On Mon, Sep 13, 2021 at 8:53 AM Yonghong Song <yhs@xxxxxx> wrote:
>
> Add BTF_KIND_TAG documentation in btf.rst.
>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
Probably worth it to mention that component_idx for tag for VAR has to be -1.
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Documentation/bpf/btf.rst | 27 ++++++++++++++++++++++++++-
> 1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
> index 846354cd2d69..9fff578a0a35 100644
> --- a/Documentation/bpf/btf.rst
> +++ b/Documentation/bpf/btf.rst
> @@ -85,6 +85,7 @@ sequentially and type id is assigned to each recognized type starting from id
> #define BTF_KIND_VAR 14 /* Variable */
> #define BTF_KIND_DATASEC 15 /* Section */
> #define BTF_KIND_FLOAT 16 /* Floating point */
> + #define BTF_KIND_TAG 17 /* Tag */
>
> Note that the type section encodes debug info, not just pure types.
> ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
> @@ -106,7 +107,7 @@ Each type contains the following common data::
> * "size" tells the size of the type it is describing.
> *
> * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
> - * FUNC and FUNC_PROTO.
> + * FUNC, FUNC_PROTO and TAG.
> * "type" is a type_id referring to another type.
> */
> union {
> @@ -465,6 +466,30 @@ map definition.
>
> No additional type data follow ``btf_type``.
>
> +2.2.17 BTF_KIND_TAG
> +~~~~~~~~~~~~~~~~~~~
> +
> +``struct btf_type`` encoding requirement:
> + * ``name_off``: offset to a non-empty string
> + * ``info.kind_flag``: 0
> + * ``info.kind``: BTF_KIND_TAG
> + * ``info.vlen``: 0
> + * ``type``: ``struct``, ``union``, ``func`` or ``var``
> +
> +``btf_type`` is followed by ``struct btf_tag``.::
> +
> + struct btf_tag {
> + __u32 component_idx;
> + };
> +
> +The ``name_off`` encodes btf_tag attribute string.
> +The ``type`` should be ``struct``, ``union``, ``func`` or ``var``.
> +If ``btf_tag.component_idx = -1``, the btf_tag attribute is
> +applied to a valid ``type``. Otherwise, the btf_tag attribute is
> +applied to a ``struct``/``union`` member or a ``func`` argument,
> +and ``btf_tag.component_idx`` should be a valid index (starting from 0)
> +pointing to a member or an argument.
> +
> 3. BTF Kernel API
> *****************
>
> --
> 2.30.2
>
