x0,<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555601 <+22>: js 0x55555555560d <phase_2+34>
0x0000555555555603 <+24>: mov %rsp,%rbp
0x0000555555555606 <+27>: mov <span class="nv">$0</span>x1,%ebx
0x000055555555560b <+32>: jmp 0x555555555620 <phase_2+53>
0x000055555555560d <+34>: call 0x555555555d4a <explode_bomb>
0x0000555555555612 <+39>: jmp 0x555555555603 <phase_2+24>
0x0000555555555614 <+41>: add <span class="nv">$0</span>x1,%ebx
0x0000555555555617 <+44>: add <span class="nv">$0</span>x4,%rbp
0x000055555555561b <+48>: cmp <span class="nv">$0</span>x6,%ebx
0x000055555555561e <+51>: je 0x555555555631 <phase_2+70>
0x0000555555555620 <+53>: mov %ebx,%eax
0x0000555555555622 <+55>: add 0x0<span class="o">(</span>%rbp<span class="o">)</span>,%eax
0x0000555555555625 <+58>: cmp %eax,0x4<span class="o">(</span>%rbp<span class="o">)</span>
0x0000555555555628 <+61>: je 0x555555555614 <phase_2+41>
0x000055555555562a <+63>: call 0x555555555d4a <explode_bomb>
0x000055555555562f <+68>: jmp 0x555555555614 <phase_2+41>
0x0000555555555631 <+70>: add <span class="nv">$0</span>x28,%rsp
0x0000555555555635 <+74>: pop %rbx
0x0000555555555636 <+75>: pop %rbp
0x0000555555555637 <+76>: ret
End of assembler dump.
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<div class="codehilite">
<pre><span></span><code> 0x00005555555555fd <+18>: cmpl <span class="nv">$0</span>x0,<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555601 <+22>: js 0x55555555560d <phase_2+34>
...
0x000055555555560d <+34>: call 0x555555555d4a <explode_bomb>
</code></pre>
</div>
<p>The program first compares if the first number is not 0. If the number is not 0, then the <code>cmpl</code> instruction returns a negative value. The <code>js</code> 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.
<div class="codehilite">
<pre><span></span><code>0x0000555555555603 <+24>: mov %rsp,%rbp
0x0000555555555606 <+27>: mov <span class="nv">$0</span>x1,%ebx
</code></pre>
</div></p>
<p><code>%rsp</code> 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 (<code>%rsp</code>) contains the address of the first number.</p>
<p>By executing <code>mov %rsp,%rbp</code> we are setting the base pointer (<code>%rbp</code>) to point to this address.</p>
<p>Now, for the second instruction <code>mov $0x1,%ebx</code>, we are initalising the <code>%ebx</code> 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.
<div class="codehilite">
<pre><span></span><code>0x000055555555560b <+32>: jmp 0x555555555620 <phase_2+53>
</code></pre>
</div></p>
<p>The program now jumps to <phase_2+53>
<div class="codehilite">
<pre><span></span><code>0x0000555555555620 <+53>: mov %ebx,%eax
0x0000555555555622 <+55>: add 0x0<span class="o">(</span>%rbp<span class="o">)</span>,%eax
0x0000555555555625 <+58>: cmp %eax,0x4<span class="o">(</span>%rbp<span class="o">)</span>
0x0000555555555628 <+61>: je 0x555555555614 <phase_2+41>
</code></pre>
</div></p>
<p>Here, the value from <code>%ebx</code> is copied to the <code>%eax</code> register. For this iteration, the value should be 1.</p>
<p>Then, the value at the memory location pointed by <code>%rbp</code> is added to the value in <code>%eax</code>. For now, 0 is added (the first number that we read).</p>
<p><code>cmp %eax,0x4(%rbp)</code> - The instruction compares the value in %eax to the value at the memory address <code>%rbp + 4</code>. 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.</p>
<p><code>je 0x555555555614 <phase_2+41></code> - The program will jump to <code>phase_2+41</code> if the previous <code>cmp</code> instruction determined the values as equal.
<div class="codehilite">
<pre><span></span><code>0x0000555555555614 <+41>: add <span class="nv">$0</span>x1,%ebx
0x0000555555555617 <+44>: add <span class="nv">$0</span>x4,%rbp
0x000055555555561b <+48>: cmp <span class="nv">$0</span>x6,%ebx
0x000055555555561e <+51>: je 0x555555555631 <phase<em>2+70>
0x0000555555555620 <+53>: mov %ebx,%eax
0x0000555555555622 <+55>: add 0x0<span class="o">(</span>%rbp<span class="o">)</span>,%eax
0x0000555555555625 <+58>: cmp %eax,0x4<span class="o">(</span>%rbp<span class="o">)</span>
0x0000555555555628 <+61>: je 0x555555555614 <phase</em>2+41>
</code></pre>
</div></p>
<p>Here, we can see that the program increments <code>%ebx</code> by 1, adds a 4 byte offset to <code>%rbp</code> (the number we will be matching now), and checks if <code>%ebx</code> is equal to 6. If it is, it breaks the loop and jumps to <code><phase_2+70></code> succesfully finishing this stage.</p>
<p>Now, given that we know the first two numbers in the sequence are <code>0 1</code>, we can calculate the other numbers by following the pattern of adding the counter and the value of the previous number.</p>
<p>Thus,</p>
<ul>
<li>3rd number = 1 (previous value) + 2 = 3</li>
<li>4th number = 3 (prev value) + 3 = 6</li>
<li>5th number = 6 (prev value) + 4 = 10</li>
<li>6th number = 10 (prev value) + 5 = 15</li>
</ul>
<div class="codehilite">
<pre><span></span><code>...
Phase <span class="m">1</span> defused. How about the next one?
<span class="m">0</span> <span class="m">1</span> <span class="m">3</span> <span class="m">6</span> <span class="m">10</span> <span class="m">15</span>
Breakpoint <span class="m">1</span>, 0x00005555555555eb <span class="k">in</span> phase_2 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> <span class="k">continue</span>
Continuing.
That<span class="err">'</span>s number <span class="m">2</span>. Keep going!
