[lttng-dev] [PATCH] Avoid using the heap for small strings with tracef/tracelog

[Mathieu Desnoyers]

----- On May 19, 2021, at 12:56 PM, lttng-dev lttng-dev at lists.lttng.org wrote:

> Try to use vsnprintf with a 512 Byte buffer on the Stack,
> if that fails allocate a larger one.

I agree with the approach.

Can we move the hardcoded "512" to a #define within src/common/tracer.h ?

Thanks,

Mathieu

> 
> Signed-off-by: Norbert Lange <nolange79 at gmail.com>
> ---
> liblttng-ust/tracef.c   | 13 ++++++++++---
> liblttng-ust/tracelog.c | 12 +++++++++---
> 2 files changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/liblttng-ust/tracef.c b/liblttng-ust/tracef.c
> index ea98e43e..18c29e25 100644
> --- a/liblttng-ust/tracef.c
> +++ b/liblttng-ust/tracef.c
> @@ -32,17 +32,24 @@
> void _lttng_ust_tracef(const char *fmt, ...)
> {
> 	va_list ap;
> -	char *msg;
> +	char local_buf[512];
> +	char *msg = local_buf;
> 	int len;
> 
> 	va_start(ap, fmt);
> -	len = vasprintf(&msg, fmt, ap);
> +	len = vsnprintf(local_buf, sizeof(local_buf), fmt, ap);
> +	if (len >= sizeof(local_buf)) {
> +		msg = (char *)malloc(len + 1);
> +		len = msg ? vsnprintf(msg, len + 1, fmt, ap) : -1;
> +	}
> +
> 	/* len does not include the final \0 */
> 	if (len < 0)
> 		goto end;
> 	__tracepoint_cb_lttng_ust_tracef___event(msg, len,
> 		LTTNG_UST_CALLER_IP());
> -	free(msg);
> end:
> +	if (msg != local_buf)
> +		free(msg);
> 	va_end(ap);
> }
> diff --git a/liblttng-ust/tracelog.c b/liblttng-ust/tracelog.c
> index 65fc87ed..a5d110fa 100644
> --- a/liblttng-ust/tracelog.c
> +++ b/liblttng-ust/tracelog.c
> @@ -35,19 +35,25 @@
> 			const char *fmt, ...) \
> 	{ \
> 		va_list ap; \
> -		char *msg; \
> +		char local_buf[512]; \
> +		char *msg = local_buf; \
> 		int len; \
> 		\
> 		va_start(ap, fmt); \
> -		len = vasprintf(&msg, fmt, ap); \
> +		len = vsnprintf(local_buf, sizeof(local_buf), fmt, ap); \
> +		if (len >= sizeof(local_buf)) { \
> +			msg = (char *)malloc(len + 1); \
> +			len = msg ? vsnprintf(msg, len + 1, fmt, ap) : -1; \
> +		} \
> 		/* len does not include the final \0 */ \
> 		if (len < 0) \
> 			goto end; \
> 		__tracepoint_cb_lttng_ust_tracelog___##level(file, \
> 			line, func, msg, len, \
> 			LTTNG_UST_CALLER_IP()); \
> -		free(msg); \
> 	end: \
> +		if (msg != local_buf) \
> +			free(msg); \
> 		va_end(ap); \
> 	}
> 
> --
> 2.30.2
> 
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> lttng-dev at lists.lttng.org
> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com