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author | Jiri Olsa <jolsa@kernel.org> | 2020-07-11 23:53:27 +0200 |
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committer | Alexei Starovoitov <ast@kernel.org> | 2020-07-13 19:42:03 +0200 |
commit | 232ce4be295743deb2cf9a7cf9c60a4fde367964 (patch) | |
tree | 2aa14d6bc698905419ca9670f381c83bb56fc340 | |
parent | bpf: Use BTF_ID to resolve bpf_ctx_convert struct (diff) | |
download | linux-232ce4be295743deb2cf9a7cf9c60a4fde367964.tar.xz linux-232ce4be295743deb2cf9a7cf9c60a4fde367964.zip |
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 <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
-rw-r--r-- | Documentation/bpf/btf.rst | 36 |
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_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 ************ |