Commit 232ce4be authored by Jiri Olsa's avatar Jiri Olsa Committed by Alexei Starovoitov
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bpf: Add info about .BTF_ids section to btf.rst 



Updating btf.rst doc with info about .BTF_ids section

Signed-off-by: default avatarJiri Olsa <jolsa@kernel.org>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Tested-by: default avatarAndrii Nakryiko <andriin@fb.com>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200711215329.41165-8-jolsa@kernel.org
parent 49f4e672

Documentation/bpf/btf.rst

+36 −0
Original line number Diff line number Diff line
@@ -691,6 +691,42 @@ kernel API, the ``insn_off`` is the instruction offset in the unit of ``struct 
bpf_insn``. For ELF API, the ``insn_off`` is the byte offset from the

beginning of section (``btf_ext_info_sec->sec_name_off``).



4.2 .BTF_ids section

====================



The .BTF_ids section encodes BTF ID values that are used within the kernel.



This section is created during the kernel compilation with the help of

macros defined in ``include/linux/btf_ids.h`` header file. Kernel code can

use them to create lists and sets (sorted lists) of BTF ID values.



The ``BTF_ID_LIST`` and ``BTF_ID`` macros define unsorted list of BTF ID values,

with following syntax::



  BTF_ID_LIST(list)

  BTF_ID(type1, name1)

  BTF_ID(type2, name2)



resulting in following layout in .BTF_ids section::



  __BTF_ID__type1__name1__1:

  .zero 4

  __BTF_ID__type2__name2__2:

  .zero 4



The ``u32 list[];`` variable is defined to access the list.



The ``BTF_ID_UNUSED`` macro defines 4 zero bytes. It's used when we

want to define unused entry in BTF_ID_LIST, like::



      BTF_ID_LIST(bpf_skb_output_btf_ids)

      BTF_ID(struct, sk_buff)

      BTF_ID_UNUSED

      BTF_ID(struct, task_struct)



All the BTF ID lists and sets are compiled in the .BTF_ids section and

resolved during the linking phase of kernel build by ``resolve_btfids`` tool.



5. Using BTF

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