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

Amery Hung
2025-11-04T23:39:59Z

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?

>
> > + 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().

>
> > + */
> > +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
> >