</code></pre>
</div>
<h2>Phase 3</h2>
<p>Let us look at the disassembled code first</p>
<div class="codehilite">
<pre><span></span><code><span class="m">0000000000001638</span> <phase_3>:
<span class="m">1638</span>: f3 0f 1e fa endbr64
163c: <span class="m">48</span> <span class="m">83</span> ec <span class="m">18</span> sub <span class="nv">$0</span>x18,%rsp
<span class="m">1640</span>: <span class="m">48</span> 8d 4c <span class="m">24</span> <span class="m">07</span> lea 0x7<span class="o">(</span>%rsp<span class="o">)</span>,%rcx
<span class="m">1645</span>: <span class="m">48</span> 8d <span class="m">54</span> <span class="m">24</span> 0c lea 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%rdx
164a: 4c 8d <span class="m">44</span> <span class="m">24</span> <span class="m">08</span> lea 0x8<span class="o">(</span>%rsp<span class="o">)</span>,%r8
164f: <span class="m">48</span> 8d <span class="m">35</span> <span class="m">60</span> 1b <span class="m">00</span> <span class="m">00</span> lea 0x1b60<span class="o">(</span>%rip<span class="o">)</span>,%rsi <span class="c1"># 31b6 <_IO_stdin_used+0x1b6></span>
<span class="m">1656</span>: b8 <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x0,%eax
165b: e8 <span class="m">80</span> <span class="nb">fc</span> ff ff call 12e0 <__isoc99_sscanf@plt>
<span class="m">1660</span>: <span class="m">83</span> f8 <span class="m">02</span> cmp <span class="nv">$0</span>x2,%eax
<span class="m">1663</span>: 7e <span class="m">20</span> jle <span class="m">1685</span> <phase_3+0x4d>
<span class="m">1665</span>: <span class="m">83</span> 7c <span class="m">24</span> 0c <span class="m">07</span> cmpl <span class="nv">$0</span>x7,0xc<span class="o">(</span>%rsp<span class="o">)</span>
166a: 0f <span class="m">87</span> 0d <span class="m">01</span> <span class="m">00</span> <span class="m">00</span> ja 177d <phase_3+0x145>
<span class="m">1670</span>: 8b <span class="m">44</span> <span class="m">24</span> 0c mov 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%eax
<span class="m">1674</span>: <span class="m">48</span> 8d <span class="m">15</span> <span class="m">55</span> 1b <span class="m">00</span> <span class="m">00</span> lea 0x1b55<span class="o">(</span>%rip<span class="o">)</span>,%rdx <span class="c1"># 31d0 <_IO_stdin_used+0x1d0></span>
167b: <span class="m">48</span> <span class="m">63</span> <span class="m">04</span> <span class="m">82</span> movslq <span class="o">(</span>%rdx,%rax,4<span class="o">)</span>,%rax
167f: <span class="m">48</span> <span class="m">01</span> d0 add %rdx,%rax
<span class="m">1682</span>: 3e ff e0 notrack jmp *%rax
<span class="m">1685</span>: e8 c0 <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
168a: eb d9 jmp <span class="m">1665</span> <phase_3+0x2d>
168c: b8 <span class="m">63</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x63,%eax
<span class="m">1691</span>: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> 3d <span class="m">02</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x23d,0x8<span class="o">(</span>%rsp<span class="o">)</span>
<span class="m">1698</span>: <span class="m">00</span>
<span class="m">1699</span>: 0f <span class="m">84</span> e8 <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> je <span class="m">1787</span> <phase_3+0x14f>
169f: e8 a6 <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
16a4: b8 <span class="m">63</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x63,%eax
16a9: e9 d9 <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> jmp <span class="m">1787</span> <phase_3+0x14f>
16ae: b8 <span class="m">61</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x61,%eax
16b3: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> <span class="m">27</span> <span class="m">01</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x127,0x8<span class="o">(</span>%rsp<span class="o">)</span>
16ba: <span class="m">00</span>
16bb: 0f <span class="m">84</span> c6 <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> je <span class="m">1787</span> <phase_3+0x14f>
16c1: e8 <span class="m">84</span> <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
16c6: b8 <span class="m">61</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x61,%eax
16cb: e9 b7 <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> jmp <span class="m">1787</span> <phase_3+0x14f>
16d0: b8 <span class="m">78</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x78,%eax
16d5: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> e7 <span class="m">02</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x2e7,0x8<span class="o">(</span>%rsp<span class="o">)</span>
16dc: <span class="m">00</span>
16dd: 0f <span class="m">84</span> a4 <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> je <span class="m">1787</span> <phase_3+0x14f>
16e3: e8 <span class="m">62</span> <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
16e8: b8 <span class="m">78</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x78,%eax
16ed: e9 <span class="m">95</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> jmp <span class="m">1787</span> <phase_3+0x14f>
16f2: b8 <span class="m">64</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x64,%eax
16f7: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> <span class="m">80</span> <span class="m">02</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x280,0x8<span class="o">(</span>%rsp<span class="o">)</span>
16fe: <span class="m">00</span>
16ff: 0f <span class="m">84</span> <span class="m">82</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> je <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1705</span>: e8 <span class="m">40</span> <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
170a: b8 <span class="m">64</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x64,%eax
170f: eb <span class="m">76</span> jmp <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1711</span>: b8 6d <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x6d,%eax
<span class="m">1716</span>: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> ff <span class="m">02</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x2ff,0x8<span class="o">(</span>%rsp<span class="o">)</span>
171d: <span class="m">00</span>
171e: <span class="m">74</span> <span class="m">67</span> je <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1720</span>: e8 <span class="m">25</span> <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
<span class="m">1725</span>: b8 6d <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x6d,%eax
172a: eb 5b jmp <span class="m">1787</span> <phase_3+0x14f>
172c: b8 <span class="m">71</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x71,%eax
<span class="m">1731</span>: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> <span class="m">75</span> <span class="m">03</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x375,0x8<span class="o">(</span>%rsp<span class="o">)</span>
<span class="m">1738</span>: <span class="m">00</span>
<span class="m">1739</span>: <span class="m">74</span> 4c je <span class="m">1787</span> <phase_3+0x14f>
173b: e8 0a <span class="m">06</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
<span class="m">1740</span>: b8 <span class="m">71</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x71,%eax
<span class="m">1745</span>: eb <span class="m">40</span> jmp <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1747</span>: b8 <span class="m">79</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x79,%eax
174c: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> <span class="m">94</span> <span class="m">02</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x294,0x8<span class="o">(</span>%rsp<span class="o">)</span>
<span class="m">1753</span>: <span class="m">00</span>
<span class="m">1754</span>: <span class="m">74</span> <span class="m">31</span> je <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1756</span>: e8 ef <span class="m">05</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
175b: b8 <span class="m">79</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x79,%eax
<span class="m">1760</span>: eb <span class="m">25</span> jmp <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1762</span>: b8 <span class="m">79</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x79,%eax
<span class="m">1767</span>: <span class="m">81</span> 7c <span class="m">24</span> <span class="m">08</span> <span class="m">88</span> <span class="m">02</span> <span class="m">00</span> cmpl <span class="nv">$0</span>x288,0x8<span class="o">(</span>%rsp<span class="o">)</span>
176e: <span class="m">00</span>
176f: <span class="m">74</span> <span class="m">16</span> je <span class="m">1787</span> <phase_3+0x14f>
<span class="m">1771</span>: e8 d4 <span class="m">05</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
<span class="m">1776</span>: b8 <span class="m">79</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x79,%eax
177b: eb 0a jmp <span class="m">1787</span> <phase_3+0x14f>
177d: e8 c8 <span class="m">05</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
<span class="m">1782</span>: b8 <span class="m">68</span> <span class="m">00</span> <span class="m">00</span> <span class="m">00</span> mov <span class="nv">$0</span>x68,%eax
<span class="m">1787</span>: <span class="m">38</span> <span class="m">44</span> <span class="m">24</span> <span class="m">07</span> cmp %al,0x7<span class="o">(</span>%rsp<span class="o">)</span>
178b: <span class="m">75</span> <span class="m">05</span> jne <span class="m">1792</span> <phase_3+0x15a>
178d: <span class="m">48</span> <span class="m">83</span> c4 <span class="m">18</span> add <span class="nv">$0</span>x18,%rsp
<span class="m">1791</span>: c3 ret
<span class="m">1792</span>: e8 b3 <span class="m">05</span> <span class="m">00</span> <span class="m">00</span> call 1d4a <explode_bomb>
<span class="m">1797</span>: eb f4 jmp 178d <phase_3+0x155>
</code></pre>
</div>
<div class="codehilite">
<pre><span></span><code>...
165b: e8 <span class="m">80</span> <span class="nb">fc</span> ff ff call 12e0 <__isoc99_sscanf@plt>
...
</code></pre>
</div>
<p>We can see that <code>scanf</code> is being called which means we need to figure out what datatype(s) the program is expecting.</p>
<p>Because I do not want to enter the solutions to phases 1 and 2 again and again, I am goig to pass a file which has these solutions.</p>
<div class="codehilite">
<pre><span></span><code>joxxxn@jupyter-nxxh6xx8:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex <span class="s1">'break phase_3'</span> -ex <span class="s1">'break explode_bomb'</span> -ex <span class="s1">'run'</span> -args ./bomb sol.txt
GNU gdb <span class="o">(</span>Ubuntu <span class="m">12</span>.1-0ubuntu1~22.04<span class="o">)</span> <span class="m">12</span>.1
Copyright <span class="o">(</span>C<span class="o">)</span> <span class="m">2022</span> Free Software Foundation, Inc.
License GPLv3+: GNU GPL version <span class="m">3</span> 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 <span class="s2">"show copying"</span> and <span class="s2">"show warranty"</span> <span class="k">for</span> details.
This GDB was configured as <span class="s2">"x86_64-linux-gnu"</span>.
Type <span class="s2">"show configuration"</span> <span class="k">for</span> 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, <span class="nb">type</span> <span class="s2">"help"</span>.
Type <span class="s2">"apropos word"</span> to search <span class="k">for</span> commands related to <span class="s2">"word"</span>...
