llvm dev - Feb 2018 - Is it possible to execute Objective-C code via LLVM JIT?

Hi Kevin

I am just pointing out that is a good place to look at how metadata is parsed
and it supports parsing metadata from pre-linked object file which is important
to JIT.

My code pretty much replicate most of the functions in MachODump in C++ and hope
it can much more reusable than the c parser. The biggest missing part is the
resilience to bad metadata due to the fact we currently don't have good way
to recover from error but I know Chris B is working on something like that. I
will definitely ask more feedback from you when I feel like my code is ready.

Thanks

Steven
> On Nov 11, 2016, at 1:38 PM, Kevin Enderby <enderby at apple.com>
wrote:
> 
> Just a comment here.  The code in tools/llvm-objdump/MachODump.cpp has
requirements beyond being able to parse the Objective-C for the machine it is
running on.  It must work cross byte sex, with differing alignment, dealing with
both Objc1 & Objc2, pointers of the non-native size and not the native
addresses and be robust enough to not crash with bad meta data.  So while it
parses the Objective-C meta data it likely is not a good model for execution.
> 
> Kev
> 
>> On Nov 11, 2016, at 1:26 PM, Steven Wu via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Hi Stanislav
>> 
>> Thanks for checking on this. If you want a quick hack, you can try to
read the code in tools/llvm-objdump/MachODump.cpp. There are some c code in
there to parse metadata.
>> 
>> My code is something I put up for my own purpose for debugging/parsing
objc metadata. The interface is a bit rough to use but the biggest problem is
testing. I have my own binary living out of tree to test the parser but it
doesn't make sense to have another parser in tree to do the same thing. I
really hope I can replace the code in MachODump, but unfortunately the current
error handling model in libObject prevents me from replicate all the functions
in MachODump. We are working towards that direction but we are not there yet. In
the meantime, I won't mind posting some patches when I have time.
>> 
>> Steven
>> 
>> 
>>> On Nov 10, 2016, at 2:03 PM, Stanislav Pankevich <s.pankevich at
gmail.com <mailto:s.pankevich at gmail.com>> wrote:
>>> 
>>> Hi Lang, Steven,
>>> 
>>> Any updates on this? Steven, is there a progress with metadata
parser for Objective-C?
>>> 
>>> Let us know if we can help somehow.
>>> 
>>> Thanks.
>>> 
>>> Stanislav
>>> 
>>>> On 22 Oct 2016, at 03:21, Lang Hames <lhames at gmail.com
<mailto:lhames at gmail.com>> wrote:
>>>> 
>>>> > So no JIT for i386?
>>>> 
>>>> I didn't realise i386 still used ObjC 1 - all my
experiments were on arm64.
>>>> 
>>>> i386 support for ObjC would be nice to have, but it's not a
blocker for adding ObjC 2 support.
>>>> 
>>>> - Lang.
>>>> 
>>>> 
>>>> On Thu, Oct 20, 2016 at 11:57 AM, Steven Wu <stevenwu at
apple.com <mailto:stevenwu at apple.com>> wrote:
>>>> 
>>>>  
>>>>> On Oct 19, 2016, at 11:28 AM, Stanislav Pankevich
<s.pankevich at gmail.com <mailto:s.pankevich at gmail.com>> wrote:
>>>>> 
>>>>> Hi Lang, Steven,
>>>>> 
>>>>> At this point it seems that the best next step is for
Steven to share his code, right? Or is there some other work that can be done in
parallel?
>>>> 
>>>> It will take some time to create patch from my existing
implementation. After that, we still need to correctly register the
implementation to objc runtime. I dont expect this to be a quick task.
>>>> 
>>>>> 
>>>>> Lang? Steven?
>>>>> 
>>>>> Here’s small note about my interest here: Alex Denisov and
I are working on a tool for mutation testing
https://github.com/AlexDenisov/mutang
<https://github.com/AlexDenisov/mutang>. Alex has made fascinating
progress towards mutation testing of C++ code and we would also like to bring
mutation testing to iOS where we are by profession. Currently inability to run
Objective-C code via JIT is a critical dependency for our progress on iOS part
so I'll be happy to help with this in any way even though my experience with
LLVM is very limited.
>>>> 
>>>> This sounds like an interesting project. Maybe you can generate
the object file and statically linked the mutated code for now?
>>>> 
>>>> Steven
>>>> 
>>>>> 
>>>>> Thanks.
>>>>> 
>>>>> Stanislav
>>>>> 
>>>>>> On 19 Oct 2016, at 01:49, Lang Hames <lhames at
gmail.com <mailto:lhames at gmail.com>> wrote:
>>>>>> 
>>>>>> Hi Steven,
>>>>>> 
>>>>>> That sounds great. The object file parser would
definitely be of interest to JIT clients. Having ObjC 1 support would be nice,
but I don't see it as a blocker to committing what you've got: all code
produced by the JIT is going to be ObjC-2.
>>>> 
>>>> So no JIT for i386? 
>>>> 
>>>> Steven
>>>> 
>>>>>> 
>>>>>> Cheers,
>>>>>> Lang.
>>>>>> 
>>>>>> On Tue, Oct 18, 2016 at 2:22 PM, Steven Wu <stevenwu
at apple.com <mailto:stevenwu at apple.com>> wrote:
>>>>>> Hi Stanislav, Lang
>>>>>> 
>>>>>> I have a version of objc metadata parser based on LLVM
which can handle objc2 metadata from MachO object and bitcode (bitcode version
is even swift compatible). I don't know if there is enough interest for me
to upstream it. It probably requires some work to clean up the interface and
complete it by adding objc1 support and other object format support.
>>>>>> 
>>>>>> Steven
>>>>>> 
>>>>>>> On Oct 18, 2016, at 2:10 PM, Lang Hames via
llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>>>>>> 
>>>>>>> Hi Stanislav,
>>>>>>> 
>>>>>>> I actually looked into this recently. It is
possible, but takes some non-trivial effort right now: You have to hook into the
JIT memory allocator, parse the objective C metadata sections in the object
file, then call the objective C runtime to register any newly added classes,
protocols and selectors.
>>>>>>> 
>>>>>>> I can't release my code as-is: I built it
against a hacked up copy of the runtime. The right way to do this would be to
write parsers for the objective C metadata, and an Objective-C registration call
that uses the parsers and calls the runtime. These should be made available
conditionally if the objective C runtime headers are available. This is
something that I'd like to get to eventually, but I have a long to-do list.
If you or anyone you know is interested in working on it I'd be happy to
provide pointers and code review.
>>>>>>> 
>>>>>>> Cheers,
>>>>>>> Lang.
>>>>>>> 
>>>>>>> On Tue, Oct 18, 2016 at 1:33 PM, Stanislav
Pankevich via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>>>>>> Hello,
>>>>>>> 
>>>>>>> Is it possible to execute Objective-C code using
LLVM JIT?
>>>>>>> 
>>>>>>> The original point for my research is this topic on
StackOverflow:
>>>>>>> 
>>>>>>> All selectors unrecognised when invoking
Objective-C methods using the LLVM ExecutionEngine
http://stackoverflow.com/questions/10375324/all-selectors-unrecognised-when-invoking-objective-c-methods-using-the-llvm-exec
<http://stackoverflow.com/questions/10375324/all-selectors-unrecognised-when-invoking-objective-c-methods-using-the-llvm-exec>
>>>>>>> 
>>>>>>> Running that example with lli produces the
following output:
>>>>>>> 
>>>>>>> 2016-10-18 21:25:58.332 lli[12970:169282] ***
NSForwarding: warning: selector (0x10356a38f) for message 'alloc' does
not match selector known to Objective C runtime (0x7fff8e93afb5)-- abort
>>>>>>> 2016-10-18 21:25:58.334 lli[12970:169282]
+[NSAutoreleasePool alloc]: unrecognized selector sent to class 0x7fff7a562130
>>>>>>> 2016-10-18 21:25:58.338 lli[12970:169282] ***
Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: '+[NSAutoreleasePool alloc]: unrecognized selector sent to class
0x7fff7a562130'
>>>>>>> 
>>>>>>> See also additional discussion here:
https://twitter.com/sbpankevich/status/783363161537798144
<https://twitter.com/sbpankevich/status/783363161537798144>.
>>>>>>> 
>>>>>>> Thanks.
>>>>>>> 
>>>>>>> Stanislav Pankevich
>>>>>>> 
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>>>>>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>>>>>> 
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>>>>>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>>>>> 
>>>>>> 
>>>>> 
>>>> 
>>>> 
>>> 
>> 
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Hi all,
> I actually looked into this recently. It is possible, but takes some
non-trivial effort right now: You have to hook into the JIT memory allocator,
parse the objective C metadata sections in the object file, then call the
objective C runtime to register any newly added classes, protocols and
selectors.
> I can't release my code as-is: I built it against a hacked up copy of
the runtime. The right way to do this would be to write parsers for the
objective C metadata, and an Objective-C registration call that uses the parsers
and calls the runtime. These should be made available conditionally if the
objective C runtime headers are available. This is something that I'd like
to get to eventually, but I have a long to-do list. If you or anyone you know is
interested in working on it I'd be happy to provide pointers and code
review.
Has any progress been made on this issue since 1 year ago?

