[cfe-dev] Diagnosing Incompatible Attributes Between a Calling Function and the Callee Function

[Zola Bridges via cfe-dev]

Hi everyone,


I'm working on adding function level Clang attributes for the Speculative
Load Hardening (SLH) feature, so devs who know what they are doing can
enable or disable SLH function by function. There are two attributes
'no_speculative_load_hardening' and 'speculative_load_hardening.'


As a part of this, I want to diagnose a special case where a function marked

'no_speculative_load_hardening' will still have
'speculative_load_hardening' enabled.


Whenever a function marked 'speculative_load_hardening' is inlined into
another function, then that function that it was inlined into will have SLH
enabled no matter what. [If you want more info on the rationale for this,
check out the patch comments here:
https://reviews.llvm.org/D54909?id=175599#inline-487979]


I want to diagnose cases similar to this example:


__attribute__((speculative_load_hardening)) inline int foo(int i) {

return i;

}


__attribute__((no_speculative_load_hardening)) int bar(int i) {

return foo(i); // Warning

}


I've thought about three different ways to implement this.


1. Add the diagnosis to Sema::ActOnFinishFunctionBody in SemaDecl.cpp. The
caller function declaration and the caller body is available. For functions
marked with SLH, I can walk the function body nodes and see whether any
function calls have the incompatible attributes of SLH + inline.


Downside: This is an expensive way to diagnose this edge case. It doesn't
seem worth the expense, so after discussing this with Aaron Ballman, I'll
leave out the diagnosis if this is the only way to do it.


2. Add the diagnosis to CheckFunctionCall or checkCall in SemaChecking.cpp


Why doesn't this work?

The context about the caller of the callee isn't available. I need to know
whether the caller has the 'no_speculative_load_hardening' attribute.


3. Add the diagnosis to Sema::ActOnCallExpr in SemaExpr.cpp


Would this work?

I'm not sure. Maybe this is a place where it would work since I have the
Scope which contains info about the caller and the FunctionDecl of the
callee which is all the info I need about the callee (hasAttr<SLH> &&
isInlineSpecified). The part I don't know how to do is distinguishing the
calls I want to check from other calls that pass through that function (I
assume constructors? Maybe other things?).


Questions

   -

   How do I distinguish between FunctionDecls that are direct function
   calls like I was to diagnose vs ones that are calls in other contexts? I
   think if I could make this distinction, then ActOnCallExpr would be a
   workable place to diagnose this. Since I don't have a good sense of what
   other calls are available, I'm having a hard time characterizing the
   distinguishing features of the calls I want to diagnose. (I'm relatively
   unknowledgeable about compilers and programming languages.)
   -

      If I only check direct function calls (which are already singled out
      in ActOnCallExpr, would that be an appropriate subset of calls for me to
      try to diagnose? How do I know whether or not that's the right subset?
      -

      If there's some documentation or book chapter I could read to get a
      better understanding of how to figure this out, please feel free to
      recommend that.
      -

   Am I missing a way to get the caller function when given a function call
   as in CheckFunctionCall?


   -

   Are there any recommendations about where else might be an appropriate
   place to diagnose this issue? Pointers about what to check into would be
   appreciated. SemaChecking.cpp seems like the right file, but I didn't see
   any other likely candidates from my inspection of the functions defined
   there.


Zola Bridges
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