Re: [PATCH bpf-next v5 4/7] libbpf: Add support for associating BPF program with struct_ops

Andrii Nakryiko
2025-11-05T00:46:33Z

On Tue, Nov 4, 2025 at 3:39 PM Amery Hung <ameryhung@gmail.com> wrote:
>
> On Tue, Nov 4, 2025 at 3:27 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Tue, Nov 4, 2025 at 9:27 AM Amery Hung <ameryhung@gmail.com> wrote:
> > >
> > > Add low-level wrapper and libbpf API for BPF_PROG_ASSOC_STRUCT_OPS
> > > command in the bpf() syscall.
> > >
> > > Signed-off-by: Amery Hung <ameryhung@gmail.com>
> > > ---
> > > tools/lib/bpf/bpf.c | 19 +++++++++++++++++++
> > > tools/lib/bpf/bpf.h | 21 +++++++++++++++++++++
> > > tools/lib/bpf/libbpf.c | 30 ++++++++++++++++++++++++++++++
> > > tools/lib/bpf/libbpf.h | 16 ++++++++++++++++
> > > tools/lib/bpf/libbpf.map | 2 ++
> > > 5 files changed, 88 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> > > index b66f5fbfbbb2..21b57a629916 100644
> > > --- a/tools/lib/bpf/bpf.c
> > > +++ b/tools/lib/bpf/bpf.c
> > > @@ -1397,3 +1397,22 @@ int bpf_prog_stream_read(int prog_fd, __u32 stream_id, void *buf, __u32 buf_len,
> > > err = sys_bpf(BPF_PROG_STREAM_READ_BY_FD, &attr, attr_sz);
> > > return libbpf_err_errno(err);
> > > }
> > > +
> > > +int bpf_prog_assoc_struct_ops(int prog_fd, int map_fd,
> > > + struct bpf_prog_assoc_struct_ops_opts *opts)
> > > +{
> > > + const size_t attr_sz = offsetofend(union bpf_attr, prog_assoc_struct_ops);
> > > + union bpf_attr attr;
> > > + int err;
> > > +
> > > + if (!OPTS_VALID(opts, bpf_prog_assoc_struct_ops_opts))
> > > + return libbpf_err(-EINVAL);
> > > +
> > > + memset(&attr, 0, attr_sz);
> > > + attr.prog_assoc_struct_ops.map_fd = map_fd;
> > > + attr.prog_assoc_struct_ops.prog_fd = prog_fd;
> > > + attr.prog_assoc_struct_ops.flags = OPTS_GET(opts, flags, 0);
> > > +
> > > + err = sys_bpf(BPF_PROG_ASSOC_STRUCT_OPS, &attr, attr_sz);
> > > + return libbpf_err_errno(err);
> > > +}
> > > diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> > > index e983a3e40d61..1f9c28d27795 100644
> > > --- a/tools/lib/bpf/bpf.h
> > > +++ b/tools/lib/bpf/bpf.h
> > > @@ -733,6 +733,27 @@ struct bpf_prog_stream_read_opts {
> > > LIBBPF_API int bpf_prog_stream_read(int prog_fd, __u32 stream_id, void *buf, __u32 buf_len,
> > > struct bpf_prog_stream_read_opts *opts);
> > >
> > > +struct bpf_prog_assoc_struct_ops_opts {
> > > + size_t sz;
> > > + __u32 flags;
> > > + size_t :0;
> > > +};
> > > +#define bpf_prog_assoc_struct_ops_opts__last_field flags
> > > +
> > > +/**
> > > + * @brief **bpf_prog_assoc_struct_ops** associates a BPF program with a
> > > + * struct_ops map.
> > > + *
> > > + * @param prog_fd FD for the BPF program
> > > + * @param map_fd FD for the struct_ops map to be associated with the BPF program
> > > + * @param opts optional options, can be NULL
> > > + *
> > > + * @return 0 on success; negative error code, otherwise (errno is also set to
> > > + * the error code)
> > > + */
> > > +LIBBPF_API int bpf_prog_assoc_struct_ops(int prog_fd, int map_fd,
> > > + struct bpf_prog_assoc_struct_ops_opts *opts);
> > > +
> > > #ifdef __cplusplus
> > > } /* extern "C" */
> > > #endif
> > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > > index fbe74686c97d..260e1feaa665 100644
> > > --- a/tools/lib/bpf/libbpf.c
> > > +++ b/tools/lib/bpf/libbpf.c
> > > @@ -13891,6 +13891,36 @@ int bpf_program__set_attach_target(struct bpf_program *prog,
> > > return 0;
> > > }
> > >
> > > +int bpf_program__assoc_struct_ops(struct bpf_program *prog, struct bpf_map *map,
> > > + struct bpf_prog_assoc_struct_ops_opts *opts)
> > > +{
> > > + int prog_fd;
> > > +
> > > + prog_fd = bpf_program__fd(prog);
> > > + if (prog_fd < 0) {
> > > + pr_warn("prog '%s': can't associate BPF program without FD (was it loaded?)\n",
> > > + prog->name);
> > > + return -EINVAL;
> > > + }
> > > +
> > > + if (prog->type == BPF_PROG_TYPE_STRUCT_OPS) {
> > > + pr_warn("prog '%s': can't associate struct_ops program\n", prog->name);
> > > + return -EINVAL;
> > > + }
> > > +
> > > + if (map->fd < 0) {
> >
> > heh, this is a bug. we use create_placeholder_fd() to create fixed FDs
> > associated with maps, and later we replace them with the real
> > underlying BPF map kernel objects. It's all details, but the point is
> > that this won't detect map that wasn't created. Use bpf_map__fd()
> > instead, it handles that correctly.
>
> I saw quite a few libbpf API doing this check (e.g., bpf_map__pin(),
> bpf_link__update_map(), bpf_map__attach_struct_ops()). Should we also
> fix them?

