[lldb-dev] lldb showing wrong type structure for virtual pointer type

jonas echterhoff via lldb-dev lldb-dev at lists.llvm.org
Thu Mar 1 02:11:10 PST 2018 

FWIW, I just out found that I can trivially work around this problem by changing the order of object files passed to the linker.

It turns out that lldb will just pick up the first type named "Transform" in the binary (regardless of namespace). Now, if I make sure that the object file containing the "correct" Transform type comes first in the linker command line, it will also be first in the binary, and it will be picked up by lldb. Now, in theory, this might cause the same problem, just the "other way around", when debugging code using the namespaced version of the type, but in our case, that should not be a problem, because the latter is not a virtual type, so lldb should not use dynamic lookup in that case.

jonas

> On Mar 1, 2018, at 3:59 AM, Jim Ingham <jingham at apple.com> wrote:
> 
> 
> 
>> On Feb 28, 2018, at 1:09 PM, jonas echterhoff <jonas at unity3d.com> wrote:
>> 
>> 
>> 
>>> On Feb 28, 2018, at 9:27 PM, Jim Ingham <jingham at apple.com> wrote:
>>> 
>>> Interesting.  
>>> 
>>> First off, you can turn off fetching dynamic values globally (including in the Xcode Locals view) by putting:
>>> 
>>> settings set target.prefer-dynamic-value no-dynamic-values
>> 
>>> in your ~/.lldbinit file.  You can toggle this on and off in a session, though Xcode won't notice you've changed the value till you cause it to refresh the locals (step or whatever).
>> 
>> This will fix the output of "frame variable". But it does not seem to fix the variable display in the UI.
> 
> They must be setting the dynamic value directly when they fetch the values.  That of course overrides the general setting.  I'll go see why they do that, but that won't help you for now.
> 
>> 
>>> We do log the process of finding the dynamic type.  You can see this by running the command:
>>> 
>>> log enable -f /tmp/lldb-object-log.txt lldb object
>>> 
>>> Probably easiest to put that in your .lldbinit.
>>> 
>>> That channel also logs when we read in modules, and so it might be a little chatty, but you should see:
>>> 
>>> <SOME_ADDRESS>: static-type = '<STATIC_TYPE>' has vtable symbol 'vtable for <DYNAMIC_CLASS>'
>>> 
>>> and then some more messages that trace our attempt to look up DYNAMIC CLASS.  If you turn on those logs, what do you see for these classes?
>> 
>> 0x000000010f62ecd0: static-type = 'Transform *' has vtable symbol 'vtable for Transform'
>> 
>> 0x000000010f62ecd0: static-type = 'Transform *' has dynamic type: uid={0x100012d7a}, type-name='Transform'
> 
> Grr...  We go from the name in the vtable symbol to the class type using Module::FindTypes, passing the exact_match flag 'cause we know this is an exact match.  Turns out the Module::FindTypes only obeys its exact_match flag if either the name you pass in starts with :: or if you can dial up the exact type kind (struct, class, enum, etc...) you are looking for.  We can't do the latter here because we don't know whether the dynamic type is a class or a struct, and we were just passing the name we got from the vtable.
> 
> I have a small fix for the dynamic type issue in r326412.  Fixing the FindTypes behavior is more involved, and I don't know whether other places rely on this misbehavior. I filed:
> 
> https://bugs.llvm.org/show_bug.cgi?id=36556
> 
> to examine that issue further.
> 
> Thanks for reporting this.
> 
> Jim
> 
> 
>> 
>> jonas
>> 
>>> 
>>> Jim
>>> 
>>> 
>>>> On Feb 28, 2018, at 12:03 PM, jonas echterhoff <jonas at unity3d.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Feb 28, 2018, at 7:14 PM, Jim Ingham <jingham at apple.com> wrote:
