Segfault when doing syscall using GNU assember

Question

I'm trying to understand why as behaves differently than nasm when doing syscalls on the assembly level. Because I'm a glutton for punishment, I'm using Intel syntax. Here's my program:

    .intel_syntax noprefix
    .section    .rodata
.LC0:
    .string "Hello world!\n"
    .text
    .globl  _start
    .type   _start, @function
_start:
    mov edx, 13
    mov ecx, OFFSET FLAT:.LC0
    mov eax, 4
    int 0x80
    ret

I assemble with as -o prog.o prog.s and link with ld -s -o prog prog.o.

But when I run it, I get:

$ ./prog 
Hello world!
Segmentation fault (core dumped)

GDB is not particularly helpful here. When I stepi on ret, it says Cannot access memory at address 0x1. Which is puzzling, because the value of ESP is:

(gdb) info registers esp
info registers esp
esp            0xbffff660       0xbffff660

Why does this program segfault?

You can't just ret from your program, you need an exit system call. The 1 is the number of arguments (argc) which happens to be on the top of the stack, so ret tries to use it as an address with obvious consequences. — Jester
– Jester, Commented Nov 7, 2017 at 23:52

user149341 · Accepted Answer · 2017-11-08 00:01:37Z

2

Because it never exits properly. _start doesn't have a parent stack frame, so returning from it will cause a crash.

You can return from main to have the standard library's _start implementation call exit for you, but if you're writing your own _start, you need to call exit yourself, as there's no parent stack frame to return to.

answered Nov 8, 2017 at 0:01

user149341

Sign up to request clarification or add additional context in comments.

5 Comments

Michael Petch Over a year ago

@DanBarowy Mostly copied? I'm venturing the mostly may be the problem. If ret was working then _start wouldn't be the entry point, main would be. And the way you link it would be different since you need the C runtime. What page/example in the book did you mostly copy from?

Michael Petch Over a year ago

My last comment is to a now deleted comment it seems ;-)

user149341 Over a year ago

@DanBarowy I'd go a step further: that book is simply crap, and should be discarded. It reads like it was written for DOS programmers, then hastily adapted to Linux at the last minute. (And many of the Linux details are seriously outdated, as the book was published in 2000…)

user149341 Over a year ago

Also, a hilarious detail: edlinas, the "no.1 Linux x86 hands-on assembler development simulator" (according to the cover, at least) is actually a QBasic program, and can only be run under DOS.

Dan Barowy Over a year ago

Yes @MichaelPetch and @duskwuff, I realized that I had replaced main with _start after I posted the comment, which lead to the aha moment where I realized that I was the idiot here. So I deleted the comment.

Collectives™ on Stack Overflow

Segfault when doing syscall using GNU assember

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related