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

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Message-ID: <CAEf4Bzbz4jY3cKGxPro7yn_2tjKqkK6P+oU8_8ZZhAfewkNEnw@mail.gmail.com>
Date: Tue, 4 Nov 2025 14:47:01 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Amery Hung <ameryhung@...il.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, alexei.starovoitov@...il.com, 
	andrii@...nel.org, daniel@...earbox.net, tj@...nel.org, martin.lau@...nel.org, 
	kernel-team@...a.com
Subject: Re: [PATCH bpf-next v5 2/7] bpf: Support associating BPF program with struct_ops

On Tue, Nov 4, 2025 at 9:27 AM Amery Hung <ameryhung@...il.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@...il.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(+)
>

[...]

>  static void bpf_struct_ops_map_free_image(struct bpf_struct_ops_map *st_map)
>  {
>         int i;
> @@ -801,6 +812,12 @@ static long bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
>                         goto reset_unlock;
>                 }
>
> +               err = bpf_prog_assoc_struct_ops(prog, &st_map->map);
> +               if (err) {
> +                       bpf_prog_put(prog);
> +                       goto reset_unlock;
> +               }
> +
>                 link = kzalloc(sizeof(*link), GFP_USER);
>                 if (!link) {

I think we need to call bpf_prog_disassoc_struct_ops() here if
kzalloc() fails (just like we do bpf_prog_put). After kzalloc link
will be put into plink list and generic clean up path will handle all
this, but not here.

>                         bpf_prog_put(prog);
> @@ -980,6 +997,8 @@ static void bpf_struct_ops_map_free(struct bpf_map *map)
>         if (btf_is_module(st_map->btf))
>                 module_put(st_map->st_ops_desc->st_ops->owner);
>
> +       bpf_struct_ops_map_dissoc_progs(st_map);
> +
>         bpf_struct_ops_map_del_ksyms(st_map);
>
>         /* The struct_ops's function may switch to another struct_ops.

[...]