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Subjectperf segmentation fault from NULL dereference
Date
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

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