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

[Stephane Eranian]

On Tue, Sep 3, 2019 at 11:26 PM Stephane Eranian <eranian@xxxxxxxxxx> 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.
>
Any comment on this patch?

> 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);
>                 evsel->core.attr.exclude_kernel = 1;
> +               evsel->core.attr.exclude_hv     = 1;
>
>                 return true;
>         }
> --
> 2.23.0.187.g17f5b7556c-goog
>