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

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Date:   Mon, 19 Aug 2019 19:34:50 -0700
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Alexei Starovoitov" <alexei.starovoitov@...il.com>
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 Mon Aug 19, 2019 at 6:26 PM Alexei Starovoitov wrote:
> 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?

Ah yes, sorry. Will add that.