On Fri, Aug 16, 2019 at 3:33 PM Daniel Xu <dxu@xxxxxxxxx> 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@xxxxxxxxx>
...
> +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?
