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

[Daniel Xu]

On Tue, Aug 13, 2019, at 2:47 PM, Song Liu wrote:
> 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
>

My (maybe flawed) understanding is that uprobes cannot really miss the same way
a kprobe can. From skimming the code a little, it seems the main reason kprobes
can miss is when the processing of one kprobe results in hitting another kprobe.
The latter cannot be handled for whatever reason. The same cannot really happen
for uprobes as kernel doesn't call into userspace. That's why I made it 0 (that and
the fact I didn't see any accounting for uprobe misses).

cc Srikar who authored the uprobe patches.

Srikar, do you mind clarifying if uprobes can miss?

Thanks,
Daniel