Re: [PATCH] libbpf: Use the correct fd when attaching to perf events

[Andrii Nakryiko]

On Fri, Mar 12, 2021 at 6:43 PM Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, Mar 12, 2021 at 06:33:01PM -0800, Andrii Nakryiko wrote:
> > On Fri, Mar 12, 2021 at 6:22 PM Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Mar 12, 2021 at 05:31:14PM -0800, Andrii Nakryiko wrote:
> > > > On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
> > > > >
> > > > > We should be using the program fd here, not the perf event fd.
> > > >
> > > > Why? Can you elaborate on what issue you ran into with the current code?
> > >
> > > bpf_link__pin() would fail with -EINVAL when using tracepoints, kprobes, or
> > > uprobes. The failure would happen inside the kernel, in bpf_link_get_from_fd()
> > > right here:
> > >         if (f.file->f_op != &bpf_link_fops) {
> > >                 fdput(f);
> > >                 return ERR_PTR(-EINVAL);
> > >         }
> >
> > kprobe/tracepoint/perf_event attachments behave like bpf_link (so
> > libbpf uses user-space high-level bpf_link APIs for it), but they are
> > not bpf_link-based in the kernel. So bpf_link__pin() won't work for
> > such types of programs until we actually have bpf_link-backed
> > attachment support in the kernel itself. I never got to implementing
> > this because we already had auto-detachment properties from perf_event
> > FD itself. But it would be nice to have that done as a real bpf_link
> > in the kernel (with all the observability, program update,
> > force-detach support).
> >
> > Looking for volunteers to make this happen ;)
> >
> >
> > >
> > > Since bpf wasn't looking for the perf event fd, I swapped it for the program fd
> > > and bpf_link__pin() worked.
> >
> > But you were pinning the BPF program, not a BPF link. Which is not
> > what should have happen.
>
> This is the code in question:
>         link = bpf_program__attach(prog);
>         // make sure `link` is valid, blah blah...
>         bpf_link__pin(link, some_path);
>
> Are you saying that this usage is incorrect?

Right, for kprobe/tracepoint/perf_event attachments it's not
supported. cgroup, xdp, raw_tracepoint and
fentry/fexit/fmod_ret/freplace (and a few more) attachments are
bpf_links in the kernel, so it works for them.

>
> Sultan