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

["Daniel Xu" <dxu@dxuuu.xyz>]

On Wed, Aug 7, 2019, at 11:57 AM, Andrii Nakryiko wrote:
> 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?

Makes sense to me. This would probably result in a more predictable API when
new types are added. I'll make it this way in V2.

> 
> >  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.

Ok.