[PATCH] D115274: [IR][RFC] Memory region declaration intrinsic

Mon Mar 21 15:36:30 PDT 2022

arichardson added a comment.

In general I like this and would be very happy if we had a better way of dealing with sub-objects in LLVM.

Looking at this from the CHERI perspective, it seems like we could enforce this intrinsic at runtime by lowering it the same way as we current do for the out-of-tree`@llvm.cheri.bounds.set()`. This intrinsic also creates a memory subregion, the only difference as far as I can see it is that negative start offsets have to be encoded by doing a negative GEP first since we only have a pointer+length argument. And of course that it is enforced at runtime by narrowing the bounds that are part of the CHERI pointer.

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Comment at: llvm/docs/LangRef.rst:20876
+to be able to annotate array bounds in C family of languages,
+which may allow alloca splitting, and better alias analysis.
+
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Do you envision this being used for all sub-object pointer creations? If so it might need a flag to disable it since it might break some C patterns such as `container_of`.

According to https://godbolt.org/z/evTbejaMf the container_of macro results in an inbounds GEP, so with sufficient inlining things might break?

About three years ago I spent quite a lot of time enforcing sub-object bounds at runtime using CHERI. Almost all code works just fine but there are things such as container_of() that require opt-out annotations. I wrote about the incompatibilities that I found in Chapter 5 of https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-949.pdf. TL;DR: not many changes needed - about 50 annotations across the entire FreeBSD source tree. Almost all annotations due to container_of or emulation of C++ inheritance in C.

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https://reviews.llvm.org/D115274