On 9/8/21 10:42 PM, Andrii Nakryiko wrote:
On Tue, Sep 7, 2021 at 4:01 PM Yonghong Song <yhs@xxxxxx> wrote:
Add BTF_KIND_TAG documentation in btf.rst.
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
Documentation/bpf/btf.rst | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)
[...]
+2.2.17 BTF_KIND_TAG
+~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+ * ``name_off``: offset to a non-empty string
+ * ``info.kind_flag``: 0 for tagging ``type``, 1 for tagging member/argument of the ``type``
+ * ``info.kind``: BTF_KIND_TAG
+ * ``info.vlen``: 0
+ * ``type``: ``struct``, ``union``, ``func`` or ``var``
+
+``btf_type`` is followed by ``struct btf_tag``.::
+
+ struct btf_tag {
+ __u32 comp_id;
+ };
+
+The ``name_off`` encodes btf_tag attribute string.
+If ``info.kind_flag`` is 1, the attribute is attached to the ``type``.
This contradicts "info.kind_flag" description above
will remove info.kind_flag stuff in the next revision.
+If ``info.kind_flag`` is 0, the attribute is attached to either a
+``struct``/``union`` member or a ``func`` argument.
+Hence the ``type`` should be ``struct``, ``union`` or
+``func``, and ``btf_tag.comp_id``, starting from 0,
+indicates which member or argument is attached with
+the attribute.
Does the kernel validate this restriction for the VAR target type?
I.e., if we have kind_flag == 0 (member of type), we should disallow
VAR, right?
Yes, I even has a selftest for that.
+
3. BTF Kernel API
*****************
--
2.30.2
