[PATCH] D41880: Adding nocf_check attribute for cf-protection fine tuning

Aaron Ballman via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Fri Feb 16 06:18:57 PST 2018

aaron.ballman added inline comments.

Comment at: test/Sema/attr-nocf_check.c:18-20
+  FuncPointerWithNoCfCheck fNoCfCheck = f; // no-warning
+  (*fNoCfCheck)();                       // no-warning
+  f = fNoCfCheck;                        // no-warning
oren_ben_simhon wrote:
> aaron.ballman wrote:
> > oren_ben_simhon wrote:
> > > aaron.ballman wrote:
> > > > oren_ben_simhon wrote:
> > > > > aaron.ballman wrote:
> > > > > > These are an error in GCC and I think we should match that behavior. https://godbolt.org/g/r3pf4X
> > > > > I will create a warning however in LLVM we don't create an error upon incompatible pointer due to function attribute types.
> > > > It should be an error -- Clang does error on this sort of thing when appropriate (which I believe it is, here). For instance, calling convention attributes do this: https://godbolt.org/g/mkTGLg
> > > In Clang there is Sema::IncompatiblePointer in case to pointers are not compatible. This flag emits warning message. In the time i check for pointer incompatibility (checkPointerTypesForAssignment()), i don;t have a handle to the attributes. Any suggestion how to implement the exception for nocf_check attribute?
> > I believe this is handled in `ASTContext::mergeFunctionType()`. See:
> > ```
> >   // Compatible functions must have compatible calling conventions
> >   if (lbaseInfo.getCC() != rbaseInfo.getCC())
> >     return QualType();
> > ```
> > Somewhere around there is likely where you should be.
> I already added there getnocfcheck.
> After double checking, I see that nocf_check behavior is identical to other function attributes.
> For some reason in the clang tests they give warning but in godbolt it gives an error.
> I am not sure what is the difference between the flags in godbolt and in my test but this is what causing the warning/error message difference.
> So basically my behavior is identical to other function type attributes (e.g. no_caller_saved_registers). I believe it is also identical to GCC but i can't prove it because i don't know the flags that godbolt is using.
You can see the flags being passed in godbolt by passing -v on the command line. FWIW, I get the same error behavior elsewhere as well:


I think you should do some more investigation into what's going on there. Ultimately, I want to avoid clang accepting the `nocf_check` attribute (even with a warning) in cases where GCC doesn't accept it, because that leads to incompatibilities when switching between the two compilers. We should accept what GCC accepts and reject what GCC rejects unless there's a good reason to deviate.



More information about the llvm-commits mailing list