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

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Message-Id: <20190904062603.90165-1-eranian@google.com>
Date:   Tue,  3 Sep 2019 23:26:03 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...e.hu, acme@...hat.com,
        jolsa@...hat.com, namhyung@...nel.org
Subject: [PATCH] perf record: fix priv level with branch sampling for paranoid=2

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@...gle.com>
---
 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