From: Jiri Olsa <jolsa@kernel.org>
To: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andriin@fb.com>
Cc: netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: [PATCH v7 bpf-next 7/9] bpf: Add info about .BTF_ids section to btf.rst
Date: Sat, 11 Jul 2020 23:53:27 +0200 [thread overview]
Message-ID: <20200711215329.41165-8-jolsa@kernel.org> (raw)
In-Reply-To: <20200711215329.41165-1-jolsa@kernel.org>
Updating btf.rst doc with info about .BTF_ids section
Acked-by: Andrii Nakryiko <andriin@fb.com>
Tested-by: Andrii Nakryiko <andriin@fb.com>
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..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
************
--
2.25.4
next prev parent reply other threads:[~2020-07-11 21:53 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-11 21:53 [PATCH v7 bpf-next 0/9] bpf: Add d_path helper - preparation changes Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 1/9] bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 2/9] bpf: Compile resolve_btfids tool at kernel compilation start Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 3/9] bpf: Add BTF_ID_LIST/BTF_ID/BTF_ID_UNUSED macros Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 4/9] bpf: Resolve BTF IDs in vmlinux image Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 5/9] bpf: Remove btf_id helpers resolving Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 6/9] bpf: Use BTF_ID to resolve bpf_ctx_convert struct Jiri Olsa
2020-07-11 21:53 ` Jiri Olsa [this message]
2020-07-11 21:53 ` [PATCH v7 bpf-next 8/9] tools headers: Adopt verbatim copy of btf_ids.h from kernel sources Jiri Olsa
2020-07-11 21:53 ` [PATCH v7 bpf-next 9/9] selftests/bpf: Add test for resolve_btfids Jiri Olsa
2020-07-13 22:07 ` [PATCH v7 bpf-next 0/9] bpf: Add d_path helper - preparation changes Alexei Starovoitov
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