Re: [PATCH] perf record: fix priv level with branch sampling for paranoid=2

[Stephane Eranian]

On Fri, Sep 20, 2019 at 12:12 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Tue, Sep 03, 2019 at 11:26:03PM -0700, Stephane Eranian wrote:
> > Now that the default perf_events paranoid level is set to 2, a regular user
> > cannot monitor kernel level activity anymore. As such, with the following
> > cmdline:
> >
> > $ perf record -e cycles date
> >
> > The perf tool first tries cycles:uk but then falls back to cycles:u
> > as can be seen in the perf report --header-only output:
> >
> >   cmdline : /export/hda3/tmp/perf.tip record -e cycles ls
> >   event : name = cycles:u, , id = { 436186, ... }
> >
> > This is okay as long as there is way to learn the priv level was changed
> > internally by the tool.
> >
> > But consider a similar example:
> >
> > $ perf record -b -e cycles date
> > Error:
> > You may not have permission to collect stats.
> >
> > Consider tweaking /proc/sys/kernel/perf_event_paranoid,
> > which controls use of the performance events system by
> > unprivileged users (without CAP_SYS_ADMIN).
> > ...
> >
> > Why is that treated differently given that the branch sampling inherits the
> > priv level of the first event in this case, i.e., cycles:u? It turns out
> > that the branch sampling code is more picky and also checks exclude_hv.
> >
> > In the fallback path, perf record is setting exclude_kernel = 1, but it
> > does not change exclude_hv. This does not seem to match the restriction
> > imposed by paranoid = 2.
> >
> > This patch fixes the problem by forcing exclude_hv = 1 in the fallback
> > for paranoid=2. With this in place:
> >
> > $ perf record -b -e cycles date
> >   cmdline : /export/hda3/tmp/perf.tip record -b -e cycles ls
> >   event : name = cycles:u, , id = { 436847, ... }
> >
> > And the command succeeds as expected.
> >
> > Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
> > ---
> >  tools/perf/util/evsel.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index 85825384f9e8..3cbe06fdf7f7 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -2811,9 +2811,11 @@ bool perf_evsel__fallback(struct evsel *evsel, int err,
> >               if (evsel->name)
> >                       free(evsel->name);
> >               evsel->name = new_name;
> > -             scnprintf(msg, msgsize,
> > -"kernel.perf_event_paranoid=%d, trying to fall back to excluding kernel samples", paranoid);
> > +             scnprintf(msg, msgsize, "kernel.perf_event_paranoid=%d, trying "
> > +                       "to fall back to excluding kernel and hypervisor "
> > +                       " samples", paranoid);
>
> extra space in here        ^
>
>         Warning:
>         kernel.perf_event_paranoid=2, trying to fall back to excluding kernel and hypervisor  samples
>
> other than that it looks good to me
>
Fixed in v2.

> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>
> thanks,
> jirka