netdev - Re: [PATCH net-next v2 1/2] bpf/tracing: allow user space to query prog array on the same tp

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Date:   Wed, 6 Dec 2017 12:56:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yonghong Song <yhs@...com>
Cc:     rostedt@...dmis.org, ast@...com, daniel@...earbox.net,
        kafai@...com, netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH net-next v2 1/2] bpf/tracing: allow user space to query
 prog array on the same tp

On Tue, Dec 05, 2017 at 10:31:28PM -0800, Yonghong Song wrote:
> Commit e87c6bc3852b ("bpf: permit multiple bpf attachments
> for a single perf event") added support to attach multiple
> bpf programs to a single perf event.
> Commit 2541517c32be ("tracing, perf: Implement BPF programs
> attached to kprobes") utilized the existing perf ioctl
> interface and added the command PERF_EVENT_IOC_SET_BPF
> to attach a bpf program to a tracepoint.
> 
> This patch adds a new ioctl
> command, given a perf event fd, to query the bpf program array
> attached to the same perf tracepoint event.
> 
> The new uapi ioctl command:
>   PERF_EVENT_IOC_QUERY_BPF
> 
> The new uapi/linux/perf_event.h structure:
>   struct perf_event_query_bpf {
>        __u64	prog_ids;
>        __u32	prog_cnt;
>   };
> 
> The usage:
>   struct perf_event_query_bpf query;
>   query.prog_ids = (__u64)usr_prog_ids_buf;
>   query.prog_cnt = usr_prog_ids_buf_len;
>   err = ioctl(pmu_efd, PERF_EVENT_IOC_QUERY_BPF, &query);
> 
> Signed-off-by: Yonghong Song <yhs@...com>
> Acked-by: Alexei Starovoitov <ast@...nel.org>

Can you please fix that example to make it clear that prog_ids is in
fact a pointer to an array of size prog_cnt. Ideally also describing
what the type of array is.

In fact, would not something like:

	struct perf_event_query_bpf {
		__u32 len;
		__u32 __reserved;
		__u64 ids[0];
	};

be a much clearer interface?

Also, you forgot to tell us why we need this interface at all.