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So I've been working with this code for some time now. Pretty new to Assembly language I'm trying to make this work using gcc and gdb in Kali Linux.

I'm unsure how to set breakpoints properly as well as get this program to get the desired output.

Please let me know if I'm using this incorrectly or missed something.

Thanks!

Here's what I got:

file name: "reveng.s"

    .global main
    .type   main, @function
    .intel_syntax noprefix
main:
fx:
        push    rbp
        mov     rbp, rsp
        mov     DWORD PTR [rbp-4], edi
        mov     eax, DWORD PTR [rbp-4]
        sal     eax, 3
        cmp     eax, 5744
        sete    al
        movzx   eax, al
        pop     rbp
        ret

After working with this file I ran this in the terminal:

sudo gcc -c reveng.s -o reveng.o
sudo gcc reveng.o -o reveng
gdb reveng

Terminal Output:

(gdb) disas main
Dump of assembler code for function main:
   0x0000555555555129 <+0>:     push   %rbp
   0x000055555555512a <+1>:     mov    %rsp,%rbp
   0x000055555555512d <+4>:     mov    %edi,-0x4(%rbp)
   0x0000555555555130 <+7>:     mov    -0x4(%rbp),%eax
   0x0000555555555133 <+10>:    shl    $0x3,%eax
   0x0000555555555136 <+13>:    cmp    $0x1670,%eax
   0x000055555555513b <+18>:    sete   %al
   0x000055555555513e <+21>:    movzbl %al,%eax
   0x0000555555555141 <+24>:    pop    %rbp
   0x0000555555555142 <+25>:    ret    
   0x0000555555555143 <+26>:    nopw   %cs:0x0(%rax,%rax,1)
   0x000055555555514d <+36>:    nopl   (%rax)
End of assembler dump.
(gdb) bt
No stack.
(gdb) b 0x000055555555513e
Function "0x000055555555513e" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (0x000055555555513e) pending.
(gdb) r
Starting program: /home/chaoswraith/Desktop/CTF/BCW2022/Practice/reveng 
[Inferior 1 (process 57477) exited normally]
(gdb) p $eax
No registers.
ChaosWraith
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  • You can use `start` to `run` and have GDB stop at the top of main. (Or after the prologue, according to some GDB heuristics.) `starti` stops at the first instruction of your process, but unless you wrote your own `_start` and linked into a static executable, that's not your code. – Peter Cordes Feb 07 '22 at 18:51

0 Answers0