yep, probably :)

>
> >
> > > + pr_warn("map '%s': can't associate BPF map without FD (was it created?)\n", map->name);
> > > + return -EINVAL;
> > > + }
> > > +
> > > + if (!bpf_map__is_struct_ops(map)) {
> > > + pr_warn("map '%s': can't associate non-struct_ops map\n", map->name);
> > > + return -EINVAL;
> > > + }
> > > +
> > > + return bpf_prog_assoc_struct_ops(prog_fd, map->fd, opts);
> > > +}
> > > +
> > > int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
> > > {
> > > int err = 0, n, len, start, end = -1;
> > > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > > index 5118d0a90e24..45720b7c2aaa 100644
> > > --- a/tools/lib/bpf/libbpf.h
> > > +++ b/tools/lib/bpf/libbpf.h
> > > @@ -1003,6 +1003,22 @@ LIBBPF_API int
> > > bpf_program__set_attach_target(struct bpf_program *prog, int attach_prog_fd,
> > > const char *attach_func_name);
> > >
> > > +struct bpf_prog_assoc_struct_ops_opts; /* defined in bpf.h */
> > > +
> > > +/**
> > > + * @brief **bpf_program__assoc_struct_ops()** associates a BPF program with a
> > > + * struct_ops map.
> > > + *
> > > + * @param prog BPF program
> > > + * @param map struct_ops map to be associated with the BPF program
> > > + * @param opts optional options, can be NULL
> > > + *
> > > + * @return error code; or 0 if no error occurred.
> >
> > we normally specify returns like so:
> >
> > @return 0, on success; negative error code, otherwise
> >
> > keep it consistent?
>
> Okay. Will change.
>
> BTW. The return comment is copied from bpf_program__set_attach_target().

well, we should strive for consistency :)

>
> >
> > > + */
> > > +LIBBPF_API int
> > > +bpf_program__assoc_struct_ops(struct bpf_program *prog, struct bpf_map *map,
> > > + struct bpf_prog_assoc_struct_ops_opts *opts);
> > > +
> > > /**
> > > * @brief **bpf_object__find_map_by_name()** returns BPF map of
> > > * the given name, if it exists within the passed BPF object
> > > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > > index 8ed8749907d4..84fb90a016c9 100644
> > > --- a/tools/lib/bpf/libbpf.map
> > > +++ b/tools/lib/bpf/libbpf.map
> > > @@ -451,4 +451,6 @@ LIBBPF_1.7.0 {
> > > global:
> > > bpf_map__set_exclusive_program;
> > > bpf_map__exclusive_program;
> > > + bpf_prog_assoc_struct_ops;
> > > + bpf_program__assoc_struct_ops;
> > > } LIBBPF_1.6.0;
> > > --
> > > 2.47.3
> > >