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bpf: Add info about .BTF_ids section to btf.rst

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Updating btf.rst doc with info about .BTF_ids section

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200711215329.41165-8-jolsa@kernel.org
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Jiri Olsa 2020-07-11 23:53:27 +02:00 committed by Alexei Starovoitov
parent 49f4e67207
commit 232ce4be29
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Documentation/bpf/btf.rst
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@ -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
************