GNU Assembler, function call using stack, seg fault

Question

I have a problem with calling a function, which uses stack. The output of the program contains a lot of trash, it's like length of the string is wrong:

.code32

.equ kernel, 0x80
.equ stdout, 0x01
.equ write, 0x04
.equ exit, 0x01

.section .data
sum:
    .ascii "text"
.equ lensum, . - sum

.section .text

    .type writetxt, @function
writetxt:
    movl $write, %eax
    movl $stdout, %ebx
    popl %ecx
    pop %edx
    int kernel
    ret

.global _start

_start:
    pushl $lensum
    pushl $sum
    call writetxt

    movl $exit, %eax
    movl $0, %ebx
    int $kernel

I know that it is something with the function return address residing on stack, but i have no clue how to fix it

Michael · Accepted Answer · 2014-06-05 20:04:57Z

1

As you've noted yourself, the return address will be at the top of the stack (i.e. at (%esp)) as you enter writetxt. The last 32-bit value you pushed before the call will be at 4(%esp), and so on.

So instead of

popl %ecx
popl %edx

you should use something like

movl 4(%esp), %ecx
movl 8(%esp), %edx

Don't forget to adjust the stack pointer afterwards. For example, you could place an addl $8, %esp after the call writetext instruction.

answered Jun 5, 2014 at 20:04

Michael

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