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In-Reply-To: <CAADnVQ+RKuJB5G+-1fjsE2xLp8CxJMmidd6Qobi_4dXQOWjrow@mail.gmail.com>
Date:   Mon, 19 Aug 2019 19:34:50 -0700
To:     "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
Cc:     "bpf" <bpf@vger.kernel.org>, "Song Liu" <songliubraving@fb.com>,
        "Yonghong Song" <yhs@fb.com>, "Andrii Nakryiko" <andriin@fb.com>,
        "Peter Zijlstra" <peterz@infradead.org>,
        "Ingo Molnar" <mingo@redhat.com>,
        "Arnaldo Carvalho de Melo" <acme@kernel.org>,
        "Alexei Starovoitov" <ast@fb.com>,
        "Alexander Shishkin" <alexander.shishkin@linux.intel.com>,
        "Jiri Olsa" <jolsa@redhat.com>,
        "Namhyung Kim" <namhyung@kernel.org>,
        "LKML" <linux-kernel@vger.kernel.org>,
        "Kernel Team" <kernel-team@fb.com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add
 PERF_EVENT_IOC_QUERY_PROBE ioctl
From:   "Daniel Xu" <dxu@dxuuu.xyz>
Message-Id: <BWE3UBBDYMGD.26324NSRV46UF@dlxu-fedora-R90QNFJV>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon Aug 19, 2019 at 6:26 PM Alexei Starovoitov wrote:
> On Fri, Aug 16, 2019 at 3:33 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> >
> > It's useful to know [uk]probe's nmissed and nhit stats. For example wit=
h
> > 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@dxuuu.xyz>
> ...
> > +int perf_kprobe_event_query(struct perf_event *event, void __user *inf=
o)
> > +{
> > +       struct perf_event_query_probe __user *uquery =3D info;
> > +       struct perf_event_query_probe query =3D {};
> > +       struct trace_event_call *call =3D event->tp_event;
> > +       struct trace_kprobe *tk =3D (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, s=
ize)))
> > +               return -EFAULT;
> > +
> > +       ncopy =3D min_t(u64, query.size, sizeof(query));
> > +       query.nhit =3D trace_kprobe_nhit(tk);
> > +       query.nmissed =3D tk->rp.kp.nmissed;
> > +
> > +       if (copy_to_user(uquery, &query, ncopy))
> > +               return -EFAULT;
>=20
> 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.