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From: Daniela <dgw@liwest.at>
To: ari <edelkind-freebsd-hackers@episec.com>
Date: Tue, 2 Mar 2004 22:10:31 +0000
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cc: hackers@freebsd.org
Subject: Re: Strange behaviour in assembly language program
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On Tuesday 02 March 2004 20:15, ari wrote:
> dgw@liwest.at said this stuff:
> > Finally I came up with the simplest ASM program that reproduces the
> > error. Here it is:
> >
> > .text
> > .global _start
> > _start:
> > 	pushl	$0
> > 	movl	$1, %eax
> > 	int	$0x80
> >
> > I looked everywhere (Developer's handbook, Google, ...) to find the
> > solution, but all resources I consulted tell me this is the right way to
> > do it. This program, however, always exits with 1 regardless of the value
> > I push.
>
> .text
> .global _start
> _start:
>         pushl   $8
>         pushl   $0
>         movl    $1, %eax
>         int     $0x80

With this suggestion, it always returns 0 instead of 1.
Shouldn't pushl place 4 bytes on the stack? It translates into the instruction 
0x6A (pushes only one byte).
BTW, when I assemble it with as(1), there is always an extra instruction after 
my code, and it's a different one each time (and it's always one that 
effectively does nothing). Who ordered that? Is it because of alignment 
constraints in the ELF file?