Reading symbols from ./bomb...
Breakpoint <span class="m">1</span> at 0x1638
Breakpoint <span class="m">2</span> at 0x1d4a
Starting program: /home/joxxxn/lab2-bomblab-navanchauhan/bombbomb/bomb sol.txt
<span class="o">[</span>Thread debugging using libthread_db enabled<span class="o">]</span>
Using host libthread_db library <span class="s2">"/lib/x86_64-linux-gnu/libthread_db.so.1"</span>.
Welcome to my fiendish little bomb. You have <span class="m">6</span> phases with
which to blow yourself up. Have a nice day!
Phase <span class="m">1</span> defused. How about the next one?
That<span class="err">'</span>s number <span class="m">2</span>. Keep going!
random string
Breakpoint <span class="m">1</span>, 0x0000555555555638 <span class="k">in</span> phase_3 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> disas
Dump of assembler code <span class="k">for</span> <span class="k">function</span> phase_3:
<span class="o">=</span>> 0x0000555555555638 <+0>: endbr64
0x000055555555563c <+4>: sub <span class="nv">$0</span>x18,%rsp
0x0000555555555640 <+8>: lea 0x7<span class="o">(</span>%rsp<span class="o">)</span>,%rcx
0x0000555555555645 <+13>: lea 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%rdx
0x000055555555564a <+18>: lea 0x8<span class="o">(</span>%rsp<span class="o">)</span>,%r8
0x000055555555564f <+23>: lea 0x1b60<span class="o">(</span>%rip<span class="o">)</span>,%rsi <span class="c1"># 0x5555555571b6</span>
0x0000555555555656 <+30>: mov <span class="nv">$0</span>x0,%eax
0x000055555555565b <+35>: call 0x5555555552e0 <__isoc99_sscanf@plt>
0x0000555555555660 <+40>: cmp <span class="nv">$0</span>x2,%eax
0x0000555555555663 <+43>: jle 0x555555555685 <phase_3+77>
0x0000555555555665 <+45>: cmpl <span class="nv">$0</span>x7,0xc<span class="o">(</span>%rsp<span class="o">)</span>
0x000055555555566a <+50>: ja 0x55555555577d <phase_3+325>
0x0000555555555670 <+56>: mov 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%eax
0x0000555555555674 <+60>: lea 0x1b55<span class="o">(</span>%rip<span class="o">)</span>,%rdx <span class="c1"># 0x5555555571d0</span>
0x000055555555567b <+67>: movslq <span class="o">(</span>%rdx,%rax,4<span class="o">)</span>,%rax
0x000055555555567f <+71>: add %rdx,%rax
0x0000555555555682 <+74>: notrack jmp *%rax
0x0000555555555685 <+77>: call 0x555555555d4a <explode_bomb>
0x000055555555568a <+82>: jmp 0x555555555665 <phase_3+45>
0x000055555555568c <+84>: mov <span class="nv">$0</span>x63,%eax
0x0000555555555691 <+89>: cmpl <span class="nv">$0</span>x23d,0x8<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555699 <+97>: je 0x555555555787 <phase_3+335>
0x000055555555569f <+103>: call 0x555555555d4a <explode_bomb>
0x00005555555556a4 <+108>: mov <span class="nv">$0</span>x63,%eax
0x00005555555556a9 <+113>: jmp 0x555555555787 <phase_3+335>
--Type <RET> <span class="k">for</span> more, q to quit, c to <span class="k">continue</span> without paging--
</code></pre>
</div>
<p><code>gdb</code> has thankfully marked the address which is being passed to <code>scanf</code>. We can access the value:</p>
<div class="codehilite">
<pre><span></span><code><span class="o">(</span>gdb<span class="o">)</span> x/1s 0x5555555571b6
0x5555555571b6: <span class="s2">"%d %c %d"</span>
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<p>BINGO! The program expects an integer, character, and another integer. Onwards.</p>
<div class="codehilite">
<pre><span></span><code> 0x0000555555555660 <+40>: cmp <span class="nv">$0</span>x2,%eax
0x0000555555555663 <+43>: jle 0x555555555685 <phase_3+77>
...
0x0000555555555685 <+77>: call 0x555555555d4a <explode_bomb>
</code></pre>
</div>
<p>The program checks whether <code>scanf</code> returns a value <= 2, if it does then it calls the <code>explode_bomb</code> function. </p>
<p><em>Note: <code>scanf</code> returns the number of fields that were succesfully converted and assigned</em></p>
<div class="codehilite">
<pre><span></span><code> 0x0000555555555665 <+45>: cmpl <span class="nv">$0</span>x7,0xc<span class="o">(</span>%rsp<span class="o">)</span>
0x000055555555566a <+50>: ja 0x55555555577d <phase_3+325>
...
0x000055555555577d <+325>: call 0x555555555d4a <explode_bomb>
</code></pre>
</div>
<p>Similarly, the program checks and ensures the returned value is not > 7.
<div class="codehilite">
<pre><span></span><code>0x0000555555555670 <+56>: mov 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%eax
0x0000555555555674 <+60>: lea 0x1b55<span class="o">(</span>%rip<span class="o">)</span>,%rdx <span class="c1"># 0x5555555571d0</span>
0x000055555555567b <+67>: movslq <span class="o">(</span>%rdx,%rax,4<span class="o">)</span>,%rax
0x000055555555567f <+71>: add %rdx,%rax
0x0000555555555682 <+74>: notrack jmp *%rax
0x0000555555555685 <+77>: call 0x555555555d4a <explode_bomb>
</code></pre>
</div></p>
<ul>
<li><code>0x0000555555555670 <+56>: mov 0xc(%rsp),%eax</code> - Moves value located at <code>0xc</code> (12 in Decimal) bytes above the stack pointer to <code>%eax</code> register. </li>
<li><code>0x0000555555555674 <+60>: lea 0x1b55(%rip),%rdx # 0x5555555571d0</code> - This instruction calculates an effective address by adding <code>0x1b55</code> to the current instruction pointer (<code>%rip</code>). The result is stored in the <code>%rdx</code> register. </li>
<li><code>0x000055555555567b <+67>: movslq (%rdx,%rax,4),%rax</code>
<ul>
<li><code>movslq</code> stands for "move with sign-extension from a 32-bit value to a 64-bit value." (if the 32-bit value is negative, the 64-bit result will have all its upper 32 bits set to 1; otherwise, they'll be set to 0). </li>
<li><code>(%rdx,%rax,4)</code> - First start with the value in the %rdx register, then add to it the value in the %rax register multiplied by 4.</li>
<li><code>%rax</code> - Destination Register</li>
</ul></li>
<li><code>0x000055555555567f <+71>: add %rdx,%rax</code> - Adds base address in <code>%rdx</code> to the offset in <code>%rax</code> </li>
<li><code>0x0000555555555682 <+74>: notrack jmp *%rax</code> - Jumps to the address stored in <code>%rax</code></li>
<li><code>0x0000555555555685 <+77>: call 0x555555555d4a <explode_bomb></code> - If we are unable to jump to the specified instruction, call <code>explode_bomb</code></li>
</ul>
<p>Let us try to run the program again with a valid input for the first number and see what the program is computing for the address.</p>
<p>I used the input: <code>3 c 123</code>.</p>
<p>To check what is the computed address, we can switch to the asm layout by running <code>layout asm</code>, and then going through instructions <code>ni</code> or <code>si</code> until we reach the line <code>movslq (%rdx,%rax,4),%rax</code></p>
<p><code>%rax</code> should hold the value 3.</p>
<pre><code>(gdb) print $rax
$1 = 3
</code></pre>
<p><img src="/assets/bomb-lab/phase-3.png" alt="Screenshot of GDB terminal depicting us checking the value of the instruction to be jumped to" /></p>
<p>We can see that this makes us jump to <code><phase_3+186></code> (Continue to step through the code by using <code>ni</code>)
<div class="codehilite">
<pre><span></span><code>0x00005555555556f2 <+186>: mov <span class="nv">$0</span>x64,%eax
0x00005555555556f7 <+191>: cmpl <span class="nv">$0</span>x280,0x8<span class="o">(</span>%rsp<span class="o">)</span>
0x00005555555556ff <+199>: je 0x555555555787 <phase<em>3+335>
0x0000555555555705 <+205>: call 0x555555555d4a <explode</em>bomb>
</code></pre>
</div></p>
<p>We see that <code>0x64</code> (Decimal 100) is being stored in <code>%eax</code>. Then, the program compares <code>0x280</code> (Decimal 640) with memory address <code>0x8</code> bytes above the stack pointer (<code>%rsp</code>). If the values are equal, then it jumps to <code><phase_3+335></code>, otherwise <code>explode_bomb</code> is called.
