perf segmentation fault from NULL dereference

From: John Garry <john.garry@huawei.com>
To: Andi Kleen <ak@linux.intel.com>, Jiri Olsa <jolsa@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Arnaldo Carvalho de Melo" <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Linuxarm <linuxarm@huawei.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Namhyung Kim <namhyung@kernel.org>
Subject: perf segmentation fault from NULL dereference
Date: Tue, 25 Sep 2018 16:53:40 +0100	[thread overview]
Message-ID: <712b7c31-f681-7737-71e7-c028b8d2bba5@huawei.com> (raw)

Hi,

I am seeing this perf crash on my arm64-based system:

root@localhost:~# ./perf_debug_ record -e armv8_pmuv3_0/br_mis_pred/ sleep 1
perf: Segmentation fault
Obtained 9 stack frames.
./perf_debug_() [0x4c5ef8]
[0xffff82ba267c]
./perf_debug_() [0x4bc5a8]
./perf_debug_() [0x419550]
./perf_debug_() [0x41a928]
./perf_debug_() [0x472f58]
./perf_debug_() [0x473210]
./perf_debug_() [0x4070f4]
/lib/aarch64-linux-gnu/libc.so.6(__libc_start_main+0xe0) [0xffff8294c8a0]
Segmentation fault (core dumped)

I find 'cycles' event is fine.

I bisected the issue to here:
commit bfd8f72c2778f5bd63dc9eb6d23bd7a0d99cff6d (HEAD, refs/bisect/bad)
Author: Andi Kleen <ak@linux.intel.com>
Date:   Fri Nov 17 13:42:58 2017 -0800

     perf record: Synthesize unit/scale/... in event update

     Move the code to synthesize event updates for scale/unit/cpus to a
     common utility file, and use it both from stat and record.

     This allows to access scale and other extra qualifiers from perf 
script.

     Signed-off-by: Andi Kleen <ak@linux.intel.com>
     Acked-by: Jiri Olsa <jolsa@kernel.org>
     Link: 
http://lkml.kernel.org/r/20171117214300.32746-2-andi@firstfloor.org
     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

I am suspicious that this is a real issue, as this patch has been in 
mainline for some time...

This simple change fixes the issue me:

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 91e6d9c..f4fd826 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -3576,7 +3576,7 @@ int perf_event__process_feature(struct perf_tool 
*tool,
      int max, err;
      u16 type;

-    if (!evsel->own_cpus)
+    if (!evsel->own_cpus || !(evsel->attr.read_format & 
PERF_FORMAT_ID)) // roundabout check for !evsel->id
          return 0;

      ev = cpu_map_data__alloc(evsel->own_cpus, &size, &type, &max);

It turns out that evsel->id is NULL on a call to 
perf_event__process_feature(), which upsets this code:

     ev->header.type = PERF_RECORD_EVENT_UPDATE;
     ev->header.size = (u16)size;
     ev->type = PERF_EVENT_UPDATE__CPUS;
     ev->id   = evsel->id[0];

Please me let me know if a valid issue so we can get a fix in.

Apologies for crying wolf if I'm off the mark.

Cheers,
John

