[PATCH] D25448: [ubsan] Disable -fsanitize=vptr checks for devirtualized calls

[John McCall via cfe-commits]

rjmccall added a comment.

In https://reviews.llvm.org/D25448#571941, @vsk wrote:

> Thanks for your feedback so far, and sorry for the delayed response.
>
> In https://reviews.llvm.org/D25448#570014, @rjmccall wrote:
>
> > Wait, can you talk me through the bug here?
>
>
> Derived inherits from Base1 and Base2. We upcast an instance of Derived to Base2, then call a method (f1). Clang figures out that the method has to be Derived::f1. Next, clang passes in a pointer to the vtable pointer for **Base1**, along with the typeinfo for **Base2**, into the sanitizer runtime. This confuses the runtime, which reports that the dynamic type of the object is "Base1", and that this does not match the expected type ("Base2").


A pointer to the vtable pointer for Base1 is a pointer to a Derived.  You have a multiple inheritance bug, or really a general inheritance bug.  It's being covered up in the case of single, non-virtual inheritance because that's the case in which a pointer to a base-class object is the same as a pointer to the derived class object.

When the devirtualizer sees what's formally a call to a method on Base2, and it decides that it can instead treat it as a call to a method on Derived, it has to adjust the 'this' pointer to point to a Derived.  (I don't know off-hand whether it does this by retroactively adjusting the pointer or whether it just avoids adjusting it in the first place.)  When it does this, it should also be changing its notion of what class the pointer points to.

> With the 'final' keyword:
> 
>   %6 = ptrtoint <2 x i32 (...)**>* %1 to i64
>   call void @__ubsan_handle_dynamic_type_cache_miss(@1 (TypeInfo for Base2), i64 %6, ...)
>    
> 
> Without the 'final' keyword:
> 
>   %6 = bitcast <2 x i32 (...)**>* %1 to %class.Derived*
>   %7 = getelementptr inbounds %class.Derived, %class.Derived* %6, i64 0, i32 1
>   %8 = ptrtoint %class.Base2* %7 to i64, !nosanitize !5
>   call void @__ubsan_handle_dynamic_type_cache_miss(@1 (TypeInfo for Base2), i64 %8, ...)
>    
> 
> 
> 
>>   Why is final-based devirtualization here different from, say, user-directed devirtualization via a qualified method name?
> 
> I'm not sure. I tested this by removing the 'final' specifier from 'Derived' and calling:
> 
>   obj.Derived::f1()
> 
> 
> In this case, clang passes the correct typeinfo (for Derived) in to the runtime.
> 
>> It sounds to me from your description that you're not sure why this is happening.  If this indeed only triggers in the presence of multiple inheritance, it might just be the case that you're doing your object-extents analysis starting from the wrong offset.
> 
> It looks like I just haven't done a good job of explaining the issue. The bug really does seem to be that clang isn't passing the right information to the ubsan runtime. However, I'm not sure what the right fix is. Do we disable sanitization in cases where we expect FP's, do we try harder to pass in the right vptr (in this case, the vptr for Base2) into the runtime, or do we try harder to pass in the right typeinfo (in this case, the typeinfo for Derived)?




https://reviews.llvm.org/D25448