Re: [PATCH bpf-next v5 2/7] bpf: Support associating BPF program with struct_ops

Song Liu
2025-11-04T22:00:08Z

> On Nov 4, 2025, at 9:26 AM, Amery Hung <ameryhung@gmail.com> wrote:
>
> Add a new BPF command BPF_PROG_ASSOC_STRUCT_OPS to allow associating
> a BPF program with a struct_ops map. This command takes a file
> descriptor of a struct_ops map and a BPF program and set
> prog->aux->st_ops_assoc to the kdata of the struct_ops map.
>
> The command does not accept a struct_ops program nor a non-struct_ops
> map. Programs of a struct_ops map is automatically associated with the
> map during map update. If a program is shared between two struct_ops
> maps, prog->aux->st_ops_assoc will be poisoned to indicate that the
> associated struct_ops is ambiguous. The pointer, once poisoned, cannot
> be reset since we have lost track of associated struct_ops. For other
> program types, the associated struct_ops map, once set, cannot be
> changed later. This restriction may be lifted in the future if there is
> a use case.
>
> A kernel helper bpf_prog_get_assoc_struct_ops() can be used to retrieve
> the associated struct_ops pointer. The returned pointer, if not NULL, is
> guaranteed to be valid and point to a fully updated struct_ops struct.
> For struct_ops program reused in multiple struct_ops map, the return
> will be NULL.
>
> To make sure the returned pointer to be valid, the command increases the
> refcount of the map for every associated non-struct_ops programs. For
> struct_ops programs, the destruction of a struct_ops map already waits for
> its BPF programs to finish running. A later patch will further make sure
> the map will not be freed when an async callback schedule from struct_ops
> is running.
>
> struct_ops implementers should note that the struct_ops returned may or
> may not be attached. The struct_ops implementer will be responsible for
> tracking and checking the state of the associated struct_ops map if the
> use case requires an attached struct_ops.
>
> Signed-off-by: Amery Hung <ameryhung@gmail.com>
> ---
> include/linux/bpf.h | 16 ++++++
> include/uapi/linux/bpf.h | 17 +++++++
> kernel/bpf/bpf_struct_ops.c | 90 ++++++++++++++++++++++++++++++++++
> kernel/bpf/core.c | 3 ++
> kernel/bpf/syscall.c | 46 +++++++++++++++++
> tools/include/uapi/linux/bpf.h | 17 +++++++
> 6 files changed, 189 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a47d67db3be5..0f71030c03e1 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1726,6 +1726,8 @@ struct bpf_prog_aux {
> struct rcu_head rcu;
> };
> struct bpf_stream stream[2];
> + struct mutex st_ops_assoc_mutex;
> + struct bpf_map *st_ops_assoc;
> };

In the bpf-oom thread, we agreed (mostly agreed?) that we will allow
attaching a struct_ops map multiple times.

To match this design, shall we associate a BPF program with a
bpf_struct_ops_link instead of bpf_map? This requires one more
pointer deref to get the pointer to the struct_ops map. But the
solution will be more future proof.

Does this make sense?

Thanks,
Song