Re: [PATCH v2 bpf-next 1/4] tracing/probe: Add PERF_EVENT_IOC_QUERY_PROBE ioctl

[Song Liu]

On Fri, Aug 9, 2019 at 2:48 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_uprobe_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_uprobe *tu = (struct trace_uprobe *)call->data;
> +       u64 nmissed, nhit;
> +
> +       if (!capable(CAP_SYS_ADMIN))
> +               return -EPERM;
> +       if (copy_from_user(&query, uquery, sizeof(query)))
> +               return -EFAULT;
> +
> +       nhit = tu->nhit;
> +       nmissed = 0;

Blindly return 0 is a little weird. Maybe return 0xffffffffffffffff so
that the user
can tell this is not a valid 0. Or some other idea?

Thanks,
Song