linux-kernel - Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add PERF_EVENT_IOC_QUERY

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Message-ID: <CAADnVQ+RKuJB5G+-1fjsE2xLp8CxJMmidd6Qobi_4dXQOWjrow@mail.gmail.com>
Date:   Mon, 19 Aug 2019 18:26:09 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     bpf <bpf@...r.kernel.org>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexei Starovoitov <ast@...com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add PERF_EVENT_IOC_QUERY_PROBE
 ioctl

On Fri, Aug 16, 2019 at 3:33 PM Daniel Xu <dxu@...uu.xyz> wrote:
>
> It's useful to know [uk]probe's nmissed and nhit stats. For example with
> tracing tools, it's important to know when events may have been lost.
> debugfs currently exposes a control file to get this information, but
> it is not compatible with probes registered with the perf API.
>
> While bpf programs may be able to manually count nhit, there is no way
> to gather nmissed. In other words, it is currently not possible to
> retrieve information about FD-based probes.
>
> This patch adds a new ioctl that lets users query nmissed (as well as
> nhit for completeness). We currently only add support for [uk]probes
> but leave the possibility open for other probes like tracepoint.
>
> Signed-off-by: Daniel Xu <dxu@...uu.xyz>
...
> +int perf_kprobe_event_query(struct perf_event *event, void __user *info)
> +{
> +       struct perf_event_query_probe __user *uquery = info;
> +       struct perf_event_query_probe query = {};
> +       struct trace_event_call *call = event->tp_event;
> +       struct trace_kprobe *tk = (struct trace_kprobe *)call->data;
> +       u64 ncopy;
> +
> +       if (!capable(CAP_SYS_ADMIN))
> +               return -EPERM;
> +       if (copy_from_user(&query, uquery,
> +                          offsetofend(struct perf_event_query_probe, size)))
> +               return -EFAULT;
> +
> +       ncopy = min_t(u64, query.size, sizeof(query));
> +       query.nhit = trace_kprobe_nhit(tk);
> +       query.nmissed = tk->rp.kp.nmissed;
> +
> +       if (copy_to_user(uquery, &query, ncopy))
> +               return -EFAULT;

shouldn't kernel update query.size before copying back?
Otherwise how user space would know which fields
were populated?