How to solve segmentation fault of certain instructions of assembly code?

I wrote some simple assembly code like below:

.global _start

.text
_start:
    call _sum
    subq $0x8, %rsp
    popq %rax
    ret
_sum:
    ret

In order to get the value of %rax after 'popq' instruction,

I assembled that code using 'as' and 'ld' command. and I started gdb debugger by putting break point at '_start'

and the result comes like below:

B+> │0x400078 <_start>       callq  0x400083 <_sum>                            
│   │0x40007d <_start+5>     sub    $0x8,%rsp                                  
│   │0x400081 <_start+9>     pop    %rax                                       
│   │0x400082 <_start+10>    retq                                              
│   │0x400083 <_sum>         retq

However, before going into pop instruction, There comes an error message saying that

Program received signal SIGSEGV, Segmentation fault. Cannot access memory at address 0x1

(when I changed the $0x8 into $0x0~$0x7, it all worked.)

It seems like at the first stage the sum function might be the problem. because It literally does nothing but return.

So, How can I modify this code to get the value of %rax after the popq instruction?

Thanks.

Solution

I think probably this question is a duplicate, but anyway, there is one problem in your code.

.global _start

.text
_start:
    call _sum
    subq $0x8, %rsp
    popq %rax
    ret # <-- return to where?
_sum:
    ret

A main in C ~~has to~~ can return because _start eventually calls main, but here, you are writing _start directly. It returns to nowhere if you put a ret.

In place of ret, put this instead.

movl $60, %eax # syscall number for sys_exit
movl $0, %edi # or whatever value you want your process
             # to return with from 0 to 255;
             # xor %edi, %edi is usually better if you want 0
syscall