Implement new attributes in the Clang compiler

[connorkuehl]

How do we add support for things like __randomize_layout and __no_randomize_layout?

[connorkuehl]

I found this in the Compiler Internals Manual. I haven't found an appropriate analog from the context of the Clang Plugin API.

[nforbus]

So we already have randstruct registered randstruct with the FrontendPluginRegistry, so we can call our plugin from command line via -plugin randstruct, is that correct?

So at this point what we need is a way for the plugin to be called from structs declared with a calling component, and attributes won't work because we're not going to modify the attr.td file (which is a route we would go if we were not doing a plugin).

Currently looking at other plugins to see how they manage this, nothing so far though.

[tim-pugh]

@jeffreytakahashi any progress on this?

[Nixoncole]

This is a lead from March 2017 that makes me believe we cannot add custom attributes without directly editing Clang. http://clang-developers.42468.n3.nabble.com/User-defined-attributes-in-clang-plugins-td4055940.html

This is a blog post that (kind of?) outlines using the clang attribute "annotate", to create a custom annotation that we can pull off at the "bitcode level": https://blog.quarkslab.com/implementing-a-custom-directive-handler-in-clang.html - I want to speak with ya'll about this tomorrow, because I don't have enough contextual knowledge to understand if we can use this.

Official docs for the llvm "intrinsic" leveraged by the blog post. I dont feel like it provided me with much information, but maybe someone can connect the dots? - http://llvm.org/docs/LangRef.html#llvm-var-annotation-intrinsic

[jeffreytakahashi]

@tim-pugh See above post from Cole, we're really struggling with this.

[tim-pugh]

https://stackoverflow.com/questions/4736738/how-can-you-add-a-new-keyword-to-clang-a-keyword-that-would-be-treated-as-main

My idea of an alternative wouldn't fix anything (it would make it more complicated actually). It would be adding a keyword to clang which they state can be more error prone.

[tim-pugh]

https://clang.llvm.org/docs/ClangPlugins.html

in here you can read a section talking about "pragmas". I'm really not sure if using this would be the correct approach, but it talks about registering them with the clang compiler via a plugin.

[jeffreytakahashi]

So after a lot of reading and a bit of poking we can add annotations to both variables and functions such that they show up in llvm metadata, however they do not when assigned to structs. In either case, if we are reading this correctly (and we would really like someone to verify that we are). I will post screenshots soon, and we will get some links up soon, but it appears that even using attribute((packed)) does not show up in the bytecode so we're not sure if it's just there in a way we cannot see.

[jeffreytakahashi]

[jeffreytakahashi]

http://bholt.org/posts/llvm-quick-tricks.html

[Nixoncole]

Found a half-baked explanation for parsing the metadata: https://stackoverflow.com/questions/15114284/llvm-get-annotations