[all-commits] [llvm/llvm-project] cef638: [Bounds-Safety] Temporarily relax a `counted_by` a...

[Dan Liew via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cef6387e52578366c2332275dad88b9953b55336
      https://github.com/llvm/llvm-project/commit/cef6387e52578366c2332275dad88b9953b55336
  Author: Dan Liew <dan at su-root.co.uk>
  Date:   2024-05-17 (Fri, 17 May 2024)

  Changed paths:
    M clang/include/clang/Basic/DiagnosticGroups.td
    M clang/include/clang/Basic/DiagnosticSemaKinds.td
    M clang/lib/Sema/SemaDeclAttr.cpp
    M clang/test/Sema/attr-counted-by-vla.c

  Log Message:
  -----------
  [Bounds-Safety] Temporarily relax a `counted_by` attribute restriction on flexible array members

In 0ec3b972e58bcbcdc1bebe1696ea37f2931287c3 an additional restriction
was added when applying the `counted_by` attribute to flexible array
members in structs. The restriction prevented the element type being
a struct that itself had a flexible array member. E.g.:

```
struct has_unannotated_VLA {
  int count;
  char buffer[];
};

struct buffer_of_structs_with_unnannotated_vla {
  int count;
  struct has_unannotated_VLA Arr[] __counted_by(count);
};
```

In this example assuming the size of `Arr` is `sizeof(struct
has_unannotated_VLA)*count` (which is what the attribute says) is wrong
because it doesn't account for the size of
`has_unannotated_VLA::buffer`. This is why this kind of code construct
was treated as an error.

However, it turns out existing Linux kernel code used the attribute
on a flexible array member in this way
(https://github.com/llvm/llvm-project/pull/90786#issuecomment-2118416515).

To unbreak the build this restriction is downgraded to a warning with
the plan to make it an error again once the errornous use of the
attribute in the Linux kernel is resolved.



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