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

On Fri, Mar 12, 2021 at 06:33:01PM -0800, Andrii Nakryiko wrote:

On Fri, Mar 12, 2021 at 6:22 PM Sultan Alsawaf [off-list ref] wrote:

quoted

On Fri, Mar 12, 2021 at 05:31:14PM -0800, Andrii Nakryiko wrote:

quoted

On Fri, Mar 12, 2021 at 1:43 PM Sultan Alsawaf [off-list ref] wrote:

quoted

From: Sultan Alsawaf <redacted>

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 ;)

quoted

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?

Sultan