How to automatically detect the OS at compile time

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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 18 May 2024, 16:56

Code:

virtual
        file "%ComSpec%%SHELL%"
        load OS_byte byte from $$
end virtual
if OS_byte = 'M'
        display 'Windows OS'
        format pe ...
else if OS_byte = 0x7f
        display 'Linux OS'
        format elf ...
else
        display 'Unknown OS'
        err
end if

The above code can automatically show the running OS without needing to set any values, or make a special included file that defines the OS.

It works because fasm will replace any unset environment variables with null.

When run in Windows, only "ComSpec" is expanded and cmd.exe is loaded.

When run in Linus, only "SHELL" is expanded and /bin/bash (or whatever your shell is) is loaded.

Then the first byte is examined to determine the file type. "M" is the DOS stub for Windows, and 0x7f is the ELF header for Linux.

Then you can have OS dependant code paths be assembled as needed.

18 May 2024, 16:56

macomics

Joined: 26 Jan 2021
Posts: 1062
Location: Russia

macomics 18 May 2024, 19:25

Then it's worth checking all 'magic' in header

It is also worth considering the case of missing values in both ComSpec and SHELL, or an incorrect value in these variables

Code:

virtual
        file "%ComSpec%%SHELL%"
    if $ > 16
        load OS_word word from $$
        load OS_dword dword from $$
    end if
end virtual
if OS_word = 'MZ'
        display 'Windows OS'
        format pe ...
else if OS_dword = 0x7f or ( 'ELF' shl 8 )
        display 'Linux OS'
        format elf ...
else
        display 'Unknown OS'
        err
end if

Code:

$ touch ./zero.txt; env SHELL="./zero.txt" fasm ostype.asm
flat assembler  version 1.73.32  (16384 kilobytes memory, x64)
Unknown OS
ostype.asm [16]:
        err
processed: err
error: error directive encountered in source file.

$ env ComSpec="./x96dbg.exe" env SHELL="" fasm ostype.asm
flat assembler  version 1.73.32  (16384 kilobytes memory, x64)
Windows OS
1 passes, 512 bytes.

$ fasm ostype.asm
flat assembler  version 1.73.32  (16384 kilobytes memory, x64)
Linux OS
1 passes, 268 bytes.

$ echo M>./test.txt; env SHELL="./test.txt" fasm ostype.asm
flat assembler  version 1.73.32  (16384 kilobytes memory, x64)
Unknown OS
ostype.asm [16]:
        err
processed: err
error: error directive encountered in source file.

18 May 2024, 19:25

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 19 May 2024, 04:01

Under WINE it fails because WINE imports the SHELL value, and also sets the ComSpec value for itself. So the end result is both values run together and it gives this mess:

Code:

C:\windows\system32\cmd.exe/bin/bash

19 May 2024, 04:01

Furs

Joined: 04 Mar 2016
Posts: 2595

Furs 19 May 2024, 19:55

Well tbh the solution is too much of a hack since you can just export those envs vars yourself, or change them even.

19 May 2024, 19:55

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 20 May 2024, 01:57

I think it is also a hack to have to create, maintain and update a separate file for each system.

Code:

; filename OS.inc

OS_Unknown      = -1
OS_Linux        = 0
OS_Windows      = 1
OS_Mac          = 2
OS_Unix         = 3
OS_DOS          = 4
OS_QNX          = 5
OS_CPM          = 6
OS_OS2          = 7
OS_Menuet       = 8

; ========================================================================
; IMPORTANT !!!!! change the line below to match the system you have !!!!!
; ========================================================================

OS = OS_        ; <--- complete this line before using !!!!!

If you are deploying from a central system then updating and distributing become awkward and annoying.

20 May 2024, 01:57

Furs

Joined: 04 Mar 2016
Posts: 2595

Furs 20 May 2024, 15:34

Usually you pass it as a command line arg or something along those lines to a compilation script. Or an env var, but one explicitly set for this purpose, not having other side-effects.

Note that there's no guarantee you're even assembling for your own platform in the first place.

20 May 2024, 15:34

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20513
Location: In your JS exploiting you and your system

revolution 20 May 2024, 15:57

If that works for you then great.

I find it extremely convenient to just go to a customer's machine (with whatever OS they might have) and type "fasm source.asm" and done.

Then there is no need to fiddle with command line options, or have to select the correct script, or make a custom OS dependant file, etc. All those things are easy to make mistakes, or forget to do.

20 May 2024, 15:57

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