[Lldb-commits] [PATCH] D132815: [LLDB] Do not dereference promise pointer in `coroutine_handle` pretty printer

[Pavel Labath via Phabricator via lldb-commits]

labath added inline comments.


================
Comment at: lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp:246-248
+  DataExtractor data(&promise_addr, sizeof(promise_addr),
+                     process_sp->GetByteOrder(),
+                     process_sp->GetAddressByteSize());
----------------
avogelsgesang wrote:
> labath wrote:
> > Have you checked there won't be a use-after-free problem here, given that this data extractor will refer to the stack object?
> > 
> > To create persistent data, you need to use the DataBufferSP constructor, but I'm wondering if we couldn't fix this by creating the (non-pointer) object using the `CreateValueObjectFromAddress` function, as above, but then actually use valobj->AddressOf as the synthetic child.
> > 
> > I am also somewhat surprised that we need to use the GetAddressOf trick here, as this seems to indicate that the coroutine contains (in the proper C "subobject" kind of way) the promise object. That's not necessarily wrong, but it makes me think we may be "breaking the cycle" at the wrong place.
> Thanks for taking a look!
> 
> > To create persistent data, you need to use the DataBufferSP constructor
> 
> good point, I will keep this in mind as a fallback in case we don't decide to follow any of the other directions you hinted at.
> 
> > wondering if we couldn't fix this by creating the (non-pointer) object using the CreateValueObjectFromAddress function, as above, but then actually use valobj->AddressOf as the synthetic child
> 
> Good idea! I will give it a try
> 
> 
> > [...] as this seems to indicate that the coroutine contains (in the proper C "subobject" kind of way) the promise object. That's not necessarily wrong, but it makes me think we may be "breaking the cycle" at the wrong place.
> 
> The physical layout of this is:
> ```
> // in the standard library
> template<typename promise_type>
> struct exception_handle<promise_type> {
>     __coro_frame<promise_type>* __hdl; // <--- this is the pointer we read with `GetCoroFramePtrFromHandle`
> };
> 
> // compiler-generated coroutine frame. Generated ad-hoc per coroutine
> struct __coro_frame<promise_type> {
>      // The ABI guaranteees that hose two pointers are always the first two pointers in the struct.
>      void (*resume)(void*); // function pointer for type erasure
>      void (*destroy)(void*); // function pointer for type erasure
>      // Next comes our promise type. This is under the control of the program author
>      promise_type promise;
>      // Next comes any compiler-generated, internal state which gets persisted across suspension points. 
>      // The functions pointed to by `resume`/`destroy` know how to interpret this part of the coroutine frame.
>      int __suspension_point_id;
>      double __some_internal_state;
>      std::string __some_other_internal_state;
>      ....
> };
> ```
> 
> The programmer (i.e., most likely the user of this pretty-printer), wrote only the "promise" explicitly in his code. Everything else is compiler-generated. As such, the lldb-user will usually look for the "promise" first, and I would like to make it easy to find it, by exposing it as a top-level children of the `exception_handle` instead of hiding it inside a sub-child.
> As such, the lldb-user will usually look for the "promise" first, and I would like to make it easy to find it, by exposing it as a top-level children of the `exception_handle` instead of hiding it inside a sub-child.

That makes sense. And I think that's a good argument for automatically "dereferencing" that object (i.e., status quo). That said, it's not fully clear to me how do we end up looping here. I take it the promise object contains a (compiler-generated ?) pointer to another __coro_frame object? What would happen if we turned *that* into a pointer?


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  https://reviews.llvm.org/D132815/new/

https://reviews.llvm.org/D132815