Lang, I have spent some time trying to follow your hints along the
lines quoted above and found that it is indeed a non-trivial effort to
implement all of this. Specifically I explored the latest objc4-723
from Apple Open Source and it looks like all of the APIs that allow
the registration of Objective-C classes, selectors, etc. are all very
private.

One year ago you said you could help anyone interested in working on
this. Let me check here again as a volunteer (if this work can ever be
accomplished by someone outside Apple).

[cutting the rest of the thread to reduce the size of the email]

Thanks,

Stanislav

On 12 Feb 2018, at 22:31, Stanislav Pankevich via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Specifically I explored the latest objc4-723
> from Apple Open Source and it looks like all of the APIs that allow
> the registration of Objective-C classes, selectors, etc. are all very
> private.
The Objective-C runtime provides public APIs for doing all of this.  They’re
even documented.  They are also more or less standard and so work with all
runtime implementations, not just the Apple one.  I was using them for JIT’d
code on macOS and FreeBSD 10 years ago.
> One year ago you said you could help anyone interested in working on
> this. Let me check here again as a volunteer (if this work can ever be
> accomplished by someone outside Apple).
As I said in the earlier thread, the best way of doing this is to add a new
subclass of CGObjCRuntime that generates the code using the public APIs.  If
you’re running in the same process as the JIT, you could register the selectors
in the host environment and just inject the values as symbols (this is what I
did).  I’d be happy to help out someone who wants to do this.

David