<div class="codehilite">
<pre><span></span><code>0x0000555555555787 <+335>: cmp %al,0x7<span class="o">(</span>%rsp<span class="o">)</span>
0x000055555555578b <+339>: jne 0x555555555792 <phase<em>3+346>
0x000055555555578d <+341>: add <span class="nv">$0</span>x18,%rsp
0x0000555555555791 <+345>: ret <br />
0x0000555555555792 <+346>: call 0x555555555d4a <explode</em>bomb>
</code></pre>
</div></p>
<p>Here, the program is comparing the value of our given character to the value stored in <code>%al</code> (lower 8 bits of <code>EAX</code>), and checks if they are not equal.</p>
<p>Knowing that the character is stored at an offset of 7 bytes to <code>%rsp</code>, we can print and check the value by running:</p>
<div class="codehilite">
<pre><span></span><code><span class="o">(</span>gdb<span class="o">)</span> x/1cw <span class="nv">$rsp</span>+7
c
<span class="o">(</span>gdb<span class="o">)</span> print <span class="nv">$al</span>
<span class="nv">$1</span> <span class="o">=</span> <span class="m">100</span>
</code></pre>
</div>
<p>We can simply lookup the <a rel="noopener" target="_blank" href="https://www.cs.cmu.edu/~pattis/15-1XX/common/handouts/ascii.html">ASCII table</a>, and see that 100 in decimal stands for the character <code>d</code>. Let us try this answer:</p>
<div class="codehilite">
<pre><span></span><code>...
That<span class="err">'</span>s number <span class="m">2</span>. Keep going!
<span class="m">3</span> d <span class="m">640</span>
Breakpoint <span class="m">1</span>, 0x0000555555555638 <span class="k">in</span> phase_3 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> <span class="k">continue</span>
Continuing.
Halfway there!
</code></pre>
</div>
<h2>Phase 4</h2>
<div class="codehilite">
<pre><span></span><code>joxxxn@jupyter-nxxh6xx8:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex <span class="s1">'break phase_4'</span> -ex <span class="s1">'break explode_bomb'</span> -ex <span class="s1">'run'</span> -args ./bomb sol.txt
GNU gdb <span class="o">(</span>Ubuntu <span class="m">12</span>.1-0ubuntu1~22.04<span class="o">)</span> <span class="m">12</span>.1
Copyright <span class="o">(</span>C<span class="o">)</span> <span class="m">2022</span> Free Software Foundation, Inc.
License GPLv3+: GNU GPL version <span class="m">3</span> 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 <span class="s2">"show copying"</span> and <span class="s2">"show warranty"</span> <span class="k">for</span> details.
This GDB was configured as <span class="s2">"x86_64-linux-gnu"</span>.
Type <span class="s2">"show configuration"</span> <span class="k">for</span> 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, <span class="nb">type</span> <span class="s2">"help"</span>.
Type <span class="s2">"apropos word"</span> to search <span class="k">for</span> commands related to <span class="s2">"word"</span>...
Reading symbols from ./bomb...
Breakpoint <span class="m">1</span> at 0x17d3
Breakpoint <span class="m">2</span> at 0x1d4a
Starting program: /home/joxxxn/lab2-bomblab-navanchauhan/bombbomb/bomb sol.txt
<span class="o">[</span>Thread debugging using libthread_db enabled<span class="o">]</span>
Using host libthread_db library <span class="s2">"/lib/x86_64-linux-gnu/libthread_db.so.1"</span>.
Welcome to my fiendish little bomb. You have <span class="m">6</span> phases with
which to blow yourself up. Have a nice day!
Phase <span class="m">1</span> defused. How about the next one?
That<span class="err">'</span>s number <span class="m">2</span>. Keep going!
Halfway there!
<span class="nb">test</span> string
Breakpoint <span class="m">1</span>, 0x00005555555557d3 <span class="k">in</span> phase_4 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> disas phase_4
Dump of assembler code <span class="k">for</span> <span class="k">function</span> phase_4:
<span class="o">=</span>> 0x00005555555557d3 <+0>: endbr64
0x00005555555557d7 <+4>: sub <span class="nv">$0</span>x18,%rsp
0x00005555555557db <+8>: lea 0x8<span class="o">(</span>%rsp<span class="o">)</span>,%rcx
0x00005555555557e0 <+13>: lea 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%rdx
0x00005555555557e5 <+18>: lea 0x1bba<span class="o">(</span>%rip<span class="o">)</span>,%rsi <span class="c1"># 0x5555555573a6</span>
0x00005555555557ec <+25>: mov <span class="nv">$0</span>x0,%eax
0x00005555555557f1 <+30>: call 0x5555555552e0 <__isoc99_sscanf@plt>
0x00005555555557f6 <+35>: cmp <span class="nv">$0</span>x2,%eax
0x00005555555557f9 <+38>: jne 0x555555555802 <phase_4+47>
0x00005555555557fb <+40>: cmpl <span class="nv">$0</span>xe,0xc<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555800 <+45>: jbe 0x555555555807 <phase_4+52>
0x0000555555555802 <+47>: call 0x555555555d4a <explode_bomb>
0x0000555555555807 <+52>: mov <span class="nv">$0</span>xe,%edx
0x000055555555580c <+57>: mov <span class="nv">$0</span>x0,%esi
0x0000555555555811 <+62>: mov 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%edi
0x0000555555555815 <+66>: call 0x555555555799 <func4>
0x000055555555581a <+71>: cmp <span class="nv">$0</span>x2,%eax
0x000055555555581d <+74>: jne 0x555555555826 <phase_4+83>
0x000055555555581f <+76>: cmpl <span class="nv">$0</span>x2,0x8<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555824 <+81>: je 0x55555555582b <phase_4+88>
0x0000555555555826 <+83>: call 0x555555555d4a <explode_bomb>
0x000055555555582b <+88>: add <span class="nv">$0</span>x18,%rsp
0x000055555555582f <+92>: ret
End of assembler dump.
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<p>Again, <code>gdb</code> has marked the string being passed to <code>scanf</code></p>
<div class="codehilite">
<pre><span></span><code><span class="o">(</span>gdb<span class="o">)</span> x/1s 0x5555555573a6
0x5555555573a6: <span class="s2">"%d %d"</span>
</code></pre>
</div>
<p>Okay, so this time we are supposed to enter 2 numbers.
<div class="codehilite">
<pre><span></span><code>0x00005555555557f6 <+35>: cmp <span class="nv">$0</span>x2,%eax
0x00005555555557f9 <+38>: jne 0x555555555802 <phase_4+47>
</code></pre>
</div></p>
<p>Checks if there were 2 values read from calling <code>scanf</code>, if not -> jump to <code><phase_4+47></code> which calls <code><explode_bomb></code>.
<div class="codehilite">
<pre><span></span><code>0x00005555555557fb <+40>: cmpl <span class="nv">$0</span>xe,0xc<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555800 <+45>: jbe 0x555555555807 <phase_4+52>
</code></pre>
</div></p>
<p>Compare <code>0xe</code> (14 in Decimal) and value stored at <code>$rsp</code> + <code>0xc</code> bytes (Decimal 12). If this condition is met (<= 14), jump to <code><phase_4+52></code>. If not, then explode bomb.</p>
<div class="codehilite">
<pre><span></span><code>...
