Hi Yonghong.
> LLVM14 added support for a new C attribute ([1])
> __attribute__((btf_tag("arbitrary_str")))
> This attribute will be emitted to dwarf ([2]) and pahole
> will convert it to BTF. Or for bpf target, this
> attribute will be emitted to BTF directly ([3]).
> The attribute is intended to provide additional
> information for
> - struct/union type or struct/union member
> - static/global variables
> - static/global function or function parameter.
>
> This new attribute can be used to add attributes
> to kernel codes, e.g., pre- or post- conditions,
> allow/deny info, or any other info in which only
> the kernel is interested. Such attributes will
> be processed by clang frontend and emitted to
> dwarf, converting to BTF by pahole. Ultimiately
> the verifier can use these information for
> verification purpose.
>
> The new attribute can also be used for bpf
> programs, e.g., tagging with __user attributes
> for function parameters, specifying global
> function preconditions, etc. Such information
> may help verifier to detect user program
> bugs.
>
> After this series, pahole dwarf->btf converter
> will be enhanced to support new llvm tag
> for btf_tag attribute. With pahole support,
> we will then try to add a few real use case,
> e.g., __user/__rcu tagging, allow/deny list,
> some kernel function precondition, etc,
> in the kernel.
We are looking into implementing this in the GCC BPF port.
Supporting the new C attribute in BPF programs as a target-specific
attribute, and the new BTF kind, is straightforward enough.
However, I am afraid it will be difficult to upstream to GCC support for
a target-independent C attribute called `btf_tag' that emits a
LLVM-specific DWARF tag. Even if we proposed to use a GCC-specific
DWARF tag like DW_TAG_GNU_annotation using the same number, or better a
compiler neutral tag like DW_TAG_annotation or DW_TAG_BPF_annotation,
adding such an attribute for all targets would still likely to be much
controversial...
Would you be open to explore other, more generic, ways to convey these
annotations to pahole, something that could be easily supported by GCC,
and potentially other C compilers?
