linux-kernel - RE: [PATCH] libbpf: Remove kprobe_event on failed kprobe_open

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Date:   Fri, 17 Jun 2022 21:13:46 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Chuang W <nashuiliang@...il.com>
Cc:     Chuang W <nashuiliang@...il.com>,
        Jingren Zhou <zhoujingren@...iglobal.com>,
        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>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: RE: [PATCH] libbpf: Remove kprobe_event on failed kprobe_open_legacy

Chuang W wrote:
> In a scenario where livepatch and aggrprobe coexist, the creating
> kprobe_event using tracefs API will succeed, a trace event (e.g.
> /debugfs/tracing/events/kprobe/XX) will exist, but perf_event_open()
> will return an error.

This seems a bit strange from API side. I'm not really familiar with
livepatch, but I guess this is UAPI now so fixing add_kprobe_event_legacy
to fail is not an option?

> 
> Signed-off-by: Chuang W <nashuiliang@...il.com>
> Signed-off-by: Jingren Zhou <zhoujingren@...iglobal.com>
> ---
>  tools/lib/bpf/libbpf.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 0781fae58a06..d0a36350e22a 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10809,10 +10809,11 @@ static int perf_event_kprobe_open_legacy(const char *probe_name, bool retprobe,
>  	}
>  	type = determine_kprobe_perf_type_legacy(probe_name, retprobe);
>  	if (type < 0) {
> +		err = type;
>  		pr_warn("failed to determine legacy kprobe event id for '%s+0x%zx': %s\n",
>  			kfunc_name, offset,
> -			libbpf_strerror_r(type, errmsg, sizeof(errmsg)));
> -		return type;
> +			libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
> +		goto clear_kprobe_event;
>  	}
>  	attr.size = sizeof(attr);
>  	attr.config = type;
> @@ -10826,9 +10827,14 @@ static int perf_event_kprobe_open_legacy(const char *probe_name, bool retprobe,
>  		err = -errno;
>  		pr_warn("legacy kprobe perf_event_open() failed: %s\n",
>  			libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
> -		return err;
> +		goto clear_kprobe_event;
>  	}
>  	return pfd;
> +
> +clear_kprobe_event:
> +	/* Clear the newly added kprobe_event */
> +	remove_kprobe_event_legacy(probe_name, retprobe);
> +	return err;
>  }
>  
>  struct bpf_link *
> -- 
> 2.34.1
>