From: Yonghong Song <yhs@fb.com>
Date: Thu, 27 Jan 2022 15:46:27 +0000 (-0800)
Subject: docs/bpf: clarify how btf_type_tag gets encoded in the type chain
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=b72903847af014342e0a22d8b595ad09a7eb45a0;p=linux.git

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 <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154627.665163-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---

diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index ab08852e53ae2..7940da9bc6c1e 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
 =================