Date: Wed, 3 Mar 2004 11:26:32 -0500 From: ari <edelkind-freebsd-hackers@episec.com> To: Daniela <dgw@liwest.at> Cc: hackers@freebsd.org Subject: Re: Strange behaviour in assembly language program Message-ID: <20040303162632.GC50518@episec.com> In-Reply-To: <200403022210.31451.dgw@liwest.at> References: <200403022046.22882.dgw@liwest.at> <20040302201554.GA50518@episec.com> <200403022210.31451.dgw@liwest.at>
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dgw@liwest.at said this stuff:
> > .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).
32-bit, 80386-based processors cannot push one byte onto the stack; they
can push only in 2- or 4-byte increments (word or double-word). While
instruction 0x6a pushes an immediate one-byte value, this is only to
save instruction space. The number is in fact pushed as a 32-bit
("sign-extended") value.
6a 08
should have the same effect as
68 08 00 00 00
On freebsd, using a native binary format, the above sample should return
8. It works properly on any system that i've checked. I'd be
interested in seeing your compiled binary if yours doesn't.
> 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?
Each section must be aligned on a 4-byte boundary (this is not specific
to ELF). This can be duplicated by adding
.align 4
as the last instruction. Because the text section is intended for
executable code, as(1) offers non-operation instructions (which should
be unnecessary in any situation where a programmer doesn't know what
he's getting himself into). Newer versions of gnu as(1) seem to pad
this with zeros, which you can duplicate with:
.align 4, 0
ari
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