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Full Discussion: Why does this example C code run and yet SHOULD either not compile or give a segmentation fault?
Top Forums Programming Why does this example C code run and yet SHOULD either not compile or give a segmentation fault? Post 303033023 by wisecracker on Thursday 28th of March 2019 03:21:04 PM
Old 03-28-2019
Apologies if this attaches to the previous post.
As a finale to this thread I decided to compile using the correct 'asm()' function __asm__ volatile() .
Code:
/* Real obfuscation scenario. */

#include <stdio.h>

int addition(int NUM1, int NUM2);
int obfuscate(int NUM1, int NUM2);

int main()
{
    int NUMBER1;
    int NUMBER2;
    int SUM;

    __asm__ volatile("nop;"
    "jmp    out;"
    "in:"
    "nop;");

    printf("Enter two integer numbers:- ");
    scanf("%d %d", &NUMBER1, &NUMBER2);

    SUM = addition(NUMBER1, NUMBER2);

    printf("SUM = %d.\n", SUM);

    return(0);
}

int addition(int NUM1,int NUM2)
{
    int RESULT;
    RESULT = NUM1 + NUM2;
    return RESULT;
}

int obfuscate(int NUM1,int NUM2)
{
    int RESULT;

    __asm__ volatile("jmp    getout;"
    "nop;"
    "out:"
    "nop;"
    "jmp    in;"
    "getout:"
    "nop;");
    /* Any number of these jimps could be used to hide stuff. */

    RESULT = NUM1 / NUM2;
    return RESULT;
}

Results, OSX 10.14.3, default bash terminal, gcc 4.2.1.
The results are similar for AMIGA OS 3.0.x gcc 2.95.3 and Linunx Mint 19 gcc 7.3.0.
Code:
Last login: Thu Mar 28 18:58:36 on console
AMIGA:amiga~> cd Desktop/Code/C
AMIGA:amiga~/Desktop/Code/C> gcc -Wall -ansi obfuscate_asm.c -o obfuscate_asm
AMIGA:amiga~/Desktop/Code/C> ./obfuscate_asm
Enter two integer numbers:- 123 456
SUM = 579.
AMIGA:amiga~/Desktop/Code/C> hexdump -C obfuscate_asm
........
00000e90  55 48 89 e5 48 83 ec 20  c7 45 fc 00 00 00 00 90  |UH..H.. .E......|
00000ea0  e9 8b 00 00 00 90 48 8d  3d cb 00 00 00 b0 00 e8  |......H.=.......|
00000eb0  92 00 00 00 48 8d 3d da  00 00 00 48 8d 75 f8 48  |....H.=....H.u.H|
00000ec0  8d 55 f4 89 45 ec b0 00  e8 7f 00 00 00 8b 7d f8  |.U..E.........}.|
00000ed0  8b 75 f4 89 45 e8 e8 25  00 00 00 48 8d 3d b9 00  |.u..E..%...H.=..|
00000ee0  00 00 89 45 f0 8b 75 f0  b0 00 e8 57 00 00 00 31  |...E..u....W...1|
00000ef0  f6 89 45 e4 89 f0 48 83  c4 20 5d c3 0f 1f 40 00  |..E...H.. ]...@.|
00000f00  55 48 89 e5 89 7d fc 89  75 f8 8b 75 fc 03 75 f8  |UH...}..u..u..u.|
00000f10  89 75 f4 8b 45 f4 5d c3  0f 1f 84 00 00 00 00 00  |.u..E.].........|
00000f20  55 48 89 e5 89 7d fc 89  75 f8 e9 07 00 00 00 90  |UH...}..u.......|
00000f30  90 e9 6f ff ff ff 90 8b  45 fc 99 f7 7d f8 89 45  |..o.....E...}..E|
00000f40  f4 8b 45 f4 5d c3 ff 25  c4 00 00 00 ff 25 c6 00  |..E.]..%.....%..|
00000f50  00 00 00 00 4c 8d 1d ad  00 00 00 41 53 ff 25 9d  |....L......AS.%.|
00000f60  00 00 00 90 68 00 00 00  00 e9 e6 ff ff ff 68 0e  |....h.........h.|
00000f70  00 00 00 e9 dc ff ff ff  45 6e 74 65 72 20 74 77  |........Enter tw|
00000f80  6f 20 69 6e 74 65 67 65  72 20 6e 75 6d 62 65 72  |o integer number|
00000f90  73 3a 2d 20 00 25 64 20  25 64 00 53 55 4d 20 3d  |s:- .%d %d.SUM =|
00000fa0  20 25 64 2e 0a 00 00 00  01 00 00 00 1c 00 00 00  | %d.............|
........
AMIGA:amiga~/Desktop/Code/C> _

Voila! No error but the jumps are still there.
 

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LEARN ABOUT LINUX

inw_p

OUTB(2) 						     Linux Programmer's Manual							   OUTB(2)

NAME

       outb, outw, outl, outsb, outsw, outsl, inb, inw, inl, insb, insw, insl, outb_p, outw_p, outl_p, inb_p, inw_p, inl_p - port I/O

DESCRIPTION

       This  family  of  functions  is	used  to do low-level port input and output.  The out* functions do port output, the in* functions do port
       input; the b-suffix functions are byte-width and the w-suffix functions word-width; the _p-suffix functions pause until the I/O completes.

       They are primarily designed for internal kernel use, but can be used from user space.

       You compile with -O or -O2 or similar.  The functions are defined as inline macros, and will not be  substituted  in  without  optimization
       enabled, causing unresolved references at link time.

       You  use  ioperm(2)  or	alternatively  iopl(2) to tell the kernel to allow the user space application to access the I/O ports in question.
       Failure to do this will cause the application to receive a segmentation fault.

CONFORMING TO

       outb() and friends are hardware-specific.  The value argument is passed first and the port argument is passed second, which is the opposite
       order from most DOS implementations.

SEE ALSO

       ioperm(2), iopl(2)

COLOPHON

       This  page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    1995-11-29								   OUTB(2)
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