VB6 Stubs BS


Author: David Zimmer
Date: 10.31.22 - 10:34am



VB6 binaries have a series of native stubs that get used for various bits of glue. I have identified 8 different styles so far:
Public Enum nativeStubTypes
    st_unknown = 0
    st_callback = 1
    st_jmpImportApi = 2
    st_property = 3
    st_pcodeMethod = 4
    st_apiDeclare = 5
    st_controlEvent = 6
    st_propNewObj = 7
    st_setEvent = 8
End Enum
One downside to old school asm programmers is they werent afraid to get cute. In some cases, they decided to reuse the same base stub, but just tack on a little extra crap as a prefix. But..instead of inserting alignment bytes to keep it tidy, they felt ok with sucking up part of an instruction into an essential nop. You were naughty and You know who you are!
'.text:00401D38 Stub_132:
'.text:00401D38 B8 3C 00 00 00   mov     eax, 3Ch ;   ; Event 39 for UserControl1
'.text:00401D3D 66 3D 33 C0      cmp     ax, 0C033h   <--bullshit so they could multiplex...
'.text:00401D41 BA 14 45 40 00   mov     edx, offset off_404514
'.text:00401D46 68 B6 10 40 00   push    offset MethCallEngine
'.text:00401D4B C3               retn
'
'.text:00401D3F Stub_244:
'.text:00401D3F 33 C0            xor     eax, eax               ;Pcode func_404514
'.text:00401D41 BA 14 45 40 00   mov     edx, offset off_404514
'.text:00401D46 68 B6 10 40 00   push    offset MethCallEngine
'.text:00401D4B C3               retn
I am writing an IDC generator to name stuff and try to get it to disasm properly...but there is no way to get IDA to handle this. nopping the 66 3D.. would ruin the point of the mov eax..so..its basically what they had to do without a jmp

Well at least they did save 9 bytes!

Dont worry I still love you guys, still OG masters in my book. (but i see what you did there!)




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