From b72903847af014342e0a22d8b595ad09a7eb45a0 Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Thu, 27 Jan 2022 07:46:27 -0800 Subject: [PATCH] docs/bpf: clarify how btf_type_tag gets encoded in the type chain Clarify where the BTF_KIND_TYPE_TAG gets encoded in the type chain, so applications and kernel can properly parse them. Signed-off-by: Yonghong Song Link: https://lore.kernel.org/r/20220127154627.665163-1-yhs@fb.com Signed-off-by: Alexei Starovoitov --- Documentation/bpf/btf.rst | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst index ab08852..7940da9 100644 --- a/Documentation/bpf/btf.rst +++ b/Documentation/bpf/btf.rst @@ -503,6 +503,19 @@ valid index (starting from 0) pointing to a member or an argument. * ``info.vlen``: 0 * ``type``: the type with ``btf_type_tag`` attribute +Currently, ``BTF_KIND_TYPE_TAG`` is only emitted for pointer types. +It has the following btf type chain: +:: + + ptr -> [type_tag]* + -> [const | volatile | restrict | typedef]* + -> base_type + +Basically, a pointer type points to zero or more +type_tag, then zero or more const/volatile/restrict/typedef +and finally the base type. The base type is one of +int, ptr, array, struct, union, enum, func_proto and float types. + 3. BTF Kernel API ================= -- 2.7.4