[Pure C][x86][RevEng] Function arguments as local variables

From C textbooks you know that you can use function arguments just like local variables. Let's try.

First example:

#include <stdio.h>

void f()
{
        int x, y;
        for (x=0; x<3; x++)
                for (y=0; y<3; y++)
                        printf ("%d %d\n", x, y);
};

void main()
{
        f();
};

Looks innocent! Let's rewrite it:

void f(int x, int y)
{
        for (x=0; x<3; x++)
                for (y=0; y<3; y++)
                        printf ("%d %d\n", x, y);
};

void main()
{
        f(0, 0);
};

Works just like the first function.

Compile both for 32-bit x86 (you may need to install additional GCC deps):

gcc -S -masm=intel -m32 fname.c

Compare (diff) two assembly outputs (some lines omitted):

% diff -u v1.s v2.s
  
...
        mov     ebp, esp
        push    ebx
-       sub     esp, 20
+       sub     esp, 4
        call    __x86.get_pc_thunk.bx
        add     ebx, OFFSET FLAT:_GLOBAL_OFFSET_TABLE_
-       mov     DWORD PTR -16[ebp], 0
+       mov     DWORD PTR 8[ebp], 0
        jmp     .L2
 .L5:
-       mov     DWORD PTR -12[ebp], 0
+       mov     DWORD PTR 12[ebp], 0
        jmp     .L3
 .L4:
        sub     esp, 4
-       push    DWORD PTR -12[ebp]
-       push    DWORD PTR -16[ebp]
+       push    DWORD PTR 12[ebp]
+       push    DWORD PTR 8[ebp]
        lea     eax, .LC0@GOTOFF[ebx]
        push    eax
        call    printf@PLT
        add     esp, 16
-       add     DWORD PTR -12[ebp], 1
+       add     DWORD PTR 12[ebp], 1
 .L3:
-       cmp     DWORD PTR -12[ebp], 2
+       cmp     DWORD PTR 12[ebp], 2
        jle     .L4
-       add     DWORD PTR -16[ebp], 1
+       add     DWORD PTR 8[ebp], 1
 .L2:
-       cmp     DWORD PTR -16[ebp], 2
+       cmp     DWORD PTR 8[ebp], 2
        jle     .L5
...

The only difference is that x/y variables are located on another side of ESP (stack pointer)!

In x64 that picture will be slightly different and somewhat harder to understand. This is why I used 32-bit code here in my example.

(the post first published at 20221225.)


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