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

[Andrii Nakryiko]

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.

>
> Sultan