Push/Pop segmentation fault at Assembly x86

Question

I\'m using elf64 to compile my assembly x86 code: I\'ve this sub-routine:

printNumber:
    mov EAX, EDX ; EDX contain some value like \"35\"         


        

Answer 1

I see a push rdx, but nowhere a pop rdx. You keep pushing values on stack and when you reach RET you return to the address which was previously the content of RDX.

Answer 2

The instruction

push rdx;

can, on itself, only cause a segmentation fault in rather rare cases: When you're running out of stack or when you've messed around with (E)SP. Since you can run to that code, I don't think you did the second, and the first is rather unrealistic, if this is all your application does. But Michael above pointed to the right direction: It's not the push instruction that's causing the segmentation fault but the missing pops before the ret. At the end of your function, the stack must contain exactly the same number of elements as at the beginning, or the ret instruction will read whatever is at the bottom of the stack and try to use it as return address -> bang.

You cannot use the stack to return values this way. You (for instance) need the calling function to allocate memory for the return data and provide it's address as argument. Read up about calling conventions and passing arguments in assembly language.

Answer 3

Shouldn't that be "PUSH EDX" instead of "PUSH RDX"? Or perhaps a PUSH dword ptr 0/PUSH EDX combo if you really need to push 64-bits? Wondering if maybe it ends up being unaligned and doesn't like it? Prior x86 modes don't care about alignment, but perhaps it does for x64 instructions?