0x0000555555555807 <+52>: mov <span class="nv">$0</span>xe,%edx
0x000055555555580c <+57>: mov <span class="nv">$0</span>x0,%esi
0x0000555555555811 <+62>: mov 0xc<span class="o">(</span>%rsp<span class="o">)</span>,%edi
0x0000555555555815 <+66>: call 0x555555555799 <func4>
0x000055555555581a <+71>: cmp <span class="nv">$0</span>x2,%eax
0x000055555555581d <+74>: jne 0x555555555826 <phase_4+83>
0x000055555555581f <+76>: cmpl <span class="nv">$0</span>x2,0x8<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555824 <+81>: je 0x55555555582b <phase_4+88>
0x0000555555555826 <+83>: call 0x555555555d4a <explode_bomb>
</code></pre>
</div>
<ul>
<li><code>0x0000555555555815 <+66>: call 0x555555555799 <func4></code> calls another function called <code>func4</code></li>
<li>The returned value is compared with <code>0x2</code>, if they are not equal then the program jumps to call <code><explode_bomb></code>. This tells us that <code>func4</code> should return 2.</li>
</ul>
<p>Let us look into <code>func4</code></p>
<div class="codehilite">
<pre><span></span><code><span class="o">(</span>gdb<span class="o">)</span> disas func4
Dump of assembler code <span class="k">for</span> <span class="k">function</span> func4:
0x0000555555555799 <+0>: endbr64
0x000055555555579d <+4>: sub <span class="nv">$0</span>x8,%rsp
0x00005555555557a1 <+8>: mov %edx,%ecx
0x00005555555557a3 <+10>: sub %esi,%ecx
0x00005555555557a5 <+12>: shr %ecx
0x00005555555557a7 <+14>: add %esi,%ecx
0x00005555555557a9 <+16>: cmp %edi,%ecx
0x00005555555557ab <+18>: ja 0x5555555557b9 <func4+32>
0x00005555555557ad <+20>: mov <span class="nv">$0</span>x0,%eax
0x00005555555557b2 <+25>: jb 0x5555555557c5 <func4+44>
0x00005555555557b4 <+27>: add <span class="nv">$0</span>x8,%rsp
0x00005555555557b8 <+31>: ret
0x00005555555557b9 <+32>: lea -0x1<span class="o">(</span>%rcx<span class="o">)</span>,%edx
0x00005555555557bc <+35>: call 0x555555555799 <func4>
0x00005555555557c1 <+40>: add %eax,%eax
0x00005555555557c3 <+42>: jmp 0x5555555557b4 <func4+27>
0x00005555555557c5 <+44>: lea 0x1<span class="o">(</span>%rcx<span class="o">)</span>,%esi
0x00005555555557c8 <+47>: call 0x555555555799 <func4>
0x00005555555557cd <+52>: lea 0x1<span class="o">(</span>%rax,%rax,1<span class="o">)</span>,%eax
0x00005555555557d1 <+56>: jmp 0x5555555557b4 <func4+27>
</code></pre>
</div>
<p>This looks like a recursive function :( (I hate recursive functions)</p>
<p>Let's annotate the instructions.</p>
<div class="codehilite">
<pre><span></span><code>endbr64
sub <span class="nv">$0</span>x8,%rsp // subtract <span class="m">8</span> bytes from the stack pointer
mov %edx,%ecx // Move the value <span class="k">in</span> register %edx to %ecx
sub %esi,%ecx // Subtract the value <span class="k">in</span> %esi from %ecx
shr %ecx // Right <span class="nb">shift</span> the value <span class="k">in</span> %ecx by one bit <span class="o">(</span>dividing the value by <span class="m">2</span><span class="o">)</span>
add %esi,%ecx // Add the value <span class="k">in</span> %esi to %ecx
cmp %edi,%ecx // Compare
ja 0x5555555557b9 <func4+32> // If %ecx > %edi -> jump to instruction at offset +32
mov <span class="nv">$0</span>x0,%eax // Move <span class="m">0</span> to %eax
jb 0x5555555557c5 <func4+44> // If %ecx < %edi -> jump to instruction at offset +44.
add <span class="nv">$0</span>x8,%rsp // add <span class="m">8</span> bytes to the stack pointer
ret // <span class="k">return</span>
lea -0x1<span class="o">(</span>%rcx<span class="o">)</span>,%edx // LEA of <span class="nv">$rxc</span> - <span class="m">1</span> into <span class="nv">$edx</span>
call 0x555555555799 <func4> // Call itself
add %eax,%eax // Double the value <span class="k">in</span> %eax
jmp 0x5555555557b4 <func4+27> // jump to the instruction at offset +27
lea 0x1<span class="o">(</span>%rcx<span class="o">)</span>,%esi
call 0x555555555799 <func4>
lea 0x1<span class="o">(</span>%rax,%rax,1<span class="o">)</span>,%eax // LEA of %rax * <span class="m">2</span> + <span class="m">1</span> into <span class="nv">$eax</span>
jmp 0x5555555557b4 <func4+27>
</code></pre>
</div>
<p>We can either try to compute the values by hand, or write a simple script in Python to get the answer.</p>
<div class="codehilite">
<pre><span></span><code><span class="k">def</span> <span class="nf">func4</span><span class="p">(</span><span class="n">edi</span><span class="p">,</span> <span class="n">esi</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">edx</span><span class="o">=</span><span class="mi">20</span><span class="p">):</span>
<span class="n">ecx</span> <span class="o">=</span> <span class="p">(</span><span class="n">edx</span> <span class="o">-</span> <span class="n">esi</span><span class="p">)</span> <span class="o">//</span> <span class="mi">2</span> <span class="o">+</span> <span class="n">esi</span>
<span class="k">if</span> <span class="n">ecx</span> <span class="o">></span> <span class="n">edi</span><span class="p">:</span>
<span class="k">return</span> <span class="mi">2</span> <span class="o">*</span> <span class="n">func4</span><span class="p">(</span><span class="n">edi</span><span class="p">,</span> <span class="n">esi</span><span class="p">,</span> <span class="n">ecx</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">ecx</span> <span class="o"><</span> <span class="n">edi</span><span class="p">:</span>
<span class="k">return</span> <span class="mi">2</span> <span class="o">*</span> <span class="n">func4</span><span class="p">(</span><span class="n">edi</span><span class="p">,</span> <span class="n">ecx</span> <span class="o">+</span> <span class="mi">1</span><span class="p">,</span> <span class="n">edx</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">15</span><span class="p">):</span> <span class="c1"># We can limit to 14</span>
<span class="k">if</span> <span class="n">func4</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">"answer is </span><span class="si">{</span><span class="n">x</span><span class="si">}</span><span class="s2">"</span><span class="p">)</span>
<span class="k">break</span>
</code></pre>
</div>
<p>Running this code, we get: <code>answer is 5</code></p>
<p>Okay, so we know that the number needed to be passed to <code>func4</code> is 5. But, what about the second digit?</p>
<p>If we go back to the code for <code><phase_4></code>, we can see that:
<div class="codehilite">
<pre><span></span><code>0x000055555555581f <+76>: cmpl <span class="nv">$0</span>x2,0x8<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555824 <+81>: je 0x55555555582b <phase_4+88>
</code></pre>
</div></p>
<p>The value at <code>$rsp+8</code> should be equal to 2. So, let us try passing <code>5 2</code> as our input.</p>
<div class="codehilite">
<pre><span></span><code>...
Phase <span class="m">1</span> defused. How about the next one?
That<span class="err">'</span>s number <span class="m">2</span>. Keep going!
Halfway there!
<span class="m">5</span> <span class="m">2</span>
Breakpoint <span class="m">1</span>, 0x00005555555557d3 <span class="k">in</span> phase_4 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> <span class="k">continue</span>
Continuing.
So you got that one. Try this one.
</code></pre>
</div>
<h2>Phase 5</h2>
<div class="codehilite">
<pre><span></span><code>So you got that one. Try this one.
