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

Mon Oct 2 21:36:34 PDT 2017

I'm catching up on this. Does this mean LLVM x64 JITTed code is not
exception friendly or you can't catch exceptions inside LLVM JITTed
code. The first one seems to indicate that the code is not ABI
friendly or that not enough information is present to notify Windows
of unwind tables.

I'll ask the question another way: Does LLVM emit enough information
so that RtlAddFunctionTable can work? Anyone successfully done this in
the wild?

On Sun, Oct 1, 2017 at 7:16 AM, Stefan Gränitz via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi, I checked last year's mails. Back then Timur and me faced the same issue
> Björn reports (nulled memory in _ThrowInfo).
>
>> 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?
>
> Igor Minin explained this was due to Data Execution Prevention (DEP) and
> gave pointers to the involved functions in the OS runtime:
> http://lists.llvm.org/pipermail/llvm-dev/2016-June/100901.html
>
> The whole thing gets hairy quickly. It seems like a combination of multiple
> problems. So let's first collect evidence and clarify the goal.
>
> Joseph Tremoulet from Microsoft partially confirmed Igor's statements:
> "I checked with some experts here, and they agree with your/Igor’s
> assessment that it’s not possible to have a dynamically generated handler on
> x86"
> "the llilc code only handles x86_64, not 32-bit x86; I know the
> implementations of WinEH are quite different between the two"
> First conclusion: No solution for JITed catch handlers on x86.
>
> I think for most of us x86 has little prio anyway, so let's focus on x86_64.
> In theory both, JITed and static catch handlers, may be possible here.
>
> I disabled DEP globally on my machine and set MEM_EXECUTE_OPTION_ENABLE
> explicitly:
> http://www.thewindowsclub.com/disable-data-execution-prevention
> https://github.com/Sh1ft0x0EF/metahook/blob/master/sys_launcher.cpp#L65
> It didn't fix anything.
> Second conclusion: DEP may be one reason, but there must be something else.
>
> My next guess was an incompatibility between Clang (we used for JITing) and
> MSVC (we used for static compilation). Joseph:
> "One thing that comes to mind is that presumably you’re not trying to make
> your language/runtime’s exceptions masquerade as MSVC++ exceptions, so I
> guess you’d want your static native code to either have an SEH
> __try/__except or compile with clang’s equivalents of MSVC’s /EHa and
> catch(…)"
>
> I tried all the combinations without success. I also compared the compiled
> code between Clang 3.8 and MSVC, which showed no reasonable difference.
> Second conclusion: No issue with code generation.
>
> At that point we ran out of time we could spend on this. So we postponed the
> feature, hoping for Microsoft to come up with a solution at some point.
> Giving it another try today, I think this looks promising:
> https://github.com/dotnet/llilc/blob/master/lib/Jit/EEMemoryManager.cpp#L250
>
> Processing .pdata section entries, it creates and allocates unwind blocks
> and funclets (handlers for catch & finally) for CLR managed exceptions. It's
> probably not directly applicable to native exception handling, but it may
> provide useful information. Also there is much more documentation available
> now:
> https://github.com/dotnet/coreclr/blob/master/Documentation/botr/clr-abi.md#exception-handling
> https://llvm.org/docs/ExceptionHandling.html#exception-handling-using-the-windows-runtime
>
> I could imagine setting up minimal tests would be a useful first step to
> distinguish issues in a reproducible way.
>
> Best
> Stefan
>
> Am 29.09.17 um 19:43 schrieb Lang Hames:
>
> 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>
>> 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
>>
>> 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 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"
>> <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.
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> --
>> https://weliveindetail.github.io/blog/
>> 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
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> https://weliveindetail.github.io/blog/
> https://cryptup.org/pub/stefan.graenitz@gmail.com
>
>
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