I am currently working on famous System Programming problem, "Bomb lab" phase2. When I disassembled phase_2 with gdb, code looks like this...
Phase_2
0x00005555555555cb <+0>: endbr64
0x00005555555555cf <+4>: push %rbp
0x00005555555555d0 <+5>: push %rbx
0x00005555555555d1 <+6>: sub $0x28,%rsp
0x00005555555555d5 <+10>: mov %fs:0x28,%rax
0x00005555555555de <+19>: mov %rax,0x18(%rsp)
0x00005555555555e3 <+24>: xor %eax,%eax
0x00005555555555e5 <+26>: mov %rsp,%rsi
0x00005555555555e8 <+29>: callq 0x555555555bd5 <read_six_numbers>
0x00005555555555ed <+34>: cmpl $0x0,(%rsp)
0x00005555555555f1 <+38>: js 0x5555555555fd <phase_2+50>
0x00005555555555f3 <+40>: mov %rsp,%rbp
0x00005555555555f6 <+43>: mov $0x1,%ebx
0x00005555555555fb <+48>: jmp 0x555555555615 <phase_2+74>
0x00005555555555fd <+50>: callq 0x555555555ba9 <explode_bomb>
0x0000555555555602 <+55>: jmp 0x5555555555f3 <phase_2+40>
0x0000555555555604 <+57>: callq 0x555555555ba9 <explode_bomb>
0x0000555555555609 <+62>: add $0x1,%ebx
0x000055555555560c <+65>: add $0x4,%rbp
0x0000555555555610 <+69>: cmp $0x6,%ebx
0x0000555555555613 <+72>: je 0x555555555621 <phase_2+86>
0x0000555555555615 <+74>: mov %ebx,%eax
0x0000555555555617 <+76>: add 0x0(%rbp),%eax
0x000055555555561a <+79>: cmp %eax,0x4(%rbp)
0x000055555555561d <+82>: je 0x555555555609 <phase_2+62>
0x000055555555561f <+84>: jmp 0x555555555604 <phase_2+57>
0x0000555555555621 <+86>: mov 0x18(%rsp),%rax
0x0000555555555626 <+91>: xor %fs:0x28,%rax
0x000055555555562f <+100>: jne 0x555555555638 <phase_2+109>
0x0000555555555631 <+102>: add $0x28,%rsp
0x0000555555555635 <+106>: pop %rbx
0x0000555555555636 <+107>: pop %rbp
0x0000555555555637 <+108>: retq
0x0000555555555638 <+109>: callq 0x555555555220 <__stack_chk_fail@plt>
I guess that in line <+62>, %ebx means index, and increments the value until the value is 6 (by line <+69>). But I don't really understand lines such as
0x000055555555560c <+65>: add $0x4,%rbp
or
0x0000555555555615 <+74>: mov %ebx,%eax
0x0000555555555617 <+76>: add 0x0(%rbp),%eax
0x000055555555561a <+79>: cmp %eax,0x4(%rbp)
I guess it is probably related to features of sequence that I have to find, but I don't understand why values such as 0x4(%rbp)is compared with %eax etc... Can someone explain?
