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

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Message-ID: <CAMB2axM8DuaCbO=jHf+XbqM_9YyTqdQpUH738c1=dnYBaXRMvg@mail.gmail.com>
Date: Tue, 4 Nov 2025 15:27:19 -0800
From: Amery Hung <ameryhung@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...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 2:47 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> 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.
>

1 point for catching a bug that was missed by AI. I will move
bpf_prog_assoc_struct_ops() after the link is successfully created.

> >                         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.
>
> [...]