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bpf,docs: Document KF_IMPLICIT_ARGS flag

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Add a section explaining KF_IMPLICIT_ARGS kfunc flag. Remove __prog
arg annotation, as it is no longer supported.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260120223027.3981805-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Ihor Solodrai
2026-01-20 14:30:26 -08:00
committed by Alexei Starovoitov
parent aed57a3638
commit 74bc4f6127
1 changed files with 32 additions and 17 deletions
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49
Documentation/bpf/kfuncs.rst
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@@ -232,23 +232,6 @@ Or::
...
}
2.3.6 __prog Annotation
---------------------------
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
is ignored, and rewritten by the verifier.
An example is given below::
__bpf_kfunc int bpf_wq_set_callback_impl(struct bpf_wq *wq,
int (callback_fn)(void *map, int *key, void *value),
unsigned int flags,
void *aux__prog)
{
struct bpf_prog_aux *aux = aux__prog;
...
}
.. _BPF_kfunc_nodef:
2.4 Using an existing kernel function
@@ -381,6 +364,38 @@ 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.
2.5.9 KF_IMPLICIT_ARGS flag
------------------------------------
The KF_IMPLICIT_ARGS flag is used to indicate that the BPF signature
of the kfunc is different from it's kernel signature, and the values
for implicit arguments are provided at load time by the verifier.
Only arguments of specific types are implicit.
Currently only ``struct bpf_prog_aux *`` type is supported.
A kfunc with KF_IMPLICIT_ARGS flag therefore has two types in BTF: one
function matching the kernel declaration (with _impl suffix in the
name by convention), and another matching the intended BPF API.
Verifier only allows calls to the non-_impl version of a kfunc, that
uses a signature without the implicit arguments.
Example declaration:
.. code-block:: c
__bpf_kfunc int bpf_task_work_schedule_signal(struct task_struct *task, struct bpf_task_work *tw,
void *map__map, bpf_task_work_callback_t callback,
struct bpf_prog_aux *aux) { ... }
Example usage in BPF program:
.. code-block:: c
/* note that the last argument is omitted */
bpf_task_work_schedule_signal(task, &work->tw, &arrmap, task_work_callback);
2.6 Registering the kfuncs
--------------------------