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

Steven Rostedt
2019-03-04T21:50:05Z

On Mon, 4 Feb 2019 15:07:24 -0600
Tom Zanussi <zanussi@kernel.org> wrote:

> 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);
> }
>

Shouldn't we use strncpy() in save_comm() too. Feels safer.

-- Steve