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cgccassemblyx86-64reverse-engineering

Redundant register values in GCC-compiled binary?

Alright, so I'm trying to learn reverse engineering and x64 assembly. As an example I wrote this little test program in C:

#include <stdio.h>

int square(int num) {
        return num * num;
}

int main() {
        int ans = square(5);
        printf("%d", ans);
}

This results in the following assembly code for the square function:

push   rbp
mov    rbp,rsp
mov    DWORD PTR [rbp-0x4],edi
mov    eax,DWORD PTR [rbp-0x4]
imul   eax,eax
pop    rbp
ret

This seems kind of weird to me, as edi is the function argument, which I believe is then stored on the stack and loaded back into eax, where it is then multiplied? Why not skip lines 3 and 4 and just imul edi, edi?
So I opened up radare2 and did just that, but now the program returns seemingly random numbers, I'm guessing memory addresses?

radare2 screenshot + patched binary running

Can someone explain to me why GCC uses this seemingly redundant register, and what I did wrong trying to patch the binary?

Solution

  • You didn't enable optimizations, so you're looking at debug code. With -O3, the output is:

    enter image description here

    One note about your question, you need to assign to eax, because eax stores the return value in an x86 application.