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

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Date:   Fri, 12 Mar 2021 18:22:17 -0800
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libbpf: Use the correct fd when attaching to perf events

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@...neltoast.com> wrote:
> >
> > From: Sultan Alsawaf <sultan@...neltoast.com>
> >
> > 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