[PATCH v5 bpf-next 7/9] bpf: Add info about .BTF.ids section to btf.rst

Jiri Olsa
2020-07-03T09:52:00Z

Updating btf.rst doc with info about .BTF.ids section

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
Documentation/bpf/btf.rst | 36 ++++++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index 4d565d202ce3..17ebd78d2c9e 100644
--- a/Documentation/bpf/btf.rst
+++ b/Documentation/bpf/btf.rst
@@ -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 ``int 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
************

--
2.25.4