[PATCH 2/2] tracing: Use strncpy instead of memcpy for string keys in hist triggers

Tom Zanussi
2019-02-04T21:07:45Z

From: Tom Zanussi <tom.zanussi@linux.intel.com>

Because there may be random garbage beyond a string's null terminator,
it's not correct to copy the the complete character array for use as a
hist trigger key. This results in multiple histogram entries for the
'same' string key.

So, in the case of a string key, use strncpy instead of memcpy to
avoid copying in the extra bytes.

Before, using the gdbus entries in the following hist trigger as an
example:

# echo 'hist:key=comm' > /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
# cat /sys/kernel/debug/tracing/events/sched/sched_waking/hist

...

{ comm: ImgDecoder #4 } hitcount: 203
{ comm: gmain } hitcount: 213
{ comm: gmain } hitcount: 216
{ comm: StreamTrans #73 } hitcount: 221
{ comm: mozStorage #3 } hitcount: 230
{ comm: gdbus } hitcount: 233
{ comm: StyleThread#5 } hitcount: 253
{ comm: gdbus } hitcount: 256
{ comm: gdbus } hitcount: 260
{ comm: StyleThread#4 } hitcount: 271

...

# cat /sys/kernel/debug/tracing/events/sched/sched_waking/hist | egrep gdbus | wc -l
51

After:

# cat /sys/kernel/debug/tracing/events/sched/sched_waking/hist | egrep gdbus | wc -l
1

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
---
kernel/trace/trace_events_hist.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index c4a667503bf0..1b349689b897 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -4695,9 +4695,10 @@ static inline void add_to_key(char *compound_key, void *key,
/* ensure NULL-termination */
if (size > key_field->size - 1)
size = key_field->size - 1;
- }

- memcpy(compound_key + key_field->offset, key, size);
+ strncpy(compound_key + key_field->offset, (char *)key, size);
+ } else
+ memcpy(compound_key + key_field->offset, key, size);
}

static void
--
2.14.1