[llvm-dev] Clang/LLVM JIT - When to use "registerEHFrames()"

Fri Sep 29 10:43:06 PDT 2017

Hi Bjoern,

I'm trying to make exceptions run. I have an Object file with a function,
> throwing a 1 and a second function which should catch the 1. Normal JITTING
> under Windows showed me, that I have an unresolved reference to the virtual
> table of type_info. Some experiments later I was able to load "msvcrt.lib"
> as an archive and could resolve the reference. Nice - but than
> "??_Etype_info@@UEAPEAXI at Z" was missing too. Ufff! I decided to ignore
> this. Because, when I try to load every .lib and .obj provided by Visual
> Studio, I get an assertion failure with "Relocation type not implemented
> yet!".

RuntimeDyldCOFF is missing a lot of relocation support. We need a COFF
expert to fix that and unfortunately I'm not one.

I decided to have a look at "_CxxThrowException". I inserted my own
> function for the JIT and had a look at the parameters. I got two of them.
> The first was the address of the Exception-Object, which was correct. The
> second is the address for the "_ThrowInfo". This address was valid too, but
> all its members - except from attributes - are null. So I can't throw this
> Exception. I tried to pass the address of typeid(1) to it, or modificate
> the call. Nothing helped.

I don't know windows exception handling well, but if it's anything like
DWARF EH then I'd be inclined to blame the missing relocations/fixups --
the _ThrowInfo struct (or whatever data-source ultimately populates it)
probably isn't being fixed up.

I have no clue and no idea anymore. So... Do you have an idea?

I'm afraid I don't personally. We need some windows linker / system experts
to take an interest in the JIT.

Back then I tried to solve a related issue: SEH exceptions thrown from code
> loaded with RuntimeDyld had to be caught in statically compiled code. It
> turned out Windows explicitly prohibits this. I got in touch with Microsoft
> people and IIRC it's due to security concerns.

Stefan -- That's an interesting restriction. :/
Does it prohibit exceptions thrown in JIT'd code from being caught also in
JIT'd code, or does it only apply if an exception crosses the boundary back
into statically compiled code?
Do you know how they were enforcing that?

Cheers,
Lang.

