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

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Date:   Mon, 4 Mar 2019 16:50:00 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tom Zanussi <zanussi@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 2/2] tracing: Use strncpy instead of memcpy for string
 keys in hist triggers

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

> From: Tom Zanussi <tom.zanussi@...ux.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@...ux.intel.com>
> Cc: Namhyung Kim <namhyung@...nel.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