netdev - Re: [PATCH bpf-next v3 2/4] bpf: Support associating BPF program with struct

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Message-ID: <CAMB2axPZaXvqvJx8qTFVe61q8-tmSdb4udyHW4PQ-iL4Am1wSA@mail.gmail.com>
Date: Fri, 17 Oct 2025 15:11:57 -0700
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 v3 2/4] bpf: Support associating BPF program with struct_ops

On Fri, Oct 17, 2025 at 3:05 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Fri, Oct 17, 2025 at 2:56 PM 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 or 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 pointer returned, if not NULL, is
> > guaranteed to be valid and point to a fully updated struct_ops struct.
> > This is done by increasing the refcount of the map for every associated
> > non-struct_ops programs. For struct_ops program reused in multiple
> > struct_ops map, the return will be NULL. 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    | 70 ++++++++++++++++++++++++++++++++++
> >  kernel/bpf/core.c              |  3 ++
> >  kernel/bpf/syscall.c           | 46 ++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 17 +++++++++
> >  6 files changed, 169 insertions(+)
> >
>
> [...]
>
> > @@ -1394,6 +1401,69 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
> >         return err;
> >  }
> >
> > +int bpf_prog_assoc_struct_ops(struct bpf_prog *prog, struct bpf_map *map)
> > +{
> > +       guard(mutex)(&prog->aux->st_ops_assoc_mutex);
> > +
> > +       if (prog->aux->st_ops_assoc && prog->aux->st_ops_assoc != map) {
> > +               if (prog->type == BPF_PROG_TYPE_STRUCT_OPS) {
> > +                       WRITE_ONCE(prog->aux->st_ops_assoc, BPF_PTR_POISON);
> > +                       return 0;
> > +               }
> > +
> > +               return -EBUSY;
> > +       }
> > +
> > +       if (prog->type != BPF_PROG_TYPE_STRUCT_OPS)
> > +               bpf_map_inc(map);
>
> if st_ops_assoc was already set to map before, we will bump refcount
> one more time here, but we'll bpf_map_put() only once in
> bpf_prog_disassoc_struct_ops(), no?

Right. It's a bug. I will fix it in the next respin. Thanks for catching it.

>
> > +
> > +       WRITE_ONCE(prog->aux->st_ops_assoc, map);
> > +       return 0;
> > +}
> > +
> > +void bpf_prog_disassoc_struct_ops(struct bpf_prog *prog)
> > +{
> > +       struct bpf_map *map;
> > +
> > +       guard(mutex)(&prog->aux->st_ops_assoc_mutex);
> > +
> > +       map = READ_ONCE(prog->aux->st_ops_assoc);
> > +       if (!map || map == BPF_PTR_POISON)
> > +               return;
> > +
> > +       if (prog->type != BPF_PROG_TYPE_STRUCT_OPS)
> > +               bpf_map_put(map);
> > +
> > +       WRITE_ONCE(prog->aux->st_ops_assoc, NULL);
> > +}
> > +
>
> [...]