On Thu, Sep 28, 2017 at 12:37 PM, Stefan Gränitz via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I tried loading the "msvcrt.lib" as a archive. That was... a bad idea! I
> get a Exception while loading:
> Assertion failed: ((int64_t)Result <= INT32_MAX) && "Relocation overflow",
> file \lib\executionengine\runtimedyld\Targets/RuntimeDyldCOFFX86_64.h,
> line 81
>
> It's a limitation of the COFF/PE format and unrelated to exceptions. This
> patch explains it and shows a workaround:
> https://github.com/weliveindetail/pj-llvm/commit/
> 97cd336d458ae9c73232d1b539ceefcdb2f5eb0f
>
> Is there no hope left?
>
> Well at least I am not aware of a solution.
>
> Am 28.09.17 um 16:04 schrieb bjoern.gaier at horiba.com:
>
> Hello Stefan,
>
> I'm happy someone replied to my problem! Many thanks! To be honest... I
> didn't understood much of your mail. I'm a beginner with the JIT - so I
> will explain what I've done.
>
> To manage the memory and resolve symbols, I'm using my own Resolver-Class,
> which overloads the allocation and the findSymbol functions. I've noticed
> today, that the "registerEHFrames" function of my class gets called
> automatically, with correct values. I'm remapping my code and the address
> are still correct. Great! But, what should I do with it? I pass the values
> to the original function, but my exception won't be caught! It's an
> exception raised inside the JITTED code and should also catched there.
>
> I tried loading the "msvcrt.lib" as a archive. That was... a bad idea! I
> get a Exception while loading:
> Assertion failed: ((int64_t)Result <= INT32_MAX) && "Relocation overflow",
> file \lib\executionengine\runtimedyld\Targets/RuntimeDyldCOFFX86_64.h,
> line 81
>
> Research didn't helped me! My code was compiled with /MD, but it didn't
> changed. So I'm still stupid D:
> The JITTED code must be loaded to shared memory later - there aren't
> libraries, so even if this would work, it wouldn't help me. I tried
> compiling my code with sjlj-exceptions. Didn't worked...
>
> Is there no hope left?
>
> Kind regards
> Björn
>
>
>
> From:        Stefan Gränitz <stefan.graenitz at gmail.com>
> <stefan.graenitz at gmail.com>
> To:        bjoern.gaier at horiba.com, llvm-dev at lists.llvm.org
> Date:        27.09.2017 23:09
> Subject:        Re: [llvm-dev] Clang/LLVM JIT - When to use
> "registerEHFrames()"
> ------------------------------
>
>
>
> Hi Björn
>
> To first answer your questionin the subject: For x86 registerEHFrames() is
> only a stub. For x86_64 registerEHFrames() is implemented properly in
> RuntimeDyldCOFFX86_64, calling MemMgr.registerEHFrames() for each EH frame
> section. It should be called and work out of the box without your
> involvement, but unfortunately it won't solve your issue. All the essential
> information is there in the comments, just check the base classes.
>
> This thread from last year helps with your unresolved symbol:
> *http://lists.llvm.org/pipermail/llvm-dev/2016-October/106458.html*
> <http://lists.llvm.org/pipermail/llvm-dev/2016-October/106458.html>
>
> Back then I tried to solve a related issue: SEH exceptions thrown from
> code loaded with RuntimeDyld had to be caught in statically compiled code.
> It turned out Windows explicitly prohibits this. I got in touch with
> Microsoft people and IIRC it's due to security concerns.
>
> Depending on your specific case, you may want to fall back to vectored
> exception handling. In my experience this was a dead end though. If you
> need a solution for arbitrary situations, you just can't jump back to a
> "safe" place to continue execution. I tried setjump (on each entry point to
> the dynamically loaded code) / longjmp (in the vectored exception handler),
> but the address was invalidated when I accessed it. I suspect it's kind of
> undefined behavior to call longjmp outside a child frame of the function
> that called setjmp. Anyway it turned all far too hacky.
>
> If you are willing to do research, compare implementations and behavior
> with the MachO and ELF versions. At least one of them works, just not on
> Windows ;)
> Also check the LLILC project: *https://github.com/dotnet/llilc*
> <https://github.com/dotnet/llilc> I heard about some solution that uses
> trampolines to push exceptions back to dynamically loaded code and handle
> them there.
>
> AND disclaimer: I did not follow recent developments in this area. If
> there's news please let me know!
>
> Cheers & Good Luck!
> Stefan
>
> Am 25.09.17 um 11:31 schrieb via llvm-dev:
> Hello friendly LLVM-World,
>
> because I don't know if I had send my problem to the correct Mailing-List,
> I will send my problem to this address too. I'm not subscribed to this
> list, so please add my in CC if you response.
>
> Kind regards
> Björn
>
>
> From:        Bjoern Gaier/HE/HORIBA
> To:        Clang Dev *<cfe-dev at lists.llvm.org>* <cfe-dev at lists.llvm.org>,
> "cfe-dev" *<cfe-dev-bounces at lists.llvm.org>*
> <cfe-dev-bounces at lists.llvm.org>
> Date:        19.09.2017 08:05
> Subject:        Clang/LLVM JIT - When to use "registerEHFrames()"
> ------------------------------
>
>
> Hello friendly Clang-World,
>
> I was experimenting with Clang and the JIT capabilities of LLVM. Most of
> my attempts were successfully but, I still fail miserably at exceptions.
> Doing research I found the function "registerEHFrames()" which should
> assist me supporting exceptions - but sadly the documentation I found
> wasn't helpful.
> I looked at into the "notifyObjectLoaded" function and discovered that
> there appear some symbol names starting with "$" - I expected them to be
> connected to my try and catch block. But what now? As usually, at this
> point I have there names, but can't get there address to register them with
> the "registerEHFrames()" function. Also the JITTER still wants an address
> for "??_7type_info@@6B@" which is the virtual table of the type_info
> struct.
>
> Confusing! So friendly Clang-World, could you please help?
>
> Not so important - but has the dragon which decorates clang and LLVM a
> name?
>
> Kind regards
> Björn
>
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789
> Geschäftsführer: Hiroshi Kawamura, Dr Hiroshi Nakamura, Markus Bode, Heiko
> Lampert, Takashi Nagano, Takeshi Fukushima.
>
> _______________________________________________
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> *llvm-dev at lists.llvm.org* <llvm-dev at lists.llvm.org>
> *http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev*
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>
> --
> *https://weliveindetail.github.io/blog/*
> <https://weliveindetail.github.io/blog/>
> *https://cryptup.org/pub/stefan.graenitz@gmail.com*
> <https://cryptup.org/pub/stefan.graenitz@gmail.com>
>
>
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789
> Geschäftsführer: Hiroshi Kawamura, Dr Hiroshi Nakamura, Markus Bode, Heiko
> Lampert, Takashi Nagano, Takeshi Fukushima.
>
>
> -- https://weliveindetail.github.io/blog/https://cryptup.org/pub/stefan.graenitz@gmail.com
>
>
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