Commit 48f5a6c4 authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov
Browse files

docs/bpf: Add documentation for BTF_KIND_TAG 



Add BTF_KIND_TAG documentation in btf.rst.

Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223103.249100-1-yhs@fb.com
parent c240ba28

Documentation/bpf/btf.rst

+28 −1
Original line number Diff line number Diff line
@@ -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 +466,32 @@ 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``.

For ``var`` type, ``btf_tag.component_idx`` must be ``-1``.

For the other three types, if the btf_tag attribute is

applied to the ``struct``, ``union`` or ``func`` itself,

``btf_tag.component_idx`` must be ``-1``. Otherwise,

the 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

*****************