[cfe-dev] BPF: adding new clang extension bpf_dominating_decl attribute

[Aaron Ballman via cfe-dev]

On Mon, Dec 20, 2021 at 7:06 PM Y Song <ys114321 at gmail.com> wrote:
>
> This is a request to add a clang extention, more specificly,
> a clang attribute named bpf_dominating_decl. This clang
> extention is intended to be used for bpf target only. Below
> I will explain in detail about this proposed attribute, why
> bpf community needs this, how it will be used and other
> aspects as described in https://clang.llvm.org/get_involved.html.

Thank you for this RFC!

> Evidence of a significant user community
> ========================================
>
> We are proposing a new clang attribute bpf_dominating_decl which
> was implemented in [1]. The feature has also been discussed in
> cfe-dev mailing list ([2]). It intended to solve the
> following use case:
>   - A tool generated vmlinux.h is used for CO-RE (compile once,
>     run everywhere) use cases.
>   - vmlinux.h contains all kernel data structures for a particular config,
>     see [3] and [4] about how it is generated and why it is important.
>   - but vmlinux.h may have type conflicts with other headers
>     user intends to use.
>
> Macros are such an example. Currently CO-RE relocation cannot
> handle macros and macros may be defined in some header files accessible
> to the user. If those header files have type conflict with vmlinux.h,
> users are forced to copy macro definitions. The same for some simple
> static inline functions defined in header files. This issue has been
> discussed before and that is why we proposed this issue. And just last
> week, it is discussed/complained again ([5]) for not able to use
> some non-kernel types with a header file which has some type conflicts
> with vmlinux.h.
>
> If it is accepted, the attribute will be used inside the vmlinux.h and
> it will be used by virtually all bpf developers and it will make bpf devlopers
> more productive by not copying macros, static inline functions or
> non-kernel types.

I'm uncomfortable with this attribute. Typically, attributes extend
rather than redefine the language. e.g., you might add attributes for
better performance or diagnostic characteristics, but you typically
should not use an attribute to redefine the basic premises of the
language.

In this particular case, the attribute is used to tell the compiler to
ignore type redefinition errors and instead pick a "dominating"
declaration for the type. While C isn't as type sensitive as C++ is,
it still has _Generic, __typeof__, and other tricks that can expose
type system shenanigans like this in surprising ways. Given that type
size information is critical for many things in C (memcpy, memcmp,
pointer arithmetic with offsetof, etc), I'm uncomfortable with the
security aspects of the likely type confusion stemming from this being
so novel in C.

That said, we do have *one* attribute that I consider to be a
"redefine the language in fundamental ways" feature --
[[clang::overloadable]] allows you to define overload sets in C, which
is a distinctly not-C thing to do because of the name mangling
involved. However, that attribute introduces the C++ semantics into C
whereas the BPF dominating declaration attribute is introducing wholly
novel semantics. So I don't really consider [[clang::overloadable]] as
direct precedent for this.

>
> A specific need to reside within the Clang tree
> ===============================================
>
> The proposed attribute will be processed by Clang frontend lex and
> sema and it would be
> best to reside within the Clang tree.

Would it be plausible/appropriate to instead run the source code
through a processing tool which emits modified source code with the
correct definitions instead of hoping this dominating declaration
works out? In this case, I think the user will get better diagnostic
behavior in the places where there *is* type confusion the compiler
can detect, but the user will at least have an easier time *debugging*
any problems from this.

Which brings up an interesting question -- how does this attribute
interact with debug information?

~Aaron

>
> A specification
> ===============
>
> The bpf_dominating_decl attribute, as its name suggested, is a bpf target
> specific attribute. The attribute is also for C language only.
>
> The syntax:
>    __attribute__((bpf_dominating_decl))
> The attribute can be attached to:
>    record, typedef and enum type declarations
>
> If a type declaration is annotated with bpf_dominating_decl attribute,
> that type declaration is considered as a dominating decl and later
> any conflict declaration, compatible or not-compatible,
> will be silently ignored.
>
> The following is a simple example,
>
>   #pragma clang attribute push(__attribute__((bpf_dominating_decl)), \
>                                apply_to = any(record, enum, type_alias))
>   typedef unsigned bob;
>   #pragma clang attribute pop
>   typedef int bob;
>
> In the above example, the second typedef will not incur a compilation
> error. The same for record types or enum values.
>
> Representation within the appropriate governing organization
> ============================================================
>
> N/A
>
> A long-term support plan
> ========================
>
> The feature will be supported for ever.
>
> A high-quality implementation
> =============================
>
> The feature has been implemented in [1] and has been code reviewed.
> One thing not in [1] but in [1]'s comment is to link
> bpf_dominating_decl attribute to CO-RE preserve_access_index attribute
> in clang frontend. That is, a type is considered dominating only if
> the type has both bpf_dominating_decl and preserve_access_index attributes.
> We can amend the implementation with this if agreed upon.
>
> A test suite
> ============
>
> The [1] has implemented necessary tests for the feature.
>
> References
> ==========
>   [1] https://reviews.llvm.org/D111307
>   [2] https://lists.llvm.org/pipermail/cfe-dev/2021-August/068696.html
>   [3] https://www.grant.pizza/blog/vmlinux-header/
>   [4] https://blog.aquasec.com/vmlinux.h-ebpf-programs
>   [5] https://lore.kernel.org/bpf/CAEf4BzZ1K9uQ-K1Q2BCSBesR3RUj_NW8uHu6NduoX7uLBdfukQ@mail.gmail.com/