[llvm-dev] lld-link with MSVC6 object files

Wed Oct 2 00:35:35 PDT 2019

On Tue, Oct 1, 2019 at 8:18 PM Paul Moran <bankybooks at gmail.com> wrote:

> I have the most edge of edge use cases :). I am recovering the lost source
> code to an application built with MSVC 6. However because I want to produce
> byte for byte exact output I need to ensure that the import table is in the
> same order as the original binary.
>

I'm not sure if I follow this part -- if you build an executable using
lld-link and compare it with an executable built with MSVC linker, they are
almost always different. lld-link doesn't attempt to produce the byte-wise
same outputs as MSVC. So, if you want to compare lld-link-produced output,
the other file needs to be built with lld-link too. But is that the case?

Since the MSVC6 linker has no way of doing this I figured I could hack this
> feature into lld-link. I need to also set the PDB path in the debug data
> but a newer version of the MS linker can do this and I believe lld-link
> already supports this too.
>
>
>
> On Tue, Oct 1, 2019 at 8:58 AM Rui Ueyama <ruiu at google.com> wrote:
>
>> Out of curiosity, why do you want to use lld-link with a compiler that
>> was released 20 years ago?
>>
>> On Tue, Oct 1, 2019 at 7:02 AM Zachary Turner via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I would expect it to be able to link the object file, even if it ignored
>>> debug info.  It's a bit strange that it complains about bad file magic.
>>>
>>> It might be tricky to get debug information working and produce a valid
>>> PDB file since that is pretty old and the format has changed both with how
>>> it was stored in the object file itself as well as the format of the PDB
>>> file.
>>>
>>> My guess is that the "magic" it's complaining about is not the magic of
>>> the object file itself but rather the first 4 bytes of the .debug$S (or was
>>> it the .debug$T?) section.  Perhaps a simple fix in this case is that
>>> instead of erroring out if we encounter an "older" magic, we just link as
>>> if debug info was not present to begin with.
>>>
>>> This will at least make it work.  If you want to actually consume the
>>> debug info though, you're in for a fun ride :)
>>>
>>> On Mon, Sep 30, 2019 at 2:19 PM Paul Moran via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> It sounds like perhaps it might mostly work with some tweaks - given
>>>> its complaining about bad file magic. I'll see if I can get lld-link to
>>>> build locally and hack out the magic checks to see if it works.
>>>>
>>>> On Mon, Sep 30, 2019 at 10:14 PM David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Mon, Sep 30, 2019 at 2:07 PM Paul Moran <bankybooks at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> MSVC 6 is 1998 not 1989 :)
>>>>>>
>>>>>
>>>>> Ah, I just glanced briefly at the Wikipedia article (
>>>>> https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B ) & misread
>>>>> the "C 6.0" and didn't notice it was distinct from "Visual C++ 6.0" -
>>>>> thanks for the catch!
>>>>>
>>>>>
>>>>>>
>>>>>> The latest MSVC linker can link these object files. Is this just
>>>>>> because it has support for C13 types and some other code path for whatever
>>>>>> MSVC6 uses? After some digging around it appears to be this format:
>>>>>>
>>>>>>
>>>>>> https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#coff-file-header-object-and-image
>>>>>>
>>>>>>
>>>>>> Which is COFF object file format? Does lld link support this format?
>>>>>>
>>>>>
>>>>> COFF is still the windows object file format, and the Windows support
>>>>> in lld is COFF support, yeah. I guess there might be some format variations
>>>>> that haven't been implemented in lld, though. It's mostly an "on demand"
>>>>> sort of approach.
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>> On Mon, Sep 30, 2019 at 7:39 PM Alexandre Ganea <
>>>>>> alexandre.ganea at ubisoft.com> wrote:
>>>>>>
>>>>>>> The CodeView library in LLVM only supports Codeview C13 types, that
>>>>>>> is, MSVC 7.0 / Visual Studio 2002 or after.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> *De :* llvm-dev <llvm-dev-bounces at lists.llvm.org> *De la part de*
>>>>>>> David Blaikie via llvm-dev
>>>>>>> *Envoyé :* September 30, 2019 2:38 PM
>>>>>>> *À :* Paul Moran <bankybooks at gmail.com>; Rui Ueyama <ruiu at google.com
>>>>>>> >
>>>>>>> *Cc :* llvm-dev at lists.llvm.org
>>>>>>> *Objet :* Re: [llvm-dev] lld-link with MSVC6 object files
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> MSVC 6 as in the Visual Studio released in 1989? Yes, I imagine
>>>>>>> that's a bit outside the intended support window.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Sep 30, 2019 at 11:18 AM Paul Moran via llvm-dev <
>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I have a question about lld-link. What obj file formats should it
>>>>>>> support? When I try to use an obj from msvc 6.0 it complains that the file
>>>>>>> magic is not valid.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> However when running  llvm-objdump it reports:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> test1.obj:      file format COFF-i386
>>>>>>>
>>>>>>> Disassembly of section .text:
>>>>>>> 0000000000000000 _main:
>>>>>>>        0:       68 00 00 00 00  pushl   $0
>>>>>>>        5:       e8 00 00 00 00  calll   0 <_main+0xa>
>>>>>>>        a:       83 c4 04        addl    $4, %esp
>>>>>>>        d:       33 c0   xorl    %eax, %eax
>>>>>>>
>>>>>>>       f:       c3      retl
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Paul
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org
>>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>
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>>
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