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

[Sultan Alsawaf]

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);
	}

Since bpf wasn't looking for the perf event fd, I swapped it for the program fd
and bpf_link__pin() worked.

Sultan