Re: about execute assembly exapmles under freebsd

From: Tetsuji \ (maverick31337_at_vfemail.net)
Date: 04/24/05

  • Next message: Tetsuji \: "Re: about execute assembly exapmles under freebsd" 
    Date: Sun, 24 Apr 2005 19:28:21 +0900
To: HHCHANG <b8701143@tmu.edu.tw>

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    HHCHANG wrote:

    > Hi, I couldn't execute assembly example under freebsd . The
    > environment and tools I used were (in IBM X31 box): 1. bash-2.05b#
    > uname -a FreeBSD sexbear.localhost 5.3-STABLE FreeBSD 5.3-STABLE
    > #3: Sun Feb 20 21:55:06 UTC 2005
    > ?root@sexbear.localhost:/usr/obj/usr/src/sys/SEXBEAR??i386 2.
    > bash-2.05b# as -v GNU assembler version 2.15 [FreeBSD] 2004-05-23
    > (i386-obrien-freebsd) using BFD version 2.15 [FreeBSD] 2004-05-23
    >
    > bash-2.05b# ld -v GNU ld version 2.15 [FreeBSD] 2004-05-23
    >
    > 3.(compile and execute the example) bash-2.05b# as -gstabs -o
    > cpuid.o cpuid.s ld -o cpuid cpuid.o cpuid (no output after
    > executing the program) gdb cpuid (gdb) run Starting program:
    > /usr/local/src/code/chap04/cpuid Program exited with code 0340.
    > (gdb) break *_start (gdb) run Program exited with code 0340.
    > ###################example in
    > book################################### #cpuid.s Sample program to
    > extract the processor Vendor ID .section .data output: .ascii "The
    > processor Vendor ID is 'xxxxxxxxxxxx'\n" .section .text .globl
    > _start _start: movl $0, %eax cpuid movl $output, %edi movl %ebx,
    > 28(%edi) movl %edx, 32(%edi) movl %ecx, 36(%edi) movl $4, %eax movl
    > $1, %ebx movl $output, %ecx movl $42, %edx int $0x80 movl $1, %eax
    > movl $0, %ebx int $0x80
    >
    > ###################example in
    > book###################################
    >
    > I viewed the tutorial: http://www.int80h.org/bsdasm/. but I
    > couldn't find any syntax error in the program. Could someone give
    > me a hint where I could find the more information? Thanks~
    >
    > Regards,

    hi,

    There are some mistakes in that code.
    1. Your code is calling systemcall in Linux mode..not in FreeBSD. In
    FreeBSD, you need to push arguments in stack as in C language.
    2. mov $output,%eax loads the "content" of $output, instead of the
    address (or pointer) of $output. So you have to take care :)
    3. An improvement can be done when you want to load 0 (zero) into a
    register, you should use "xor %eax,%eax" or "sub %eax,%eax" because it
    will make your code shorter and faster.

    So I made a working code, t.s;
    - ------t.s------------
    .section .data
    output:
       .ascii "The processor Vendor ID is 'xxxxxxxxxxxx'\n"
    .section .text
    .globl _start
    _start:
       xor %eax, %eax
       cpuid
       lea output, %edi # load address of output in edi
       movl %ebx, 28(%edi)
       movl %edx, 32(%edi)
       movl %ecx, 36(%edi)
       pushl $42
       lea output, %eax
       pushl %eax
       pushl $1
       mov $4,%eax
       push %eax
       int $0x80
       add $16,%esp
       xor %eax,%eax
       push %eax # this is shorter than "pushl $0"
       inc %eax # put $1 in %eax. This is faster and
    shorter.
       push %eax
       int $0x80
    - ----------end of t.s--------

    It is assembled and works like this

    - ------cut--------
    freebsd53:~/tmp% as t.s -o t.o
    freebsd53:~/tmp% ld t.o -o t
    freebsd53:~/tmp% ./t
    The processor Vendor ID is 'GenuineIntel'
    - ------------------

    I made a small homepage about Linux shellcode (assembler code
    utilities for hacking.)
    http://shellcode.4pu.com/

    Have fun!!
    btw my father was born in Taiwan. My last name should be "Lai"
    instead of "Rai"

    - --
    Tetsuji 'Maverick' Rai
    PGP Key fingerprint = 2021 6BF9 CEA3 73DE FF17 B326 F4DA F04E F784 3B85
    gpg fingerprint
    Aviation Jokes: http://www.geocities.com/tetsuji_rai/
    Profile http://maverick.ns1.name/
    http://maverick.IsASecret.com/
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