<span class="nb">test</span> string
Breakpoint <span class="m">1</span>, 0x0000555555555830 <span class="k">in</span> phase_5 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> disas phase_5
Dump of assembler code <span class="k">for</span> <span class="k">function</span> phase_5:
<span class="o">=</span>> 0x0000555555555830 <+0>: endbr64
0x0000555555555834 <+4>: push %rbx
0x0000555555555835 <+5>: sub <span class="nv">$0</span>x10,%rsp
0x0000555555555839 <+9>: mov %rdi,%rbx
0x000055555555583c <+12>: call 0x555555555b10 <string_length>
0x0000555555555841 <+17>: cmp <span class="nv">$0</span>x6,%eax
0x0000555555555844 <+20>: jne 0x55555555588b <phase_5+91>
0x0000555555555846 <+22>: mov <span class="nv">$0</span>x0,%eax
0x000055555555584b <+27>: lea 0x199e<span class="o">(</span>%rip<span class="o">)</span>,%rcx <span class="c1"># 0x5555555571f0 <array.0></span>
0x0000555555555852 <+34>: movzbl <span class="o">(</span>%rbx,%rax,1<span class="o">)</span>,%edx
0x0000555555555856 <+38>: and <span class="nv">$0</span>xf,%edx
0x0000555555555859 <+41>: movzbl <span class="o">(</span>%rcx,%rdx,1<span class="o">)</span>,%edx
0x000055555555585d <+45>: mov %dl,0x9<span class="o">(</span>%rsp,%rax,1<span class="o">)</span>
0x0000555555555861 <+49>: add <span class="nv">$0</span>x1,%rax
0x0000555555555865 <+53>: cmp <span class="nv">$0</span>x6,%rax
0x0000555555555869 <+57>: jne 0x555555555852 <phase_5+34>
0x000055555555586b <+59>: movb <span class="nv">$0</span>x0,0xf<span class="o">(</span>%rsp<span class="o">)</span>
0x0000555555555870 <+64>: lea 0x9<span class="o">(</span>%rsp<span class="o">)</span>,%rdi
0x0000555555555875 <+69>: lea 0x1943<span class="o">(</span>%rip<span class="o">)</span>,%rsi <span class="c1"># 0x5555555571bf</span>
0x000055555555587c <+76>: call 0x555555555b31 <strings_not_equal>
0x0000555555555881 <+81>: <span class="nb">test</span> %eax,%eax
0x0000555555555883 <+83>: jne 0x555555555892 <phase_5+98>
0x0000555555555885 <+85>: add <span class="nv">$0</span>x10,%rsp
0x0000555555555889 <+89>: pop %rbx
0x000055555555588a <+90>: ret
0x000055555555588b <+91>: call 0x555555555d4a <explode_bomb>
0x0000555555555890 <+96>: jmp 0x555555555846 <phase_5+22>
0x0000555555555892 <+98>: call 0x555555555d4a <explode_bomb>
0x0000555555555897 <+103>: jmp 0x555555555885 <phase_5+85>
End of assembler dump.
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<div class="codehilite">
<pre><span></span><code>...
0x000055555555583c <+12>: call 0x555555555b10 <string_length>
0x0000555555555841 <+17>: cmp <span class="nv">$0</span>x6,%eax
0x0000555555555844 <+20>: jne 0x55555555588b <phase_5+91>
...
0x000055555555588b <+91>: call 0x555555555d4a <explode_bomb>
...
</code></pre>
</div>
<p>First things first, these instructions check to make sure the passed string is of length 6, otherwise <code>explode_bomb</code> is called.</p>
<p>We can also see a similar pattern compared to Phase 2, where we had a loop:</p>
<ul>
<li>The looping part:
<ul>
<li><code>mov $0x0,%eax</code> - Initialise <code>%eax</code> and set it to 0 (our counter/iterator)</li>
<li><code>movzbl (%rbx,%rax,1),%edx</code> - Access <code>%rbx + 1 * %rax</code> and store it in <code>%edx</code></li>
<li><code>and $0xf,%edx</code> - Take the least significant 4 bits of the byte.</li>
<li><code>movzbl (%rcx,%rdx,1),%edx</code> - Use the 4 bits as an index into another array and load the corresponding byte into <code>%edx</code></li>
<li><code>mov %dl,0x9(%rsp,%rax,1)</code> - Store the transformed byte into a buffer on the stack</li>
<li><code>add $0x1,%rax</code> - Increment <code>%rax</code></li>
<li><code>cmp $0x6,%rax</code> - If the index is not yet 6, loop again</li>
</ul></li>
<li><code>movb $0x0,0xf(%rsp)</code> - Null-terminate the transformed string</li>
<li><code>lea 0x9(%rsp),%rdi</code> and <code>lea 0x1943(%rip),%rsi</code> </li>
<li><code>all 0x555555555b31 <strings_not_equal></code> check if the two strings loaded up just before this are equal or not.</li>
</ul>
<p>We can check the reference string we need, which <code>gdb</code> has marked as <code># 0x5555555571bf</code>, and the lookup table marked as <code># 0x5555555571f0 <array.0></code></p>
<div class="codehilite">
<pre><span></span><code><span class="o">(</span>gdb<span class="o">)</span> x/s 0x5555555571bf
0x5555555571bf: <span class="s2">"bruins"</span>
<span class="o">(</span>gdb<span class="o">)</span> x/s 0x5555555571f0
0x5555555571f0 <array.0>: <span class="s2">"maduiersnfotvbylSo you think you can stop the bomb with ctrl-c, do you?"</span>
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<p>To summarize the transformation process:</p>
<ul>
<li>The function takes each byte of the string</li>
<li>It keeps only the least significant 4 bits of each byte</li>
<li>It uses these 4 bits as an index into the lookup table (<code>array.0</code>)</li>
<li>The value from the array is then stored in a buffer</li>
</ul>
<p>Here's how the transformation process can be reversed for each character in "bruins":
1. Find the index of <code>b</code> in the lookup table (in our case, it is 13 since we index starting 0)
2. Calculate binary representation of this index (in our case 13 can be written as 1101 in binary)
3. Find ASCII character whose least significant 4 bits match (in our case, <code>m</code> has binary representation <code>01101101</code>)</p>
<p>Repeat for all 6 characters</p>
<p><em>Hint: Using an <a rel="noopener" target="_blank" href="http://sticksandstones.kstrom.com/appen.html">ASCII - Binary Table</a> can save you time.</em> </p>
<p>Thus, we can have the following transformation:</p>
<pre><code>b -> m
r -> f
u -> c
i -> d
n -> h
s -> g
</code></pre>
<p>Let us try out this answer:</p>
<div class="codehilite">
<pre><span></span><code>...
That<span class="err">'</span>s number <span class="m">2</span>. Keep going!
Halfway there!
So you got that one. Try this one.
mfcdhg
Breakpoint <span class="m">1</span>, 0x0000555555555830 <span class="k">in</span> phase_5 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> <span class="k">continue</span>
Continuing.
Good work! On to the next...
</code></pre>
</div>
<p>Awesome!</p>
<h2>Phase 6</h2>
<div class="codehilite">
<pre><span></span><code>Good work! On to the next...
