[cfe-dev] Calling virtual elf functions under windows -> Adding ASM code

[Lang Hames via cfe-dev]

Hi Bjoern,

It looks like you are casting a regular function pointer (returned from
getFunctionAddress) to a virtual method pointer. This is undefined behavior.

The easiest way to achieve the effect you want would be to add a trampoline
to your source (or at the IR level):

extern "C" SimpleResult call_Interface_init(Interface *Instance) { return
Instance->init(); }

Then you can just use:

auto CallInit =
(SimpleResult(*)(Interface*))engine->getFunctionAddress("call_Interface_init");
CallInit(inter);

Cheers,
Lang.


On Tue, Feb 20, 2018 at 1:42 AM, via cfe-dev <cfe-dev at lists.llvm.org> wrote:

> Still no progress with this problem. Only that non-member functions seems
> to be working...
> Also:
> Even when I set the target triple of the Module (when the file was parsed
> at runtime) to "COFF" nothing changes...
>
>
> From:        Bjoern Gaier/HE/HORIBA
> To:        llvm-dev at lists.llvm.org
> Cc:        cfe-dev at lists.llvm.org
> Date:        08.02.2018 12:28
> Subject:        Re: Calling virtual elf functions under windows -> Adding
> ASM code
> ------------------------------
>
>
> Hello everyone,
>
> I'm sorry for "flooding" your inbox, but I wanted to extend my message
> which is shown below. The problem is still not solved and I'm more confused
> then before.
> I compiled the code - the one seen in the pictures  to elf bc and to msvc
> (pecoff) bc. Then I used llc to generate assembler output.
>
> In my opinion the involved functions are acting exactly the same. There
> are only some registers swapped. I should mention: when using the msvc bc
> code, then the calls via the interface are working.
>
> I attach the assembly code of the involved code to. Could please anyone
> explain me the difference, which could cause the crash?
>
> Kind regards
> Björn
>
>
>
>
> From:        Bjoern Gaier/HE/HORIBA
> To:        llvm-dev at lists.llvm.org, cfe-dev at lists.llvm.org
> Date:        07.02.2018 11:47
> Subject:        Calling virtual elf functions under windows
> ------------------------------
>
>
> Hello everyone reading this,
>
> I'm totally confused and need an explanation. I'm also not sure if it's a
> clang or a LLVM subject.
>
> I'm working with Visual Studio 2015, LLVM 5.0.1 and Windows 7 64bit.
>
> I have a simple project. With 2 Header files and 1 cpp file.
> "Interface.h"
> This file defines a simple class with two pure virtual member functions.
>
> "SimpleResult.h"
> This file defines a struct holding an integer
>
> "CM_Elf.cpp"
> This file defines a class (CM_Elf) implementing the interface from
> Interface.h
> Also there is an instance of the class (named "elf").
>
> I compile the CM_Elf.cpp to a CM_Elf.bc file via clang-cl but with target
> "x86_64-pc-windows-elf"
>
>
> In another application I use LLVM to parse the BC file and use the
> ExecutionEngine to get some addresses.
> What I want to archive is:
> I want to call the implemented member functions of "elf".
>
> 1.) I execute all the global constructors collected by the ExecutionEngine
> -> The constructor of elf gets called
> 2.) I use ExecutionEngine to get the address of "elf"
> 3.) I use the interface to execute Interface::init
> -> Init gets called
> 4.) I use the interface to execute Interface::shutdown
> -> CRASH!
>
> When I swap the calls to init and shutdown, then everything works. So the
> virtual call to init seems to corrupt my stack - maybe it's the return
> value. Could it be a calling convention problem? But my target is still
> "windows" only msvc changed to elf...
> When I compile CM_Elf for target "x86_64-pc-windows-msvc19.0.24215" then
> it works.
>
> When I get the address of CM_Elf::init and CM_Elf::shutdown via the
> ExecutionEngine and call them without using the virtual table, then again
> everything works.
>
> SO! Is it not possible to call virtual elf functions via there interface?
> But why does normal functions work? Also I reach the functions which are
> stored in the vTable of CM_Elf so the vTable is correct.
>
> I append some pictures of the code... I hope someone could help me with
> this.
>
> Kind regards
> Björn
>
>
>
>
>
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>
>
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789
> Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode,
> Heiko Lampert, Hiroshi Kawamura, Takashi Nagano, Takeshi Fukushima.
>
>
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