Re: [PATCH v3 bpf-next 2/4] libbpf: Add helpers to extract perf fd from bpf_link

[Andrii Nakryiko]

On Fri, Aug 16, 2019 at 3:32 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> It is sometimes necessary to perform ioctl's on the underlying perf fd.
> There is not currently a way to extract the fd given a bpf_link, so add a
> a pair of casting and getting helpers.
>
> The casting and getting helpers are nice because they let us define
> broad categories of links that makes it clear to users what they can
> expect to extract from what type of link.
>
> Acked-by: Song Liu <songliubraving@xxxxxx>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---

This looks great, thanks a lot!

I think you might have a conflict with dadb81d0afe7 ("libbpf: make
libbpf.map source of truth for libbpf version") in libbpf.map, so you
might need to pull, rebase and re-post rebased version. But in any
case:

Acked-by: Andrii Nakryiko <andriin@xxxxxx>

>  tools/lib/bpf/libbpf.c   | 21 +++++++++++++++++++++
>  tools/lib/bpf/libbpf.h   | 13 +++++++++++++
>  tools/lib/bpf/libbpf.map |  4 ++++
>  3 files changed, 38 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 2233f919dd88..41588e13be2b 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4876,6 +4876,7 @@ int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
>
>  struct bpf_link {
>         int (*destroy)(struct bpf_link *link);
> +       enum bpf_link_type type;
>  };
>
>  int bpf_link__destroy(struct bpf_link *link)
> @@ -4909,6 +4910,24 @@ static int bpf_link__destroy_perf_event(struct bpf_link *link)
>         return err;
>  }
>
> +const struct bpf_link_fd *bpf_link__as_fd(const struct bpf_link *link)
> +{
> +       if (link->type != LIBBPF_LINK_FD)
> +               return NULL;
> +
> +       return (struct bpf_link_fd *)link;
> +}
> +
> +enum bpf_link_type bpf_link__type(const struct bpf_link *link)
> +{
> +       return link->type;
> +}
> +
> +int bpf_link_fd__fd(const struct bpf_link_fd *link)
> +{
> +       return link->fd;
> +}
> +
>  struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
>                                                 int pfd)
>  {
> @@ -4932,6 +4951,7 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
>         if (!link)
>                 return ERR_PTR(-ENOMEM);
>         link->link.destroy = &bpf_link__destroy_perf_event;
> +       link->link.type = LIBBPF_LINK_FD;
>         link->fd = pfd;
>
>         if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
> @@ -5225,6 +5245,7 @@ struct bpf_link *bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
>         link = malloc(sizeof(*link));
>         if (!link)
>                 return ERR_PTR(-ENOMEM);
> +       link->link.type = LIBBPF_LINK_FD;
>         link->link.destroy = &bpf_link__destroy_fd;
>
>         pfd = bpf_raw_tracepoint_open(tp_name, prog_fd);
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index e8f70977d137..2ddef5315ff9 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -166,7 +166,20 @@ LIBBPF_API int bpf_program__pin(struct bpf_program *prog, const char *path);
>  LIBBPF_API int bpf_program__unpin(struct bpf_program *prog, const char *path);
>  LIBBPF_API void bpf_program__unload(struct bpf_program *prog);
>
> +enum bpf_link_type {
> +       LIBBPF_LINK_FD,
> +};
> +
>  struct bpf_link;
> +struct bpf_link_fd;
> +
> +/* casting APIs */
> +LIBBPF_API const struct bpf_link_fd *
> +bpf_link__as_fd(const struct bpf_link *link);
> +
> +/* getters APIs */
> +LIBBPF_API enum bpf_link_type bpf_link__type(const struct bpf_link *link);
> +LIBBPF_API int bpf_link_fd__fd(const struct bpf_link_fd *link);
>
>  LIBBPF_API int bpf_link__destroy(struct bpf_link *link);
>
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 4e72df8e98ba..ee9945177100 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -186,4 +186,8 @@ LIBBPF_0.0.4 {
>  } LIBBPF_0.0.3;
>
>  LIBBPF_0.0.5 {
> +       global:
> +               bpf_link__type;
> +               bpf_link__as_fd;
> +               bpf_link_fd__fd;
>  } LIBBPF_0.0.4;
> --
> 2.20.1
>