<span class="nb">test</span> string
Breakpoint <span class="m">1</span>, 0x0000555555555899 <span class="k">in</span> phase_6 <span class="o">()</span>
<span class="o">(</span>gdb<span class="o">)</span> disas phase_6
Dump of assembler code <span class="k">for</span> <span class="k">function</span> phase_6:
<span class="o">=</span>> 0x0000555555555899 <+0>: endbr64
0x000055555555589d <+4>: push %r15
0x000055555555589f <+6>: push %r14
0x00005555555558a1 <+8>: push %r13
0x00005555555558a3 <+10>: push %r12
0x00005555555558a5 <+12>: push %rbp
0x00005555555558a6 <+13>: push %rbx
0x00005555555558a7 <+14>: sub <span class="nv">$0</span>x68,%rsp
0x00005555555558ab <+18>: lea 0x40<span class="o">(</span>%rsp<span class="o">)</span>,%rax
0x00005555555558b0 <+23>: mov %rax,%r14
0x00005555555558b3 <+26>: mov %rax,0x8<span class="o">(</span>%rsp<span class="o">)</span>
0x00005555555558b8 <+31>: mov %rax,%rsi
0x00005555555558bb <+34>: call 0x555555555d97 <read_six_numbers>
0x00005555555558c0 <+39>: mov %r14,%r12
0x00005555555558c3 <+42>: mov <span class="nv">$0</span>x1,%r15d
0x00005555555558c9 <+48>: mov %r14,%r13
0x00005555555558cc <+51>: jmp 0x555555555997 <phase_6+254>
0x00005555555558d1 <+56>: call 0x555555555d4a <explode_bomb>
0x00005555555558d6 <+61>: jmp 0x5555555559a9 <phase_6+272>
0x00005555555558db <+66>: add <span class="nv">$0</span>x1,%rbx
0x00005555555558df <+70>: cmp <span class="nv">$0</span>x5,%ebx
0x00005555555558e2 <+73>: jg 0x55555555598f <phase_6+246>
0x00005555555558e8 <+79>: mov 0x0<span class="o">(</span>%r13,%rbx,4<span class="o">)</span>,%eax
0x00005555555558ed <+84>: cmp %eax,0x0<span class="o">(</span>%rbp<span class="o">)</span>
0x00005555555558f0 <+87>: jne 0x5555555558db <phase_6+66>
0x00005555555558f2 <+89>: call 0x555555555d4a <explode_bomb>
0x00005555555558f7 <+94>: jmp 0x5555555558db <phase_6+66>
0x00005555555558f9 <+96>: mov 0x8<span class="o">(</span>%rsp<span class="o">)</span>,%rdx
0x00005555555558fe <+101>: add <span class="nv">$0</span>x18,%rdx
0x0000555555555902 <+105>: mov <span class="nv">$0</span>x7,%ecx
0x0000555555555907 <+110>: mov %ecx,%eax
0x0000555555555909 <+112>: sub <span class="o">(</span>%r12<span class="o">)</span>,%eax
0x000055555555590d <+116>: mov %eax,<span class="o">(</span>%r12<span class="o">)</span>
0x0000555555555911 <+120>: add <span class="nv">$0</span>x4,%r12
0x0000555555555915 <+124>: cmp %r12,%rdx
0x0000555555555918 <+127>: jne 0x555555555907 <phase_6+110>
0x000055555555591a <+129>: mov <span class="nv">$0</span>x0,%esi
0x000055555555591f <+134>: mov 0x40<span class="o">(</span>%rsp,%rsi,4<span class="o">)</span>,%ecx
0x0000555555555923 <+138>: mov <span class="nv">$0</span>x1,%eax
0x0000555555555928 <+143>: lea 0x3d01<span class="o">(</span>%rip<span class="o">)</span>,%rdx <span class="c1"># 0x555555559630 <node1></span>
--Type <RET> <span class="k">for</span> more, q to quit, c to <span class="k">continue</span> without paging--
0x000055555555592f <+150>: cmp <span class="nv">$0</span>x1,%ecx
0x0000555555555932 <+153>: jle 0x55555555593f <phase_6+166>
0x0000555555555934 <+155>: mov 0x8<span class="o">(</span>%rdx<span class="o">)</span>,%rdx
0x0000555555555938 <+159>: add <span class="nv">$0</span>x1,%eax
0x000055555555593b <+162>: cmp %ecx,%eax
0x000055555555593d <+164>: jne 0x555555555934 <phase_6+155>
0x000055555555593f <+166>: mov %rdx,0x10<span class="o">(</span>%rsp,%rsi,8<span class="o">)</span>
0x0000555555555944 <+171>: add <span class="nv">$0</span>x1,%rsi
0x0000555555555948 <+175>: cmp <span class="nv">$0</span>x6,%rsi
0x000055555555594c <+179>: jne 0x55555555591f <phase_6+134>
0x000055555555594e <+181>: mov 0x10<span class="o">(</span>%rsp<span class="o">)</span>,%rbx
0x0000555555555953 <+186>: mov 0x18<span class="o">(</span>%rsp<span class="o">)</span>,%rax
0x0000555555555958 <+191>: mov %rax,0x8<span class="o">(</span>%rbx<span class="o">)</span>
0x000055555555595c <+195>: mov 0x20<span class="o">(</span>%rsp<span class="o">)</span>,%rdx
0x0000555555555961 <+200>: mov %rdx,0x8<span class="o">(</span>%rax<span class="o">)</span>
0x0000555555555965 <+204>: mov 0x28<span class="o">(</span>%rsp<span class="o">)</span>,%rax
0x000055555555596a <+209>: mov %rax,0x8<span class="o">(</span>%rdx<span class="o">)</span>
0x000055555555596e <+213>: mov 0x30<span class="o">(</span>%rsp<span class="o">)</span>,%rdx
0x0000555555555973 <+218>: mov %rdx,0x8<span class="o">(</span>%rax<span class="o">)</span>
0x0000555555555977 <+222>: mov 0x38<span class="o">(</span>%rsp<span class="o">)</span>,%rax
0x000055555555597c <+227>: mov %rax,0x8<span class="o">(</span>%rdx<span class="o">)</span>
0x0000555555555980 <+231>: movq <span class="nv">$0</span>x0,0x8<span class="o">(</span>%rax<span class="o">)</span>
0x0000555555555988 <+239>: mov <span class="nv">$0</span>x5,%ebp
0x000055555555598d <+244>: jmp 0x5555555559c4 <phase_6+299>
0x000055555555598f <+246>: add <span class="nv">$0</span>x1,%r15
0x0000555555555993 <+250>: add <span class="nv">$0</span>x4,%r14
0x0000555555555997 <+254>: mov %r14,%rbp
0x000055555555599a <+257>: mov <span class="o">(</span>%r14<span class="o">)</span>,%eax
0x000055555555599d <+260>: sub <span class="nv">$0</span>x1,%eax
0x00005555555559a0 <+263>: cmp <span class="nv">$0</span>x5,%eax
0x00005555555559a3 <+266>: ja 0x5555555558d1 <phase_6+56>
0x00005555555559a9 <+272>: cmp <span class="nv">$0</span>x5,%r15d
0x00005555555559ad <+276>: jg 0x5555555558f9 <phase_6+96>
0x00005555555559b3 <+282>: mov %r15,%rbx
0x00005555555559b6 <+285>: jmp 0x5555555558e8 <phase_6+79>
0x00005555555559bb <+290>: mov 0x8<span class="o">(</span>%rbx<span class="o">)</span>,%rbx
0x00005555555559bf <+294>: sub <span class="nv">$0</span>x1,%ebp
0x00005555555559c2 <+297>: je 0x5555555559d5 <phase_6+316>
0x00005555555559c4 <+299>: mov 0x8<span class="o">(</span>%rbx<span class="o">)</span>,%rax
0x00005555555559c8 <+303>: mov <span class="o">(</span>%rax<span class="o">)</span>,%eax
0x00005555555559ca <+305>: cmp %eax,<span class="o">(</span>%rbx<span class="o">)</span>
--Type <RET> <span class="k">for</span> more, q to quit, c to <span class="k">continue</span> without paging--
0x00005555555559cc <+307>: jge 0x5555555559bb <phase_6+290>
0x00005555555559ce <+309>: call 0x555555555d4a <explode_bomb>
0x00005555555559d3 <+314>: jmp 0x5555555559bb <phase_6+290>
0x00005555555559d5 <+316>: add <span class="nv">$0</span>x68,%rsp
0x00005555555559d9 <+320>: pop %rbx
0x00005555555559da <+321>: pop %rbp
0x00005555555559db <+322>: pop %r12
0x00005555555559dd <+324>: pop %r13
0x00005555555559df <+326>: pop %r14
0x00005555555559e1 <+328>: pop %r15
0x00005555555559e3 <+330>: ret
End of assembler dump.
