On Tue, Sep 14, 2021 at 3:30 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> 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], [4]).
> 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.
>
> In the rest of the series, Patches 1-2 had
> kernel support. Patches 3-4 added
> libbpf support. Patch 5 added bpftool
> support. Patches 6-10 added various selftests.
> Patch 11 added documentation for the new kind.
>
> [1] https://reviews.llvm.org/D106614
> [2] https://reviews.llvm.org/D106621
> [3] https://reviews.llvm.org/D106622
> [4] https://reviews.llvm.org/D109560
>
> Changelog:
> v2 -> v3:
> - put NR_BTF_KINDS and BTF_KIND_MAX into enum as well
> - check component_idx earlier (check_meta stage) in kernel
> - add more tests
> - fix misc nits
Applied. Please send an update to selftests/bpf/README.
Since folks will be puzzled with messages:
progs/tag.c:23:20: warning: unknown attribute 'btf_tag' ignored
[-Wunknown-attributes]
Even with old clang:
./test_progs -t tag
#21 btf_tag:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
The test probably should fail with old clang ?
