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

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Date:   Fri, 12 Mar 2021 17:31:14 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Sultan Alsawaf <sultan@...neltoast.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 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?

>
> Fixes: 63f2f5ee856ba ("libbpf: add ability to attach/detach BPF program to perf event")
> Signed-off-by: Sultan Alsawaf <sultan@...neltoast.com>
> ---
>  tools/lib/bpf/libbpf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d43cc3f29dae..3d20d57d4af5 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -9538,7 +9538,7 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
>         if (!link)
>                 return ERR_PTR(-ENOMEM);
>         link->detach = &bpf_link__detach_perf_event;
> -       link->fd = pfd;
> +       link->fd = prog_fd;
>
>         if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
>                 err = -errno;
> --
> 2.30.2
>