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<p>Again, we see the familiar <code>read_six_digits</code> function.</p>
<p>Let us analyse this function in chunks:
<div class="codehilite">
<pre><span></span><code>0x00005555555558bb <+34>: call 0x555555555d97 <read<em>six</em>numbers>
0x00005555555558c0 <+39>: mov %r14,%r12
0x00005555555558c3 <+42>: mov <span class="nv">$0</span>x1,%r15d
0x00005555555558c9 <+48>: mov %r14,%r13
0x00005555555558cc <+51>: jmp 0x555555555997 <phase_6+254>
</code></pre>
</div></p>
<ol>
<li>Read six numbers</li>
<li>Initialise Registers:
2.1. <code>mov %r14,%r12</code>: <code>%r14</code> should be pointing to the location of the stack where the numbers were read into. This address is copied onto <code>%r12</code>
2.2. <code>mov $0x1,%r15d</code>: The value <code>1</code> is moved into <code>%r15</code> register (probably acting like a counter)
2.3. <code>mov %r14,%r13</code>: The value is also copied to <code>%r13</code></li>
<li><p>Jump to start of loop:</p>
<div class="codehilite">
<pre><span></span><code>0x0000555555555997 <+254>: mov %r14,%rbp
0x000055555555599a <+257>: mov <span class="o">(</span>%r14<span class="o">)</span>,%eax
0x000055555555599d <+260>: sub <span class="nv">$0</span>x1,%eax
0x00005555555559a0 <+263>: cmp <span class="nv">$0</span>x5,%eax
0x00005555555559a3 <+266>: ja 0x5555555558d1 <phase_6+56>
</code></pre>
</div></li>
<li><p>Initialise register and point to first number in sequence</p></li>
<li>Adjust number(s):
2.1. <code>mov (%r14),%eax</code> -> load the current number in the sequence
2.2. <code>sub $0x1,%eax</code> -> decrement number by 1</li>
<li>Validation
3.1. <code>cmp $0x5,%eax</code>: This compares the adjusted value in <code>%eax</code> with 5.
3.2. <code>ja 0x5555555558d1 <phase_6+56></code>: jump if given value is > 5 or < 0</li>
</ol>
<p>=> All numbers should be between 1 and 6.
<div class="codehilite">
<pre><span></span><code>0x00005555555559a9 <+272>: cmp <span class="nv">$0</span>x5,%r15d
0x00005555555559ad <+276>: jg 0x5555555558f9 <phase_6+96>
</code></pre>
</div></p>
<p>This checks if the value stored in <code>%r15</code> is > 5, if it is then it jumps somewhere else. This validates our assumption that <code>%r15</code> is acting as a counter.
<div class="codehilite">
<pre><span></span><code>0x00005555555559b3 <+282>: mov %r15,%rbx
0x00005555555559b6 <+285>: jmp 0x5555555558e8 <phase_6+79>
</code></pre>
</div></p>
<p>Let us jump to +79
<div class="codehilite">
<pre><span></span><code>0x00005555555558e8 <+79>: mov 0x0<span class="o">(</span>%r13,%rbx,4<span class="o">)</span>,%eax
0x00005555555558ed <+84>: cmp %eax,0x0<span class="o">(</span>%rbp<span class="o">)</span>
0x00005555555558f0 <+87>: jne 0x5555555558db <phase<em>6+66>
0x00005555555558f2 <+89>: call 0x555555555d4a <explode</em>bomb>
0x00005555555558f7 <+94>: jmp 0x5555555558db <phase_6+66>
</code></pre>
</div></p>
<p>This section deals with checking if all the numbers in the sequence are unique or not. Thus, we need to ensure out 6 digits are unique
<div class="codehilite">
<pre><span></span><code>0x00005555555558db <+66>: add <span class="nv">$0</span>x1,%rbx // Increments by <span class="m">1</span>
0x00005555555558df <+70>: cmp <span class="nv">$0</span>x5,%ebx
0x00005555555558e2 <+73>: jg 0x55555555598f <phase<em>6+246> // Jump <span class="k">if</span> > <span class="m">5</span> <span class="o">(</span>Loop iterations are <span class="nb">complete</span><span class="o">)</span>
0x00005555555558e8 <+79>: mov 0x0<span class="o">(</span>%r13,%rbx,4<span class="o">)</span>,%eax
0x00005555555558ed <+84>: cmp %eax,0x0<span class="o">(</span>%rbp<span class="o">)</span>
0x00005555555558f0 <+87>: jne 0x5555555558db <phase</em>6+66> // Again, check <span class="k">if</span> the number being seen is unique
</code></pre>
</div></p>
<p>Now we know that the numbers are unique, between 1-6 (inclusive).</p>
<p>After stepping through the instructions, we can also see that the numbers are being transformed:
* By subtracting it from 7 (mov $0x7,%ecx followed by sub (%r12),%eax)
* This effectively maps the numbers as follows: 1 to 6, 2 to 5, 3 to 4, 4 to 3, 5 to 2, and 6 to 1.</p>
<p>Let us try to figure out what <code>0x0000555555555928 <+143>: lea 0x3d01(%rip),%rdx # 0x555555559630 <node1></code> is:</p>
<div class="codehilite">
<pre><span></span><code><span class="o">(</span>gdb<span class="o">)</span> x/30wx 0x555555559630
0x555555559630 <node1>: 0x000000d9 0x00000001 0x55559640 0x00005555
0x555555559640 <node2>: 0x000003ab 0x00000002 0x55559650 0x00005555
0x555555559650 <node3>: 0x0000014f 0x00000003 0x55559660 0x00005555
0x555555559660 <node4>: 0x000000a1 0x00000004 0x55559670 0x00005555
0x555555559670 <node5>: 0x000001b3 0x00000005 0x55559120 0x00005555
0x555555559680 <host_table>: 0x555573f5 0x00005555 0x5555740f 0x00005555
0x555555559690 <host_table+16>: 0x55557429 0x00005555 0x00000000 0x00000000
0x5555555596a0 <host_table+32>: 0x00000000 0x00000000
<span class="o">(</span>gdb<span class="o">)</span> x/30wx 0x555555559120
0x555555559120 <node6>: 0x000002da 0x00000006 0x00000000 0x00000000
0x555555559130: 0x00000000 0x00000000 0x00000000 0x00000000
0x555555559140 <userid>: 0x61767861 0x38383535 0x00000000 0x00000000
0x555555559150 <userid+16>: 0x00000000 0x00000000 0x00000000 0x00000000
0x555555559160 <userid+32>: 0x00000000 0x00000000 0x00000000 0x00000000
0x555555559170 <userid+48>: 0x00000000 0x00000000 0x00000000 0x00000000
0x555555559180 <userid+64>: 0x00000000 0x00000000 0x00000000 0x00000000
0x555555559190 <userid+80>: 0x00000000 0x00000000
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre>
</div>
<p>It appears that this is a linked list. With roughly the following structure:</p>
<div class="codehilite">
<pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">node</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">value</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">index</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">node</span><span class="w"> </span><span class="o">*</span><span class="n">next</span><span class="p">;</span><span class="w"></span>
<span class="p">};</span><span class="w"></span>
</code></pre>
</div>
<p>Let us convert the values into decimal:</p>
<pre><code>0x000000d9 -> 217
0x000003ab -> 939
0x0000014f -> 335
0x000000a1 -> 161
0x000001b3 -> 435
0x000002da -> 730
</code></pre>
<p><strong>Missing Notes</strong></p>
<p>To re-arrange this linked list in descending order, we would arrange it as follows:</p>
<pre><code>Node 2 -> Node 6 -> Node 5 -> Node 3 -> Node 1 -> Node 4
</code></pre>
<p>Since we also need to apply the transformation: <code>7 - x</code>:</p>
<pre><code>(7-2) -> (7-6) -> ... -> (7-4)
</code></pre>
<p>Final answer: <code>5 1 2 4 6 3</code></p>
<p>Let us try the answer:</p>
<pre><code>...
That's number 2. Keep going!
Halfway there!
So you got that one. Try this one.
Good work! On to the next...
5 1 2 4 6 3
Breakpoint 1, 0x0000555555555899 in phase_6 ()
(gdb) continue
Continuing.
Congratulations! You've defused the bomb!
Your instructor has been notified and will verify your solution.
[Inferior 1 (process 1754) exited normally]
</code></pre>
<p>But, what about the secret phase?</p>
<blockquote>If you have scrolled this far, consider subscribing to my mailing list <a href="https://listmonk.navan.dev/subscription/form">here.</a> You can subscribe to either a specific type of post you are interested in, or subscribe to everything with the "Everything" list.</blockquote>
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