Re: [PATCH 2/3] libbpf: Add helper to extract perf fd from bpf_link

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

On Tue, Aug 6, 2019 at 4:42 PM Daniel Xu <dxu@dxuuu.xyz> 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
> helper for it.
> ---

So I've been going back and forth with this approach and the
alternative one, and I think I'm leaning towards the alternative one
still.

I think it's better to have a broad "categories" of bpf_links, e.g.:

- FD-based bpf_link (which is the only one we have right now):
bpf_link_fd. It's not just for perf FD-based ones, raw tracepoint is
not, but it's still FD-based;
- for cgroup-related links (once they are added), it will be
bpf_link_cg (or something along the lines);
- there probably should be separate XDP-related bpf_link with device
ID/name inside;
- etc, whatever we'll need.

Then we can have a set of casting APIs and getter APIs that extract
useful information from specific type of bpf_link. We can also add
direct bpf_link creation API (e.g., from known FD), for cases where it
makes sense.

So something like (in libbpf.h):

struct bpf_link_fd;
struct bpf_link_cg;

/* casting APIs */
const struct bpf_link_fd *bpf_link__as_fd(const struct bpf_link *link);
const struct bpf_link_cg *bpf_link__as_cg(const struct bpf_link *link);

/* getters APIs */
int bpf_link_fd__fd(const struct bpf_link_fd *link);
int bpf_link_cg__cgroup_fd(const struct bpf_link_cg *link);

/* link factories (in addition to attach APIs) */
const struct bpf_link_fd *bpf_link__from_fd(int fd);
const struct bpf_link_cg *bpf_link__from_cg(int cg_fd, /* whatever
else necessary */);

I think this way it becomes obvious what you can expect to get of each
possible type of bpf_link and you'll have to explicitly cast to the
right type. Yet we still hide implementation details, allow no-brainer
bpf_link__destroy regardless of specific type of link (which probably
will be a common case).

Thoughts?

>  tools/lib/bpf/libbpf.c   | 13 +++++++++++++
>  tools/lib/bpf/libbpf.h   |  1 +
>  tools/lib/bpf/libbpf.map |  5 +++++
>  3 files changed, 19 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ead915aec349..8469d69448ae 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4004,6 +4004,19 @@ static int bpf_link__destroy_perf_event(struct bpf_link *link)
>         return err;
>  }
>
> +int bpf_link__get_perf_fd(struct bpf_link *link)

this seems like a bit too specific name (and we should avoid "get"
words, as we do in a bunch of other libbpf APIs for getters). Maybe
just `bpf_link__fd`? This especially makes sense with a "file-based
bpf_link" abstraction I proposed above.

> +{
> +       struct bpf_link_fd *l = (void *)link;
> +
> +       if (!link)
> +               return -1;
> +
> +       if (link->destroy != &bpf_link__destroy_perf_event)
> +               return -1;
> +
> +       return l->fd;
> +}
> +
>  struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
>                                                 int pfd)
>  {
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 8a9d462a6f6d..5391ac95e4fa 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -168,6 +168,7 @@ LIBBPF_API void bpf_program__unload(struct bpf_program *prog);
>  struct bpf_link;
>
>  LIBBPF_API int bpf_link__destroy(struct bpf_link *link);
> +LIBBPF_API int bpf_link__get_perf_fd(struct bpf_link *link);
>
>  LIBBPF_API struct bpf_link *
>  bpf_program__attach_perf_event(struct bpf_program *prog, int pfd);
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index f9d316e873d8..0f844ce29b04 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -184,3 +184,8 @@ LIBBPF_0.0.4 {
>                 perf_buffer__new_raw;
>                 perf_buffer__poll;
>  } LIBBPF_0.0.3;
> +
> +LIBBPF_0.0.5 {
> +       global:
> +               bpf_link__get_perf_fd;
> +} LIBBPF_0.0.4;
> --
> 2.20.1
>