Re: [PATCH bpf-next v7 4/8] libbpf: Create a bpf_link in bpf_map__attach_struct_ops().

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Mar 15, 2023 at 7:37 PM Kui-Feng Lee <kuifeng@meta.com> wrote:
>
> bpf_map__attach_struct_ops() was creating a dummy bpf_link as a
> placeholder, but now it is constructing an authentic one by calling
> bpf_link_create() if the map has the BPF_F_LINK flag.
>
> You can flag a struct_ops map with BPF_F_LINK by calling
> bpf_map__set_map_flags().
>
> Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
> ---
>  tools/lib/bpf/libbpf.c | 90 +++++++++++++++++++++++++++++++-----------
>  1 file changed, 66 insertions(+), 24 deletions(-)
>

[...]

> -               if (!prog)
> -                       continue;
> +       link->link.detach = bpf_link__detach_struct_ops;
>
> -               prog_fd = bpf_program__fd(prog);
> -               kern_data = st_ops->kern_vdata + st_ops->kern_func_off[i];
> -               *(unsigned long *)kern_data = prog_fd;
> +       if (!(map->def.map_flags & BPF_F_LINK)) {
> +               /* w/o a real link */
> +               link->link.fd = map->fd;
> +               link->map_fd = -1;
> +               return &link->link;
>         }
>
> -       err = bpf_map_update_elem(map->fd, &zero, st_ops->kern_vdata, 0);
> -       if (err) {
> -               err = -errno;
> +       fd = bpf_link_create(map->fd, -1, BPF_STRUCT_OPS, NULL);

pass 0, not -1. BPF APIs have a convention that fd=0 means "no fd was
provided". And actually kernel should have rejected this -1, so please
check why that didn't happen in your testing, we might be missing some
kernel validation.


> +       if (fd < 0) {
>                 free(link);
> -               return libbpf_err_ptr(err);
> +               return libbpf_err_ptr(fd);
>         }
>
> -       link->detach = bpf_link__detach_struct_ops;
> -       link->fd = map->fd;
> +       link->link.fd = fd;
> +       link->map_fd = map->fd;
>
> -       return link;
> +       return &link->link;
>  }
>
>  typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
> --
> 2.34.1
>