path: root/Documentation/bpf
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authorJiri Olsa <>2020-07-11 23:53:27 +0200
committerAlexei Starovoitov <>2020-07-13 10:42:03 -0700
commit232ce4be295743deb2cf9a7cf9c60a4fde367964 (patch)
tree2aa14d6bc698905419ca9670f381c83bb56fc340 /Documentation/bpf
parent49f4e6720748c778795df62d222d0200b61345be (diff)
bpf: Add info about .BTF_ids section to btf.rst
Updating btf.rst doc with info about .BTF_ids section Signed-off-by: Jiri Olsa <> Signed-off-by: Alexei Starovoitov <> Tested-by: Andrii Nakryiko <> Acked-by: Andrii Nakryiko <> Link:
Diffstat (limited to 'Documentation/bpf')
1 files changed, 36 insertions, 0 deletions
diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index 4d565d202ce3..b5361b8621c9 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 ``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(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