How to compile a file with extrn?

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jorido

Joined: 23 Jan 2017
Posts: 53

jorido

Say, I want to use this http://man7.org/linux/man-pages/man3/socket.3p.html or this http://man7.org/linux/man-pages/man3/getaliasent_r.3.html function or both:

Code:

extrn socket
extrn getaliasent_r

How should I compile and link a source assembly file in this case?

And how do I know where those functions are defined so that I'll be able to link those files to my assembly file?

31 Jan 2017, 16:34

zhak

Joined: 12 Apr 2005
Posts: 501
Location: Belarus

zhak

There's an example included in fasm package: libcdemo or elfexe/dynamic

31 Jan 2017, 18:54

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

This?

Code:

interpreter '/lib64/ld-linux-x86-64.so.2'
needed 'libc.so.6'
import printf,exit

But where is "extrn"? And what if my file format isn't "executable 3" either?

31 Jan 2017, 21:47

system error

Joined: 01 Sep 2013
Posts: 670

system error

"extrn" is for static linking
"import" is for dynamic linking

dynamic linking results in direct executable. Linking is done internally by FASM. Thus, "executable 3" is required.

Code:

format elf64 executable 3
include 'import64.inc'
entry main


interpreter '/lib64/ld-linux-x86-64.so.2' 
needed 'libc.so.6' 
import printf,exit


segment readable writeable
hello db 'Hello World',0ah,0


segment readable executable
main:

        mov rdi,hello
        call [printf]       ;dynamic linking to printf in memory


        mov rdi,0
        call [exit]

Didn't test it, but should work.

01 Feb 2017, 02:01

system error

Joined: 01 Sep 2013
Posts: 670

system error

I've read your other posts and I take it that you come from NASM background. Well, this is one area where NASM is not familiar with - dynamic linking. Get used to it and welcome to FASM.

01 Feb 2017, 02:10

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

Thanks, how about static linking?

01 Feb 2017, 04:25

system error

Joined: 01 Sep 2013
Posts: 670

system error

For static linking, the step is the same as with NASM. If you're looking for source comparison, here's the static version

Code:

format ELF64
public main

extrn printf

section '.data' writeable
hello db 'hello world',0ah,0

section '.code' executable
main:

        mov     rdi,hello
        call    printf
        ret

;link against: gcc -m64 yourfile.o -o yourfile

I didn't test this bcoz I don't have linux at this moment, but the code should work. Even if it doesn't, just pay attention to the structural differences (section, format, public etc)

01 Feb 2017, 13:52

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

Why not ld for linking?

Is this a correct way?

Code:

ld hello.o -dynamic-linker /lib64/ld-linux-x86-64.so.2 -lc -melf_x86_64

01 Feb 2017, 14:50

system error

Joined: 01 Sep 2013
Posts: 670

system error

jorido

if you're linking to dynamic library, just use the first example.

if you're linking to static library, you can use pretty much anything you know of using.

asking people about static and extrn thingies and yet using --dynamic-linker switch is not obviously smart from source structure point of view, because dynamic and static sources are different at the source code level. (segment vs section, ELF64 vs ELF64 executable 3, public vs entry etc).

If you trying to force NASM's way of doing things upon FASM, then you'll not learn anything new today.

01 Feb 2017, 15:38

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

So why not ld?

01 Feb 2017, 16:05

system error

Joined: 01 Sep 2013
Posts: 670

system error

@jorido

If I use ld, then I cannot use "format ELF64 executable 3".

I don't need "ld" to create an executable directly via FASM to access "libc.so.6". Why should I?

01 Feb 2017, 16:19

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

Alright.
What if there were other "so" library, would you still be able to use gcc?

Also:
include 'import64.inc' error: file not found.

Where should I get it?

01 Feb 2017, 18:15

system error

Joined: 01 Sep 2013
Posts: 670

system error

import64.inc is in the /examples/elfexe/dynamic/

you'd probably need "elf.inc" as well. Just make sure you have these two available in a the same path every time you're compiling.

In the examples folder, there are also examples of using both ld and gcc if you wished to perform static linking instead (like you always do in NASM).

Good luck

01 Feb 2017, 18:46

jorido

Joined: 23 Jan 2017
Posts: 53

jorido

thanks.

01 Feb 2017, 18:58

moveax41h

Joined: 18 Feb 2018
Posts: 59

moveax41h

I got a question here for Linux. I'm getting an error from fasm saying this is not a legal instruction:

Code:

interpreter '/lib64/ld-linux-x86-64.so.2' 
needed 'libc.so.6'

On Windows, I used to do something like

Code:

library 'kernel32.dll',
import 'printf', \
'scanf'

etc

I want to dynamically link those libc functions in on Linux.

30 Jul 2018, 20:01

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18109
Location: In your JS exploiting you and your system

revolution

"interpreter" and "needed" are macros, not instructions or directives. You will need to have some other macro file included to process those two lines.

30 Jul 2018, 21:15

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