[PATCH bpf-next v1 6/8] bpf,docs: Document KF_MAGIC_ARGS flag and __magic annotation

Ihor Solodrai
2025-10-29T19:01:58Z

Add sections explaining KF_MAGIC_ARGS kfunc flag and __magic argument
annotation. Mark __prog annotation as deprecated.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
Documentation/bpf/kfuncs.rst | 49 +++++++++++++++++++++++++++++++++++-
1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index e38941370b90..1a261f84e157 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -160,7 +160,7 @@ Or::
...
}

-2.2.6 __prog Annotation
+2.2.6 __prog Annotation (deprecated, use __magic instead)
---------------------------
This annotation is used to indicate that the argument needs to be fixed up to
the bpf_prog_aux of the caller BPF program. Any value passed into this argument
@@ -177,6 +177,37 @@ An example is given below::
...
}

+.. _magic_annotation:
+
+2.2.7 __magic Annotation
+---------------------------
+This annotation is used in kfuncs with KF_MAGIC_ARGS flag to indicate the
+arguments that are omitted in the BPF signature of the kfunc. The actual
+values for __magic arguments are set by the verifier at load time, and
+depend on the argument type.
+
+Currently only ``struct bpf_prog_aux *`` type is supported.
+
+Example declaration:
+
+.. code-block:: c
+
+ __bpf_kfunc int bpf_wq_set_callback(struct bpf_wq *wq,
+ int (callback_fn)(void *map, int *key, void *value),
+ unsigned int flags,
+ struct bpf_prog_aux *aux__magic)
+ {
+ ...
+ }
+
+Example usage:
+
+.. code-block:: c
+
+ /* note the last argument is omitted */
+ if (bpf_wq_set_callback(wq, callback_fn, 0))
+ return -1;
+
.. _BPF_kfunc_nodef:

2.3 Using an existing kernel function
@@ -374,6 +405,22 @@ encouraged to make their use-cases known as early as possible, and participate
in upstream discussions regarding whether to keep, change, deprecate, or remove
those kfuncs if and when such discussions occur.

+2.4.10 KF_MAGIC_ARGS flag
+------------------------------------
+
+The KF_MAGIC_ARGS flag is used to indicate that the BPF signature of the kfunc
+is different from it's kernel signature, and the values for arguments annotated
+with __magic suffix are provided at load time by the verifier.
+
+A kfunc with KF_MAGIC_ARGS flag therefore has two types in BTF: one function
+matching the kernel declaration (with _impl suffix by convention), and another
+matching the intended BPF API.
+
+Verifier allows calls to both _impl and non-_impl versions of a magic kfunc.
+
+Note that only arguments of particular types can be __magic.
+See :ref:`magic_annotation`.
+
2.5 Registering the kfuncs
--------------------------

--
2.51.1