[fasmg] Getting Mach-O section offset

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adragomir

Joined: 04 Feb 2015
Posts: 2
Location: Romania

adragomir 04 Jun 2025, 12:56

I am trying to add a new style of `entry` to the current `macho.inc` format for Mach-O x64. The "LC_UNIXTHREAD" command is deprecated, and I am trying to replace it with a LC_MAIN command, but the problem is that I don't know how to get the parameter value for the entry point LC_MAIN command, which points to the start point in the format "file (__TEXT) offset of main()"

I have tried something like

Code:

macro entry2? regs&
  MachO.command LC_MAIN
    dq regs ; entryoff - this is wrong, I need the address from the start of __TEXT, and I don't know how to get it. 
    dq 0 ; initial stack size
  MachO.text
end macro

I have tried multiple ways of saving the section start in the file to a variable, of trying to load from it (load value from MachO:MachO.segment#1#_section#1.offset), but I guess I do not really understand the code in the macho.inc file.

Any help would be appreciated.

04 Jun 2025, 12:56

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8427
Location: Kraków, Poland

Tomasz Grysztar 04 Jun 2025, 13:35

Try this:

Code:

postpone
        repeat MachO.SEGMENT_NUMBER
                load SEGNAME from MachO:MachO.segment#%.segname
                if SEGNAME = '__TEXT'
                        load VMADDR from MachO:MachO.segment#%.vmaddr
                        __TEXT_VMADDR := VMADDR
                        break
                end if
        end repeat
end postpone

It defines a __TEXT_VMADDR constant. Is that what you were looking for?

04 Jun 2025, 13:35

adragomir

Joined: 04 Feb 2015
Posts: 2
Location: Romania

adragomir 12 Jun 2025, 18:47

Yes ! This worked. I added this postpone statement somewhere and then I do something like

Code:

macro entry2? regs&
  MachO.command LC_MAIN
    dq regs - __TEXT_VMADDR ; entryoff
    dq 0 ; initial stack size
  MachO.text
end macro

Thank you so much !

12 Jun 2025, 18:47

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