>>>>> 
>>>>> Jonas,
>>>>> 
>>>>> What are you using to inspect the this pointer?
>>>> 
>>>> Normally, the Xcode debugger UI.
>>>> 
>>>>> You can use "frame variable" (the equivalent of gdb's "info locals") which just relies on debug info or the expression evaluator e.g. "print".  Do both methods show the same problem?
>>>> 
>>>> (lldb) frame variable this
>>>> (Scripting::UnityEngine::Transform *) this = 0x000000010fe2eb20
>>>> 
>>>> That gives me the wrong namespace
>>>> 
>>>> (lldb) print this
>>>> (Scripting::UnityEngine::Transform *) $4 = 0x000000010fe2eb20
>>>> 
>>>> That also gives me the wrong namespace
>>>> 
>>>> But:
>>>> 
>>>> (lldb) print *this
>>>> (Transform) $5 = {
>>>> [...]
>>>> 
>>>> gives me the correct (global) namespace.
>>>> 
>>>> Also:
>>>> 
>>>> (lldb) frame variable -d no-dynamic-values this
>>>> (Transform *) this = 0x000000010fe2eb20
>>>> 
>>>> gives me the correct namespace.
>>>> 
>>>>> 
>>>>> Also note that lldb by default will try to discern the full dynamic type of the variables it prints.  You can disable this by doing:
>>>>> 
>>>>> (lldb) expr -d no-dynamic-values -- this
>>>>> 
>>>>> or equivalently:
>>>>> 
>>>>> (lldb) frame variable -d no-dynamic-values this
>>>>> 
>>>>> Is it the dynamic value resolution that's causing the incorrect printing?
>>>> 
>>>> Yes, both of those above give me the correct types!
>>>> 
>>>> Now, this is already very helpful - Thank you! 
>>>> This means I can get correct values using the lldb console. If there was some way to make the Xcode UI show the correct values, that would be even better.
>>>> 
>>>> jonas
>>>> 
>>>>> 
>>>>> Jim
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>>> On Feb 28, 2018, at 3:03 AM, jonas echterhoff via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Feb 28, 2018, at 11:19 AM, Dmitry Antipov <dantipov at nvidia.com> wrote:
>>>>>>> 
>>>>>>> On 02/28/2018 11:31 AM, jonas echterhoff via lldb-dev wrote:
>>>>>>> 
>>>>>>>> I'm using lldb-900.0.64.
>>>>>>>      ^^^^^^^^^^^^^^
>>>>>>>      ??????????????
>>>>>>> Latest official release is 5.0.1; also there are 6.0.0 (at -rc3, the next release)
>>>>>>> and 7.0.0 (a.k.a SVN trunk). What's the 'version' output of your LLDB prompt?
>>>>>> 
>>>>>> It is what I posted:
>>>>>> 
>>>>>> jechter$ lldb --version
>>>>>> lldb-900.0.64
>>>>>> Swift-4.0
>>>>>> 
>>>>>> Maybe Apple uses a different versioning scheme for lldb distributed with their toolchains?
>>>>>>> 
>>>>>>>> Unfortunately, I have not yet succeeded in coming up with a small, independent repro case which shows this problem.
>>>>>>> 
>>>>>>> IIUC this is it:
>>>>>> 
>>>>>> [...]
>>>>>> 
>>>>>>> Here 'this' is different between calls to obj2.f () and obj2.g () (0x00007fffffffdb50 vs.
>>>>>>> 0x00007fffffffdb40), and objects are shown as different as well - {111, 222} vs. {333, 444}.
>>>>>> 
>>>>>> Thanks. What you are showing there seems very peculiar. 
>>>>>> 
>>>>>> But I don't think it's the same problem as I have (and also, using the same steps on my machine does not repro the problem you showed - I get the same value for "this" and it's members between the calls to S::B::f and S::B::g).
>>>>>> 
>>>>>> My problem was not about showing a wrong object (My "this" pointer value was correct), but about showing a wrong type representation of the correct object data.
>>>>>> 
>>>>>> jonas
>>>>>> 
>>>>>> _______________________________________________
>>>>>> lldb-dev mailing list
>>>>>> lldb-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev

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