[analyzer] Extend ownership_returns attribute to cover output parameters

[PengZheng]

I'm currently working on adding clang memory ownership annotations to Apache Celix: apache/celix#806

Is it possible to extend ownership_returns so that we can mark functions returning allocations through their output parameters?

I can work on this if some guidance is provided.

[llvmbot]

@llvm/issue-subscribers-clang-static-analyzer

Author: None (PengZheng)

Details

I'm currently working on adding clang memory ownership annotations to Apache Celix: https://github.com/apache/celix/pull/806

Is it possible to extend ownership_returns so that we can mark functions returning allocations through their output parameters?

I can work on this if some guidance is provided.

[steakhal]

Hey, sorry for being late to reply here.
I welcome you to work on this. I'll try to guide you.

Currently, the attribute only works for return types, as you noted.
I think your use-case of output parameters makes sense, although might deserve a different attribute - a 4th one besides holds, returns and takes. Maybe call it for not ownership_outputs so something silly like that for now.

When I prepped for OwnershipAttr::Returns I got hits from clang/lib/Sema/SemaDeclAttr.cpp and clang/lib/StaticAnalyzer/Checkers/MallocChecker.cpp. So these are like the files you want to look at besides the file that defines this attribute:
clang/include/clang/Basic/Attr.td

def Ownership : InheritableAttr {
  let Spellings = [Clang<"ownership_holds">, Clang<"ownership_returns">,
                   Clang<"ownership_takes">];
  let Accessors = [Accessor<"isHolds", [Clang<"ownership_holds">]>,
                   Accessor<"isReturns", [Clang<"ownership_returns">]>,
                   Accessor<"isTakes", [Clang<"ownership_takes">]>];
  let AdditionalMembers = [{
    enum OwnershipKind { Holds, Returns, Takes };
    OwnershipKind getOwnKind() const {
      return isHolds() ? Holds :
             isTakes() ? Takes :
             Returns;
    }
  }];
  let Args = [IdentifierArgument<"Module">,
              VariadicParamIdxArgument<"Args">];
  let Subjects = SubjectList<[HasFunctionProto]>;
  let Documentation = [OwnershipDocs];
}

Mind that contributions must include tests. I'd suggest looking at the git blame for the relevant lines to fish for examples.
And ofc. any behaviour changes, such as new attributes must be documented at the right places so everything remains up to date. These are not relevant right now, as you should focus on some proof of concept first to get feedback. Once you pass that one, you can start to finalise the contribution.

I you are in doubt, don't hesitate to reply here.