On Mon, Jan 27, 2025 at 3:40 PM Ihor Solodrai <ihor.solodrai@xxxxxxxxx> wrote:
>
> Explain the meaning of kind_flag in BTF type_tags and decl_tags.
> Update uapi btf.h kind_flag comment to reflect the changes.
>
> Signed-off-by: Ihor Solodrai <ihor.solodrai@xxxxxxxxx>
> ---
> Documentation/bpf/btf.rst | 25 +++++++++++++++++++++----
> include/uapi/linux/btf.h | 3 ++-
> tools/include/uapi/linux/btf.h | 3 ++-
> 3 files changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
> index 2478cef758f8..48d9b33d59db 100644
> --- a/Documentation/bpf/btf.rst
> +++ b/Documentation/bpf/btf.rst
> @@ -102,7 +102,8 @@ Each type contains the following common data::
> * bits 24-28: kind (e.g. int, ptr, array...etc)
> * bits 29-30: unused
> * bit 31: kind_flag, currently used by
> - * struct, union, fwd, enum and enum64.
> + * struct, union, enum, fwd, enum64,
> + * decl_tag and type_tag
tabs instead of spaces (docs seem to be using spaces)
> */
> __u32 info;
> /* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
> @@ -478,7 +479,7 @@ No additional type data follow ``btf_type``.
>
> ``struct btf_type`` encoding requirement:
> * ``name_off``: offset to a non-empty string
> - * ``info.kind_flag``: 0
> + * ``info.kind_flag``: 0 or 1
> * ``info.kind``: BTF_KIND_DECL_TAG
> * ``info.vlen``: 0
> * ``type``: ``struct``, ``union``, ``func``, ``var`` or ``typedef``
> @@ -489,7 +490,6 @@ No additional type data follow ``btf_type``.
> __u32 component_idx;
> };
>
> -The ``name_off`` encodes btf_decl_tag attribute string.
> The ``type`` should be ``struct``, ``union``, ``func``, ``var`` or ``typedef``.
> For ``var`` or ``typedef`` type, ``btf_decl_tag.component_idx`` must be ``-1``.
> For the other three types, if the btf_decl_tag attribute is
> @@ -499,12 +499,21 @@ the attribute is applied to a ``struct``/``union`` member or
> a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a
> valid index (starting from 0) pointing to a member or an argument.
>
> +If ``info.kind_flag`` is 0, then this is a normal decl tag, and the
> +``name_off`` encodes btf_decl_tag attribute string.
> +
> +If ``info.kind_flag`` is 1, then the decl tag represents an arbitrary
> +__attribute__. In this case, ``name_off`` encodes a string
> +representing the attribute-list of the attribute specifier. For
> +example, for an ``__attribute__((aligned(4)))`` the string's contents
> +is ``aligned(4)``.
> +
> 2.2.18 BTF_KIND_TYPE_TAG
> ~~~~~~~~~~~~~~~~~~~~~~~~
>
> ``struct btf_type`` encoding requirement:
> * ``name_off``: offset to a non-empty string
> - * ``info.kind_flag``: 0
> + * ``info.kind_flag``: 0 or 1
> * ``info.kind``: BTF_KIND_TYPE_TAG
> * ``info.vlen``: 0
> * ``type``: the type with ``btf_type_tag`` attribute
> @@ -522,6 +531,14 @@ type_tag, then zero or more const/volatile/restrict/typedef
> and finally the base type. The base type is one of
> int, ptr, array, struct, union, enum, func_proto and float types.
>
> +Similarly to decl tags, if the ``info.kind_flag`` is 0, then this is a
> +normal type tag, and the ``name_off`` encodes btf_type_tag attribute
> +string.
> +
> +If ``info.kind_flag`` is 1, then the type tag represents an arbitrary
> +__attribute__, and the ``name_off`` encodes a string representing the
> +attribute-list of the attribute specifier.
> +
> 2.2.19 BTF_KIND_ENUM64
> ~~~~~~~~~~~~~~~~~~~~~~
>
> diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
> index ec1798b6d3ff..266d4ffa6c07 100644
> --- a/include/uapi/linux/btf.h
> +++ b/include/uapi/linux/btf.h
> @@ -36,7 +36,8 @@ struct btf_type {
> * bits 24-28: kind (e.g. int, ptr, array...etc)
> * bits 29-30: unused
> * bit 31: kind_flag, currently used by
> - * struct, union, enum, fwd and enum64
> + * struct, union, enum, fwd, enum64,
> + * decl_tag and type_tag
> */
> __u32 info;
> /* "size" is used by INT, ENUM, STRUCT, UNION, DATASEC and ENUM64.
> diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
> index ec1798b6d3ff..266d4ffa6c07 100644
> --- a/tools/include/uapi/linux/btf.h
> +++ b/tools/include/uapi/linux/btf.h
> @@ -36,7 +36,8 @@ struct btf_type {
> * bits 24-28: kind (e.g. int, ptr, array...etc)
> * bits 29-30: unused
> * bit 31: kind_flag, currently used by
> - * struct, union, enum, fwd and enum64
> + * struct, union, enum, fwd, enum64,
> + * decl_tag and type_tag
> */
> __u32 info;
> /* "size" is used by INT, ENUM, STRUCT, UNION, DATASEC and ENUM64.
> --
> 2.48.1
>
