[llvm-dev] LLVM plugin: access function signature of a function pointer parameter

[Johannes Doerfert via llvm-dev]

On 2/8/21 1:00 PM, Marc wrote:
> As an additional question is there a way how I can detect if a call
> parameter is actually a function pointer?

Check the type, 
isa<FunctionType>(CI->getArgOperand(idx)->getType()->getPointerElementType()), 
though, pointer types are not binding.


>
> On 08.02.21 18:30, Johannes Doerfert wrote:
>> Going through the callee value, as you did.:
>>
>> Value *Callee = CI->getCalledValue();
> getCalledValue() seems to have been renamed to getCalledOperand()
>
>> PointerType *CalleePtrTy = dyn_cast<PointerType>(Callee->getType());
>> if (!CalleePtrTy) ...
>> FunctionType *CalleeFnTy =
>> dyn_cast<FunctionType>(CalleePtrTy->getPointerElementType());
>> if (!CalleeFnTy) ...
>> if (CalleeFnTy->getNumParams() > idx)
>>    return CalleFnTy->getParamTy(idx);
> that would be getParamType(1), it returns
> void (i32)*
>
> ... and I am back at square one :-(

I don't follow. It returns the parameter at index `idx` of the callee.
If your callee is a function that takes a function as `idx`'th argument, 
it might look like nothing happened, but the above should peel one level 
as requested.

~ Johannes


>
> Regards,
> Marc
>
>> On 2/8/21 10:20 AM, Marc via llvm-dev wrote:
>>> Hi,
>>>
>>> I am unable to find any information on how to extract the function
>>> signature from calls that receive a function pointer. e.g.:
>>>
>>> C code:
>>>     signal(SIG_TERM, a_function_ptr);
>>>
>>> IR:
>>>     %call = call void (i32)* @signal(i32 4, void (i32)* %1) #3, !dbg !26
>>>
>>> And Value* of callInst->GetOperand(1) resulting in: void (i32)*
>>>
>>> How can I extract that (i32) information, so basically the function
>>> parameter signature?
>>>
>>> I want to add a feature into an llvm plugin that parses all calls,
>>> identifies function pointers and then dumps the function signature.
>>> (Yes if a function point is hidden to be a size_t* bitcast and
>>> transfered I will not be able to see this, but in cases where this is
>>> visible I would like to get that information.
>>>
>>> Thank you!
>>>
>>> Regards,
>>> Marc
>>>