What is the decompiled (C) code construct of this assembly x86 code?

Question

This code compares every char of a string (located at ebp+arg_0) with different constants (ASCII chars) like 'I', 'o' and 'S'. I guess, based on other code parts, this code is originally written in C.

This compare-code-part looks very inefficient to. My question, how do you think this code will look in C? What code construct was used originally? my thoughts so far

• It's not a for loop. Because i don't see any upward jump and stop-condition.

• It's not a while/case/switch code construct

• My best guess is that this are a lot of consecutive if/else statements. Can you help?

Yes it's part of a challenge, i already have the flag/solution, no worries about that. Just trying to better understand the code.

Answer 1

It's not a for loop. Because i don't see any upward jump and stop-condition.

Correct.

It's not a while/case/switch code constuct

It can't be, it compares different indicies of the array.

My best guess is that this are a lot of consecutive if/elses. Can you help?

Looks like it could be this code:

void f(const char* arg_0) {
    if(arg_0[4] == 'I' && arg_0[5] == 'o' && arg_0[6] == 'S') {
        printf("Gratz man :)");
        exit(0); //noreturn, hence your control flow ends here in the assembly
    }
    puts("Wrong password"); // Or `printf("Wrong password\n");` which gets optimized to `puts`
    // leave, retn
}

This is how gcc compiles it without optimizations:

.LC0:
        .string "Gratz man :)"
.LC1:
        .string "Wrong password"
f(char const*):
        push    ebp
        mov     ebp, esp
        sub     esp, 8
        mov     eax, DWORD PTR [ebp+8]
        add     eax, 4
        movzx   eax, BYTE PTR [eax]
        cmp     al, 73
        jne     .L2
        mov     eax, DWORD PTR [ebp+8]
        add     eax, 5
        movzx   eax, BYTE PTR [eax]
        cmp     al, 111
        jne     .L2
        mov     eax, DWORD PTR [ebp+8]
        add     eax, 6
        movzx   eax, BYTE PTR [eax]
        cmp     al, 83
        jne     .L2
        sub     esp, 12
        push    OFFSET FLAT:.LC0
        call    printf
        add     esp, 16
        sub     esp, 12
        push    0
        call    exit
.L2:
        sub     esp, 12
        push    OFFSET FLAT:.LC1
        call    puts
        add     esp, 16
        nop
        leave
        ret

Looks very similar to your disassembled code.

This compare-code-part looks very inefficient

Looks like it was compiled without optimizations. With optimizations enabled, gcc compiled the code to:

.LC0:
        .string "Gratz man :)"
.LC1:
        .string "Wrong password"
f(char const*):
        sub     esp, 12
        mov     eax, DWORD PTR [esp+16]
        cmp     BYTE PTR [eax+4], 73
        jne     .L2
        cmp     BYTE PTR [eax+5], 111
        je      .L5
.L2:
        mov     DWORD PTR [esp+16], OFFSET FLAT:.LC1
        add     esp, 12
        jmp     puts
.L5:
        cmp     BYTE PTR [eax+6], 83
        jne     .L2
        sub     esp, 12
        push    OFFSET FLAT:.LC0
        call    printf
        mov     DWORD PTR [esp], 0
        call    exit

Not sure why gcc decided to jump down and back up again instead of a straight line of jnes. Also, the ret is gone, your printf got tail-call-optimized, i.e. a jmp printf istead of a call printf followed by a ret.