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cmd/cgo: cgo_import_dynamic doesn't work on PE dlls (Windows) #9095

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steeve opened this Issue Nov 12, 2014 · 3 comments

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@steeve
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steeve commented Nov 12, 2014

go version go1.4beta1 linux/amd64

With some make-fu is it possible to have SWIG+CGO build a DLL out of a package. This
fixes the issue of external linking not available in Windows.

Basically, applying the following patches to SWIG does the following:
- Mark all structs as __packed__ [1]
- Bind crosscall2 and _cgo_* via DllMain (instead of extern) [2]
- Mark all wrap functions as SWIGEXPORT [3]
- Mark all the functions in the _gc.c file as dynimport name name "mylib.dll"

Once the file is generated/compiled, to get a .dll, I do the following:
- unpack the library
- repack the library minus the _wrap object file
- cc -shared the _wrap object file into a .dll (with linked needed libraries)

Everything does compile after that with no issues.

The issue is that cgo_import_dynamic doesn't properly resolve the symbol in the DLL, but
in the IAT section of the .exe file. This causes the executing program to crash.

I traced the program with IDA and confirmed that. See the thread of go-nuts with lots of
additional informations and IDA outputs [4].

I have no idea why C code using this pragma works fine, but Go code using the variable
generated to the symbol doesn't. Maybe a bug in the PE linker?

Thank you

[1] steeve/swig@e70fce1
[2] steeve/swig@27bb9bd
[3] steeve/swig@2c55397
[4] https://groups.google.com/forum/#!topic/golang-nuts/-M-8X-PLvOw
@ianlancetaylor

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ianlancetaylor commented Nov 12, 2014

Comment 1:

Labels changed: added repo-main, release-none, os-windows.

@alexbrainman

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alexbrainman commented Nov 26, 2014

Comment 2:

>> ... The issue is that cgo_import_dynamic doesn't properly resolve the symbol in the
DLL, but in the IAT section of the .exe file. This causes the executing program to crash.
I am not clear about what you are trying to do. Please, provide a small program to
demonstrate. Something I can try here. Thank you.
Alex
@minux

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minux commented Nov 26, 2014

Comment 3:

I don't think that could work around the issue of not having external linking support.
If you are that familiar with internals of PE/COFF, I'd suggest you tackle the external
linking support, rather than make very complicated workaround that is not guaranteed
to work with future versions of Go.
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