perf record: Fix priv level with branch sampling for paranoid=2

This change “perf record: Fix priv level with branch sampling for paranoid=2″ in Linux kernel is authored by Stephane Eranian <eranian [at] google.com> on Fri Sep 20 16:03:56 2019 -0700.

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.

V2 fix a white space.

Committer testing:

After aplying the patch we get:

  [acme@quaco ~]$ perf record -b -e cycles date
  WARNING: Kernel address maps (/proc/{kallsyms,modules}) are restricted,
  check /proc/sys/kernel/kptr_restrict and /proc/sys/kernel/perf_event_paranoid.

  Samples in kernel functions may not be resolved if a suitable vmlinux
  file is not found in the buildid cache or in the vmlinux path.

  Samples in kernel modules won't be resolved at all.

  If some relocation was applied (e.g. kexec) symbols may be misresolved
  even with a suitable vmlinux or kallsyms file.

  Mon 23 Sep 2019 11:00:59 AM -03
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.005 MB perf.data (14 samples) ]
  [acme@quaco ~]$ perf evlist -v
  cycles:u: size: 112, { sample_period, sample_freq }: 4000, sample_type: IP|TID|TIME|PERIOD|BRANCH_STACK, read_format: ID, disabled: 1, inherit: 1, exclude_kernel: 1, exclude_hv: 1, mmap: 1, comm: 1, freq: 1, enable_on_exec: 1, task: 1, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, ksymbol: 1, bpf_event: 1, branch_sample_type: ANY
  [acme@quaco ~]$

That warning about restricted kernel maps will be suppressed in a follow
up patch, as perf_event_attr.exclude_kernel is set, i.e. no samples for
the kernel will be taken and thus no need for those maps.

Signed-off-by: Stephane Eranian <eranian@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20190920230356.41420-1-eranian@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 4ec8d98.

There are 6 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 8e335d1..502bc3d 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2535,9 +2535,11 @@ 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;
 	}

The commit for this change in Linux stable tree is 4ec8d98 (patch).

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